Nitrogen dioxide DOAS measurements from ground and space: comparison of zenith scattered sunlight ground-based measurements and OMI data in Central Mexico

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Skiba, D. Parra-Guevara" "autores" => array:2 [ 0 => array:2 [ "nombre" => "Y.N." "apellidos" => "Skiba" ] 1 => array:2 [ "nombre" => "D." "apellidos" => "Parra-Guevara" ] ] ] ] ] "idiomaDefecto" => "en" "EPUB" => "https://multimedia.elsevier.es/PublicationsMultimediaV1/item/epub/S0187623613710841?idApp=UINPBA00004N" "url" => "/01876236/0000002600000003/v2_201505081749/S0187623613710841/v2_201505081749/en/main.assets" ] "en" => array:18 [ "idiomaDefecto" => true "titulo" => "Nitrogen dioxide DOAS measurements from ground and space: comparison of zenith scattered sunlight ground-based measurements and OMI data in Central Mexico" "tieneTextoCompleto" => true "paginas" => array:1 [ 0 => array:2 [ "paginaInicial" => "401" "paginaFinal" => "414" ] ] "autores" => array:2 [ 0 => array:4 [ "autoresLista" => "C. Rivera" "autores" => array:1 [ 0 => array:4 [ "nombre" => "C." "apellidos" => "Rivera" "email" => array:1 [ 0 => "claudia.rivera@atmosfera.unam.mx" ] "referencia" => array:1 [ 0 => array:2 [ "etiqueta" => "*" "identificador" => "cor0005" ] ] ] ] "afiliaciones" => array:1 [ 0 => array:2 [ "entidad" => "Facultad de Ciencias, Universidad Nacional Autónoma de México, Circuito Exterior s/n, Ciudad Universitaria, 04510 México, D.F." "identificador" => "aff0005" ] ] "correspondencia" => array:1 [ 0 => array:3 [ "identificador" => "cor0005" "etiqueta" => "*" "correspondencia" => "Corresponding author." ] ] ] 1 => array:3 [ "autoresLista" => "W. Stremme, M. Grutter" "autores" => array:2 [ 0 => array:2 [ "nombre" => "W." "apellidos" => "Stremme" ] 1 => array:2 [ "nombre" => "M." "apellidos" => "Grutter" ] ] "afiliaciones" => array:1 [ 0 => array:2 [ "entidad" => "Centro de Ciencias de la Atmósfera, Universidad Nacional Autónoma de México, Circuito de la Investigación Científica s/n, Ciudad Universitaria, 04510 México, D.F." "identificador" => "aff0010" ] ] ] ] "resumenGrafico" => array:2 [ "original" => 0 "multimedia" => array:7 [ "identificador" => "fig0040" "etiqueta" => "Fig. 8" "tipo" => "MULTIMEDIAFIGURA" "mostrarFloat" => true "mostrarDisplay" => false "figura" => array:1 [ 0 => array:4 [ "imagen" => "gr8.jpeg" "Alto" => 767 "Ancho" => 1915 "Tamanyo" => 193287 ] ] "descripcion" => array:1 [ "en" => "(a) Frequency distribution of the air masses 2.25 h before measured at UN AM computed from wind data taken at RAMA stations; (b) distribution pattern of the averaged NO2 total columns 2.25 h before were measured at UNAM" ] ] ] "textoCompleto" => "1IntroductionMexico City is the third largest city of the world, with more than 20 million inhabitants (<a class="elsevierStyleCrossRef" href="#bib0215">UN, 2012</a>). It is located at 19.4° N latitude and approximately at an elevation of 2200 m above sea level (masl). For several years Mexico City has experienced severe air quality problems, which have been addressed by authorities trough the implementation of various programs. While the countermeasures and technological improvements have effectively decreased the amount of some pollutants, the city continues to experience pollution episodes resulting on the frequent violation of air quality standards.Nitrogen dioxide (NO2) plays a major role in tropospheric and stratospheric chemistry (<a class="elsevierStyleCrossRef" href="#bib0080">Crutzen, 1979</a>). In polluted regions such as Mexico City, tropospheric NO2 concentrations can be highly variable in time and space and are influenced by both natural and anthropogenic emissions. In Mexico City, most of the NO2 present in the atmosphere comes from fossil fuel combustion from transportation and partly from industrial activities. NO2 levels in Mexico City have decreased by 30% between 1990 and 2008, and the average surface concentration registered in 2008 by four stations around the Universidad Nacional Autónoma de México (UNAM) campus in Mexico City—where the ground-based measurements presented in this study were conducted—was 56.8 ppb (<a class="elsevierStyleCrossRef" href="#bib0200">SMA-GDF, 2010</a>).Satellites provide global observations, allowing to retrieve information from nearly every region in the globe (<a class="elsevierStyleCrossRef" href="#bib0190">Rees, 2001</a>). There are a number of instruments measuring from space, covering a wide range of wavelengths, with different spatial resolution and viewing geometries. In this work we have focused on the Ozone Monitoring Instrument (OMI) (<a class="elsevierStyleCrossRef" href="#bib0155">Levelt et al. 2006a</a>, <a class="elsevierStyleCrossRef" href="#bib0160">b</a>), a UV-VIS spectrometer on board NASA’s Earth Observing System-Aura satellite, able to provide a daily global coverage.Data from the OMI instrument have been used in an extensive number of studies involving different atmospheric gases as NO2, O3, SO2, HCHO, among others (<a class="elsevierStyleCrossRef" href="#bib0010">Bhartia, 2002</a>; <a class="elsevierStyleCrossRef" href="#bib0060">Chance, 2002</a>). The NO2 data products provide information about total and tropospheric NO2 columns. The NO2 dataset in particular, has been validated in different studies. <a class="elsevierStyleCrossRef" href="#bib0050">Celarier et al. (2006)</a> compared NO2 columns from OMI with NO2 columns measured by the Systéme d’Analyse par Observation Zénithale (SAOZ) network, finding good agreement between the two of them although the measurements were not done at the same time. As a continuation, the different measurements and validation activities for OMI NO2 stratospheric, tropospheric and total columns using ground- and aircraft-based measurements were summarized by <a class="elsevierStyleCrossRef" href="#bib0055">Celarier et al. (2008)</a>. Good agreement was reported between OMI and nearby instruments, finding correlations higher than 0.6 with ground-based instruments, whereas OMI NO2 stratospheric and total columns were found to be underestimated by 14 and 15-30%, respectively.During the Dutch Aerosol and Nitrogen Dioxide Experiments for Validation of OMI and SCIA-MACHY (DANDELIONS) 2005 and 2006 field experiments in the Netherlands, <a class="elsevierStyleCrossRef" href="#bib0025">Brinksma et al. (2008)</a> found good agreement between tropospheric from OMI and MAX-DOAS (Multi-AXis), as well as between total NO2 from OMI and direct sun observations. <a class="elsevierStyleCrossRef" href="#bib0015">Boersma et al. (2008)</a> compared OMI NO2 tropospheric columns with in situ aircraft measurements during the INTEX-B campaign, the results showed good correlation with no significant bias between the two data sets. <a class="elsevierStyleCrossRef" href="#bib0140">Irie et al. (2008)</a> used MAX-DOAS measurements to validate OMI NO2 columns during the Mount Tai Experiment 2006 finding that OMI data may have a positive bias of 20% over the North China Plain. <a class="elsevierStyleCrossRef" href="#bib0110">Gruzdev and Elokhov (2010)</a> found that tropospheric OMI NO2 columns measured over Zvenigorod, Russia were approximately 40% lower than ground-based twilight measurements. <a class="elsevierStyleCrossRef" href="#bib0230">Vlemmix et al. (2010)</a> compared tropospheric ground-based measured NO2 columns with OMI-satellite tropospheric data finding no significant difference and a correlation of 0.88.<a name="p403"></a><a class="elsevierStyleCrossRef" href="#bib0135">Ionov et al. (2008)</a> conducted a geophysical validation of OMI NO2 vertical column measurements with collocated ground-based observations at two stations in central Asia and Europe finding that mid-latitude OMI stratospheric NO2 column data underestimates ground-based measurements.Direct-Sun mode measurements have been conducted as well in order to validate OMI NO2 columns. <a class="elsevierStyleCrossRef" href="#bib0240">Wenig et al. (2008)</a> used direct-Sun mode Brewer measurements to conduct a validation study with OMI tropospheric NO2 columns concluding that monthly averages of coinciding measurements yielded a correlation of 0.9, however OMI data was 25% lower than the ground-based Brewer measurements. <a class="elsevierStyleCrossRef" href="#bib0115">Herman et al. (2009)</a> conducted intercomparisons of NO2 columns measured with ground-based direct solar irradiance measurements to OMI NO2 columns finding good agreement (correlation of 0.73).Regional and global models have also been used to compare and validate OMI NO2 tropospheric columns (<a class="elsevierStyleCrossRef" href="#bib0145">Kim et al, 2009</a>; <a class="elsevierStyleCrossRef" href="#bib0125">Herron-Thorpe et al., 2010</a>; <a class="elsevierStyleCrossRef" href="#bib0130">Huijnen et al, 2010</a>; <a class="elsevierStyleCrossRef" href="#bib0095">Dirksen et al, 2011</a>) and to estimate NOx emissions from different sources (<a class="elsevierStyleCrossRef" href="#bib0165">Lin, 2012</a>). Recently, <a class="elsevierStyleCrossRef" href="#bib0005">Beirle et al. (2011)</a> studied NOx emissions and lifetimes from megacities and power plants, while <a class="elsevierStyleCrossRef" href="#bib0220">Valin et al. (2011)</a> used the super-zoom mode of OMI to observe NO2slant columns from point sources and two cities, capturing significant spatial variability at fine scales.In this work, the OMI NO2 data product (OMI/ Aura nitrogen dioxide total and tropospheric column 1-orbit L2 swath 13×24 km, data set version 003) has been analyzed and compared to ground-based measurements conducted at UNAM campus in Mexico City. NO2 column maps were reconstructed from the OMI data product using an algorithm based on the optimal estimation approach with a weighted assignment of the measurement onto a fine grid, in order to identify the spatial features of its distribution with higher resolution. Focus has been given to the study of NO2 mainly to its role as a catalyst in the production of tropospheric ozone (O3), which is responsible for one of the most severe air quality problems in the Mexico City metropolitan area (MCMA).2Data set2.1OMIOMI is a nadir viewing spectrometer covering a spectral region of 264-504 nm, conducting atmospheric chemistry measurements with a resolution of 0.42-0.63 nm since 2004 on board NASA’s Earth Observation System (EOS)-Aura satellite. Aura follows a sun-synchronous polar orbit (705 km altitude), with an ascending local equator crossing time at 13:45. OMIs observations provide complete global coverage in one day with a nominal ground footprint of 13×24 km2 at nadir (<a class="elsevierStyleCrossRef" href="#bib0180">OMI Team, 2009</a>). It measures a number of air quality components such as NO2, SO2, BrO, HCHO, and aerosols (<a class="elsevierStyleCrossRef" href="#bib0155">Levelt et al. 2006a</a>, <a class="elsevierStyleCrossRef" href="#bib0160">b</a>).The OMI operational algorithm is described in detail by <a class="elsevierStyleCrossRef" href="#bib0030">Bucsela et al. (2006)</a>. For the evaluation of spectra recorded by OMI, a similar DOAS method (<a class="elsevierStyleCrossRef" href="#bib0185">Platt and Stutz, 2008</a>) is used as for the ground instruments. In this method a spectral fit is applied to all measured spectra. The evaluation of spectra results on slant column densities (SCDs) for each viewing pixel. The computed SCDs are then converted to vertical column densities (VCDs) using an air mass factor (AMF). The AMF is the ratio between the retrieved SCD and the VCD. The AMF concept was introduced by <a class="elsevierStyleCrossRef" href="#bib0205">Solomon et al. (1987)</a> and is dependent on clouds and aerosols, as well as viewing geometries and surface albedo.In this work, the OMI/Aura nitrogen dioxide total and tropospheric column 1-orbit L2 swath 13×24 km, data set version 003, level 2, algorithm version 1.1.4.4 data product was downloaded from NASA’s Goddard Earth Sciences Data and Information Services Center (GESDISC) using the MIRADOR 1.42 tool (<a href="http://disc.sci.gsfc.nasa.gov/Aura/data-holdings/OMI">http://disc.sci.gsfc.nasa.gov/Aura/data-holdings/OMI</a>). We have analyzed the dataset and only considered total NO2 columns from cloud fraction below 20% over the period 2006-2011. This dataset has been reprocessed and released on December 2011 (<a class="elsevierStyleCrossRef" href="#bib0070">Claas, 2012</a>).2.1.1Reconstruction of the mean column NO2 distributionThe mean column NO2 distribution over a fine grid (of the order of about 2 km) can be reconstructed from measurements done on a larger footprint (here 13×24 km2 for OMI), solving the mathematically ill-posed problem Y = Kx + ε. In this equation, Y represents the vector containing all OMI NO2 measurements, x is the solution vector which contains the estimation of the mean column NO2 distribution on the chosen grid, ε represents the deviation between a column measured/retrieved from the space<a name="p404"></a> measurement and the estimated mean value. ε is mainly dependent on the day-to-day variance, but also on the seasonal variance and measurement precision. The matrix K describes how the measurements are related with the mean gas horizontal distribution. It represents a forward model that reproduces mainly the footprint of the OMI instrument (13×24 km2 at nadir). The reconstruction uses a Tihkonov-smoothing-constraint, similar as the one used for profile retrievals or in image restoration. Further details of this method can be found in <a class="elsevierStyleCrossRef" href="#bib0210">Stremme et al. (2013)</a>.2.2Zenith scattered sunlight DOASGround-based DOAS measurements were conducted at the UNAM campus in Mexico City (19.33° N, 99.18° W) between August 2010 and December 2011. Scattered light was collected by means of a telescope and transferred into an Ocean Optics TR2000 spectrometer with a spectral resolution of ~0.6 nm and wavelength coverage of 291-501 nm. Spectra were taken every minute using the DOASIS software (<a class="elsevierStyleCrossRef" href="#bib0150">Kraus, 2006</a>) and were evaluated using the QDOAS (version 1.00) software (<a class="elsevierStyleCrossRef" href="#bib0105">Fayt et al., 2011</a>). Differential cross-sections of NO2 at 298 K (<a class="elsevierStyleCrossRef" href="#bib0225">Vandaele et al., 1998</a>), O3 at 221 K and 241 K (<a class="elsevierStyleCrossRef" href="#bib0035">Burrows et al, 1999</a>), the oxygen dimer O4 (<a class="elsevierStyleCrossRef" href="#bib0120">Hermans et al, 1999</a>) and a Ring spectrum, generated at 273 K from a high resolution Kurucz file using the QDOAS software (<a class="elsevierStyleCrossRef" href="#bib0105">Fayt et al., 2011</a>) were included in the analysis. The oxygen dimer O4 was included in the evaluation procedure in order to detect enhancement on the light path of the measurements since it is expected to have a constant concentration in the atmosphere. Evaluations were conducted in the 405 and 465 nm wavelength ranges with a fixed reference spectrum measured on 21 December 2011 at 14:21 local time (LT) under clear sky and high ventilation conditions for the whole time series. Geometrical AMFs were calculated and used to convert SCDs to VCDs for further comparison with OMI results. For this it was considered that the light path through a trace gas layer varies with 1/cos θ, where θ is the solar zenith angle (<a class="elsevierStyleCrossRef" href="#bib0065">Chen et al, 2009</a>). It should be noted that this simplified approach without the use of a full radiative transfer calculation that effectively incorporates scattering processes, reflection on the earth’s surface, refraction, the curvature of the earth, the vertical distribution of trace gases and the influence of aerosols leads to additional errors discussed in detail by <a class="elsevierStyleCrossRef" href="#bib0185">Platt and Stutz (2008)</a>.2.3Surface measurementsSince 1986, a network of 34 stations run by the local government (Red Automática de Monitoreo Atmosférico, RAMA) reports continuous and permanent measurements of concentrations of O3, sulfur dioxide (SO2), nitrogen oxides (NOx), carbon monoxide (CO) and particulate matter PM10 and PM2.5 to evaluate air quality in the metropolitan area. All data generated by RAMA are freely available online at <a href="http://www.calidadaire.df.gob.mx/">http://www.calidadaire.df.gob.mx/</a>. NO2 is determined using a nitrogen oxides analyzer based on the chemilumi-nescence measurement technique. For this study NO2 concentration results generated during 2010 and 2011 by Pedregal (19.32° N, 99.20° W), Tlalpan (19.25 N, 99.18 W), Santa Úrsula (19.31°N, 99.14° W), Merced (19.42° N, 99.11° W) and FES Acatlán (19.48° N, 99.24° W) stations situated close to the study site were used to calculate NO2 columns, using mixing layer heights (<a class="elsevierStyleCrossRef" href="#sec0035">section 2.4</a>) as well as pressure and temperature data (<a class="elsevierStyleCrossRef" href="#sec0040">section 2.5</a>).2.4LIDARA Vaisala CL31 ceilometer, an active instrument based on the LIDAR (light detection and ranging) measurement technique, was used to measure back-scattered aerosol density at the UNAM campus, allowing the estimation of mixing layer heights. The measurement principle involves the transmission of pulsed laser radiation into the atmosphere, which is further backscattered and detected with a certain time delay. The measurement range of the instrument used in this field study was from 0 up to 7500 m above the ground with a 10 m resolution. The LIDAR wavelength of 905 nm was provided by an InGaAs MOCVD laser diode, 110 ns, 1.2 μJ per pulse, and repetition rate of 8192 Hz. The mixing height was retrieved using the CL31 MLH software based on the gradient method, which is focused on selecting the maximum of the negative gradient of the backscatter coefficient (β) to be the top of the mixed layer (-dβ/dx). A more detailed description of this methodology can be found in <a class="elsevierStyleCrossRef" href="#bib0175">Münkel et al. (2007)</a>.2.5Meteorological parametersMeteorological parameters were recorded by a Davis Vantage Pro 2 station installed at the UNAM campus, forming part of the Programa de Estaciones Meteorológicas del Bachillerato Universitario (PEMBU) network. From the dataset recorded by<a name="p405"></a> the meteorological station, pressure and temperature were used for the calculation of ground-based NO2 columns based on NO2 concentrations measured by the RAMA network.3Results and discussion3.1Ground-based NO2 columns<a class="elsevierStyleCrossRef" href="#fig0005">Figure 1</a> shows differential NO2columns measured at UNAM between August 2010 and December 2011. The gaps in the graph represent periods where no measurements were conducted, either due to experimental problems, or periods of cloudiness or rain identified through the analysis of the data itself using evaluation results of the oxygen dimer. Only measurements conducted between 14:00 and 15:00 LT are plotted in order to depict columns coincident with OMI overpasses (~14:21 LT) over Mexico City. Large variability on day-to-day NO2columns was measured, illustrating the dynamics of fresh emissions and rapid conversion of pollutants occurring in the study area. The ground-based average NO2 column values and standard deviation measured from August 2010 to December 2011 between 14:00 and 15:00 LT was (5.61 ± 3.32) ×1016 molecules/cm2.<elsevierMultimedia ident="fig0005"></elsevierMultimedia>The average diurnal behavior of differential NO2 columns measured at UNAM on the same measurement period are presented in <a class="elsevierStyleCrossRef" href="#fig0010">Figure 2</a>. The maximum of the measured NO2 differential columns is found around noon, which is consistent with the reported hourly NOx emissions for the MCMA by the official inventory (<a class="elsevierStyleCrossRef" href="#bib0200">SMA-GDF, 2010</a>).<elsevierMultimedia ident="fig0010"></elsevierMultimedia>3.2NO2 spatial distributionThe average distribution of NO2 total columns derived from OMI measurements conducted between<a name="p406"></a> June 2006 and December 2011 over the Mexico basin is depicted in <a class="elsevierStyleCrossRef" href="#fig0015">Figure 3</a>. Higher NO2 total columns are observed in the downtown area of Mexico City, which gradually decrease with distance from the city center. The average and standard deviation of all OMI NO2 columns measured over UNAM during 2010-2011 at its overpass time, ~14:21, is (1.18 ± 1.1)×1016 molecules/cm2. While <a class="elsevierStyleCrossRef" href="#fig0015">Figure 3</a> is simply the average of the columns falling on each grid point, <a class="elsevierStyleCrossRef" href="#fig0020">Figure 4</a> shows a reconstruction of the mean column gas distribution according to the methodology described in <a class="elsevierStyleCrossRef" href="#sec0020">section 2.1.1</a>. The reconstructed value of the average over UNAM for the same time period is 1.7×1016 molecules/cm2. A clear difference can be observed between the average and reconstructed column gas distributions with the average column being smaller over UNAM. This bias is present during all seasons, it might originate from the sensitivity of the space-borne measurements to pollution near the surface and is discussed in more detail in <a class="elsevierStyleCrossRef" href="#sec0070">section 3.3</a>.<elsevierMultimedia ident="fig0015"></elsevierMultimedia><elsevierMultimedia ident="fig0020"></elsevierMultimedia>3.2.1Three main areas of interest in central MexicoFrom <a class="elsevierStyleCrossRef" href="#fig0015">Figures 3</a> and <a class="elsevierStyleCrossRef" href="#fig0020">4</a>, three main areas of interest in central Mexico can be identified: the Tula industrial area to the north, a dominating blur centered around Mexico City, and the Cuernavaca valley to the south. However, the reconstructed map in <a class="elsevierStyleCrossRef" href="#fig0020">Figure 4</a> allows to better appreciate the finely defined areas where either NO2 columns are enhanced or diminished over the studied area. Lacustrian and mountainous areas inside the MCMA are, for instance, more clearly observed in <a class="elsevierStyleCrossRef" href="#fig0020">Figure 4</a> than in <a class="elsevierStyleCrossRef" href="#fig0015">Figure 3</a>. Also, the industrial and more densely populated areas are better defined as well.Unique distribution structures of NO2 columns above the MCMA were identified through the reconstruction of mean NO2 column distribution in <a class="elsevierStyleCrossRef" href="#fig0025">Figure 5a</a>. NO2 column enhancements can be observed on the northern and central parts of the city, which are characterized by intense and heavy traffic (central and northern parts, respectively) and industrial activities (northern part). On the southern, eastern and western sides, the NO2 columns tend to decrease with distance of the city center, while on the northern part, the enhancement of NO2 columns is even extended towards the Tula industrial area (<a class="elsevierStyleCrossRef" href="#fig0025">Fig. 5b</a>), from which a clear footprint was observed and differentiated from the MCMA.<elsevierMultimedia ident="fig0025"></elsevierMultimedia>The region around Tula shown in <a class="elsevierStyleCrossRef" href="#fig0025">Figure 5b</a>, is located in the state of Hidalgo, ~50 km northwest of Mexico City. It is characterized by the presence of a power plant, a refinery, several cement works and other manufacturing industries. The OMI footprint clearly shows higher NO2 column values close to the industries. As in the case of the MCMA, the NO2 column enhancement gradually decreases with distance of the pollution sources with an increase of NO2 columns towards Mexico City. From <a class="elsevierStyleCrossRef" href="#fig0025">Figure 5b</a>, an empiric dispersion pattern of the plume of the Tula industrial complex towards the southeast can be inferred. This result is consistent with previous studies using<a name="p407"></a> dispersion models addressing the transport of the emissions from the Tula industrial complex towards the MCMA (<a class="elsevierStyleCrossRef" href="#bib0045">CCA-UNAM, 2008</a>; <a class="elsevierStyleCrossRef" href="#bib0085">de Foy et al., 2009</a>).The third identified area in central Mexico where a clear, though less intense NO2 column enhancement is observed, is the Cuernavaca valley shown in <a class="elsevierStyleCrossRef" href="#fig0025">Figure 5c</a>. This result was somewhat unexpected when compared to the relatively small column enhancements observed above the cities of Puebla and Toluca, which are similar to Cuernavaca in terms of population and industrial activity. Cuernavaca valley, however, is much lower in altitude (< 1500 masl), host of a large number of sugar cane plantations. During the industrial processing and harvesting intense biomass burning is practiced, which could explain the increased NO2 column values observed with the OMI instrument. Another possible explanation is the impact of emissions by the surrounding states. Recently <a class="elsevierStyleCrossRef" href="#bib0195">Salcedo et al. (2012)</a> reported an efficient transport of ozone and its precursors to this area by the surrounding Mexico City and Puebla state, while <a class="elsevierStyleCrossRef" href="#bib0170">Melamed et al. (2009)</a> detected during a study in 2006, NO2 transport events southeast of Mexico City from and towards the Cuautla region.<a name="p408"></a>3.2.2Seasonal variabilityAn additional approach was used analyzing OMI data into three different seasons: (1) dry and cold covering the months of November, December, January and February; (2) dry and warm for the months of March, April and May; and (3) the rainy season including the months of June, July, August, September and October. <a class="elsevierStyleCrossRef" href="#fig0030">Figure 6</a> depicts NO2 total column distributions reconstructed from OMI measurements conducted between June 2006 and December 2011 for the dry and cold (<a class="elsevierStyleCrossRef" href="#fig0030">Fig. 6a</a>), dry and warm (<a class="elsevierStyleCrossRef" href="#fig0030">Fig. 6b</a>) and rainy seasons (<a class="elsevierStyleCrossRef" href="#fig0030">Fig. 6c</a>). The same scale is used in all three plots for comparison. Higher NO2 columns were measured during the dry and cold season, followed by the dry and warm period and finally the lowest NO2 columns were found during the raining season. For the dry-cold and dry-warm seasons the pattern is confirmed by the ground-based measurements at UNAM (<a class="elsevierStyleCrossRef" href="#fig0035">Fig. 7</a>). <a class="elsevierStyleCrossRef" href="#bib0235">Wallace and Kanaroglou (2009)</a> have reported sensitivity of OMI to subtle temperature changes, while <a class="elsevierStyleCrossRef" href="#bib0020">Boersma et al. (2009)</a> observed NO2 columns 1.6-2.7 times higher in winter than in summer in Israeli cities. The atmospheric stability characteristic of the dry and cold season promotes a larger number of molecules to be present in the studied area, while during the raining season, water is mainly responsible for the removal of pollutants in the atmosphere.<elsevierMultimedia ident="fig0030"></elsevierMultimedia><elsevierMultimedia ident="fig0035"></elsevierMultimedia>The NO2 total column distribution reconstructed during the rainy season (<a class="elsevierStyleCrossRef" href="#fig0030">Fig. 6c</a>) allows to better identify the presence of areas with NO2 column enhancements over the cities of Toluca and Puebla, whose contribution is somewhat lost by the large<a name="p409"></a> column densities measured over the MCMA, the Tula industrial area or even the Cuernavaca valley during other months of the year. Also, a transport between the MCMA and Cuernavaca is visible during the dry seasons both through the Cuautla and Cuernavaca sides, where the highway connecting both cities runs. This confirms the pollution transport highlighted in the previous section and addressed by previous studies (<a class="elsevierStyleCrossRef" href="#bib0170">Melamed et. al, 2009</a>; <a class="elsevierStyleCrossRef" href="#bib0195">Salcedo et al, 2012</a>).3.3Comparison of ground-based and space-borne measurementsA comparison of direct coincident measurements was done for NO2 ground-based and OMI measured columns. All ground-based measurements conducted during the overpass time of OMI over UNAM (~14:21 LT) were used with a tolerance of ± 20 min. OMI measurements having a pixel center within a certain radius are used. The number of coincident days for the direct comparison is increasing when using larger radius. Ground-based and OMI measurements conducted on the coincident days and times were averaged and are reported in <a class="elsevierStyleCrossRef" href="#tbl0005">Table I</a> along with its standard deviation, correlation coefficient, linear-fit offset and linear-fit slope. In general, ground-based measured NO2 columns were 2.7-6 times higher than OMI measured columns, which could be explained by the spatial discrepancy between the ground-based measurements (basically local) and the satellite averaged measurements. The<a name="p410"></a> highest correlation coefficient (0.807) was found when comparing OMI data with a pixel center falling within a 20 km radius around UNAM; however, good correlation was also observed for data with pixel centers of 15 to 50 km radius around the ground-based site.<elsevierMultimedia ident="tbl0005"></elsevierMultimedia>The averages from OMI NO2 columns shown in <a class="elsevierStyleCrossRef" href="#tbl0005">Table I</a>, however, are underestimated since the instrument’s footprint of 13×24 km2 at nadir covers a large area and the UNAM site is within a region with large gradients in its horizontal distribution. A better representation of the fine structure is obtained by the reconstruction shown in <a class="elsevierStyleCrossRef" href="#fig0020">Figure 4</a> according to the methodology described in <a class="elsevierStyleCrossRef" href="#sec0020">section 2.1.1</a>. The reconstructed value of the average over UNAM for the entire time period is 1.7×1016 molecules/cm2. This value is approximately three times lower than the average ground-based NO2 column measured over UNAM.Furthermore, the satellite observation might not be sensitive enough close to the surface, where most of the pollution is present. A sensitivity of 0.5 would actually explain the difference by a factor of 2 between the mean values of the ground-based and space-borne DOAS measurements. The surface albedo of Mexico City in the 450 nm spectral region was measured by <a class="elsevierStyleCrossRef" href="#bib0040">Castro et al. (2001)</a> and recently during the MILAGRO field experiment by <a class="elsevierStyleCrossRef" href="#bib0075">Coddington et al. (2008)</a> and found to be in average around 0.05 and 0.07 respectively. Therefore, the estimated sensitivity of space-borne DOAS retrievals on surface near NO2 anomalies (local pollution in the mixing layer) should range between 0.2 and 0.7, which correspond to the two examples for surface albedo, 0.02 and 0.15, showed by <a class="elsevierStyleCrossRef" href="#bib0100">Eskes and Boersma (2003)</a>.Monthly average comparisons were also made between ground-based measurements, OMI and RAMA calculated NO2 columns (<a class="elsevierStyleCrossRef" href="#fig0035">Fig. 7</a>). Again, only ground-based measurements within ± 20 min of the OMI overpass time (~14:21 LT) above UNAM are kept. OMI averages were restricted to columns with pixel centers within ±0.1 degrees latitude/ longitude of UNAM. Ground-based reported RAMA NO2 columns were calculated from the average concentrations measured by five stations located near UNAM, multiplied by the measured mixing layer height (<a class="elsevierStyleCrossRef" href="#sec0035">section 2.4</a>) and correcting for pressure and temperature (<a class="elsevierStyleCrossRef" href="#sec0040">section 2.5</a>). Amonthly cycle was clearly identified, however, it would seem to differ slightly depending on the origin of the data. The calculated columns from surface RAMA data were the highest, followed by the ground-based DOAS measurements and finally the OMI data yielded the lowest values. This result was expected since the ground-based RAMA stations sample NO2 at a single sampling point very close to the sources and/ or when it has just been recently converted. The DOAS ground-based measurements are conducted close to the sources as well, however, its location and vegetation around the UNAM campus could partly dampen the detection of NO2 at the current sampling point. Furthermore, while the ground-based DOAS instrument is sampling at a single point, the observed columns by OMI are effectively averaged over the instrument’s field of view (13×24 km2 at nadir) yielding lower NO2 columns than the other two instruments probably because of the averaging of polluted and unpolluted areas.3.4Spatial distribution of NO2 columns reconstructed from UNAM measurementsAn additional analysis was made in the interest of gaining more information regarding the possible origin of the NO2 columns measured over UNAM, especially due to the inhomogeneous distribution of NO2 columns in the city, attested by OMI measurements. The averaged surface wind speed and wind direction measured at five stations near UNAM were used to compute the statistical wind propagation pattern that indicates the probability that a column measured at UNAM is originated or transported from a certain area of the city. Wind directions were analyzed on 30° intervals, and a probability was assigned to each sector according to the relative frequencies. Each sector was differentiated into three regions according to the intervals of certain probability (15.85,68.3, and 15.85%) related to different intervals of confidence of a log-normal distribution. Accordingly, wind speeds were expected to follow a log-normal distribution as well, described by the median and the log-normal standard deviation (<a class="elsevierStyleCrossRef" href="#fig0040">Fig. 8a</a>).<elsevierMultimedia ident="fig0040"></elsevierMultimedia>The false color of each region in <a class="elsevierStyleCrossRef" href="#fig0040">Figure 8a</a> indicates the probability that an airmass will be measured at UNAM with the certain time delay of 2.25 h. This time delay used for the visualization (<a class="elsevierStyleCrossRef" href="#fig0040">Fig. 8</a>) is the estimated lifetime of NO2 in the mixing layer with a typical temperature of the Mexico City boundary layer and according to <a class="elsevierStyleCrossRef" href="#bib0090">Dils (2008)</a>. The measured NO2 columns were then distributed to the corresponding<a name="p411"></a> sectors using wind direction and wind speed at the time when measurements were conducted. The distribution pattern of the averaged NO2 total columns reconstructed from the measurements at UNAM and wind measurements is presented in <a class="elsevierStyleCrossRef" href="#fig0040">Figure 8b</a>. The result of this approach indicates a tendency to measure higher NO2 columns when the wind direction is predominant from the north-northwest, coinciding with the most industrialized and populated area of the city.4ConclusionsNO2 columns were measured at UNAM in Mexico City and compared to space-borne observations conducted by the OMI instrument. Space-borne NO2 columns were lower than ground-based measurements, which presented large day-to-day variations. The difference between space-borne and ground-based columns could be explained by the strong horizontal inhomogeneity of the lower layer of the measured NO2 columns and by the difference in horizontal sensitivity of the two techniques (local versus averaging over 13×24 km2). More instruments located in and around the MCMA would allow to better evaluate the spatial distribution obtained from satellite measurements. A network of several MAX-DOAS has already been installed starting the fall of 2012, which will result in better ground-based coverage and accuracy of the reported columns.A reconstructed distribution of weighted NO2 columns over a fine grid was made over a large region in central Mexico, allowing to successfully define three main areas with enhanced pollution: the dominating metropolitan area of Mexico City, the heavily industrialized Tula region and the Cuernavaca valley. Other mayor urban areas such as Toluca and Puebla showed in average relatively lower column densities than Cuernavaca. The considerably higher altitude of Toluca probably favors the photolytic destruction of NO2, and this area does not appear to receive as much pollution from Mexico City, as the Cuernavaca valley. The importance of considering changes in albedo, aerosols load and their refractions indexes in NO2 photolysis rates should also be noted. The unanticipated enhancement of NO2 columns above the Cuernavaca valley, corroborates results from previous studies conducted by <a class="elsevierStyleCrossRef" href="#bib0170">Melamed et al. (2009)</a> and <a class="elsevierStyleCrossRef" href="#bib0195">Salcedo et al. (2012)</a>, where a possible transport of pollutants from the MCMA have been discussed. A plausible source arising from the biomass burning activities in that region is suggested for the high NO2 columns detected.From the reconstructed maps it is also possible to observe an important interconnection between the MCMA and the Tula industrial region, where emissions from one can evidently affect the other and the transport of pollutants between them most probably occurs. In addition, distinct seasonal variability of NO2 columns over central Mexico was found, concurring with previous studies, with higher NO2 columns measured during the dry and cold season, followed by the dry and warm period and finally the lowest NO2 columns were found during the rainy<a name="p412"></a> season. Furthermore, the spatial distribution of NO2 columns reconstructed from measurements conducted at UNAM indicate a tendency to measure higher NO2 columns when the wind direction is predominant from the north-northwest, coinciding with the most industrialized area of the city." "textoCompletoSecciones" => array:1 [ "secciones" => array:9 [ 0 => array:3 [ "identificador" => "xres499034" "titulo" => "Resumen" "secciones" => array:1 [ 0 => array:1 [ "identificador" => "abst0005" ] ] ] 1 => array:3 [ "identificador" => "xres499033" "titulo" => "Abstract" "secciones" => array:1 [ 0 => array:1 [ "identificador" => "abst0010" ] ] ] 2 => array:2 [ "identificador" => "xpalclavsec520466" "titulo" => "Keywords" ] 3 => array:2 [ "identificador" => "sec0005" "titulo" => "Introduction" ] 4 => array:3 [ "identificador" => "sec0010" "titulo" => "Data set" "secciones" => array:5 [ 0 => array:3 [ "identificador" => "sec0015" "titulo" => "OMI" "secciones" => array:1 [ 0 => array:2 [ "identificador" => "sec0020" "titulo" => "Reconstruction of the mean column NO distribution" ] ] ] 1 => array:2 [ "identificador" => "sec0025" "titulo" => "Zenith scattered sunlight DOAS" ] 2 => array:2 [ "identificador" => "sec0030" "titulo" => "Surface measurements" ] 3 => array:2 [ "identificador" => "sec0035" "titulo" => "LIDAR" ] 4 => array:2 [ "identificador" => "sec0040" "titulo" => "Meteorological parameters" ] ] ] 5 => array:3 [ "identificador" => "sec0045" "titulo" => "Results and discussion" "secciones" => array:4 [ 0 => array:2 [ "identificador" => "sec0050" "titulo" => "Ground-based NO columns" ] 1 => array:3 [ "identificador" => "sec0055" "titulo" => "NO spatial distribution" "secciones" => array:2 [ 0 => array:2 [ "identificador" => "sec0060" "titulo" => "Three main areas of interest in central Mexico" ] 1 => array:2 [ "identificador" => "sec0065" "titulo" => "Seasonal variability" ] ] ] 2 => array:2 [ "identificador" => "sec0070" "titulo" => "Comparison of ground-based and space-borne measurements" ] 3 => array:2 [ "identificador" => "sec0075" "titulo" => "Spatial distribution of NO columns reconstructed from UNAM measurements" ] ] ] 6 => array:2 [ "identificador" => "sec0080" "titulo" => "Conclusions" ] 7 => array:2 [ "identificador" => "xack161229" "titulo" => "Acknowledgements" ] 8 => array:1 [ "titulo" => "References" ] ] ] "pdfFichero" => "main.pdf" "tienePdf" => true "fechaRecibido" => "2012-08-30" "fechaAceptado" => "2013-02-22" "PalabrasClave" => array:1 [ "en" => array:1 [ 0 => array:4 [ "clase" => "keyword" "titulo" => "Keywords" "identificador" => "xpalclavsec520466" "palabras" => array:6 [ 0 => "DOAS" 1 => "OMI" 2 => "nitrogen dioxide" 3 => "ground-based" 4 => "space-borne" 5 => "central Mexico" ] ] ] ] "tieneResumen" => true "resumen" => array:2 [ "es" => array:2 [ "titulo" => "Resumen" "resumen" => "El uso de datos satelitales en combinación con mediciones realizadas en superficie puede proporcionar información valiosa acerca de la química atmosférica y la calidad del aire. En este estudio se comparan mediciones en superficie de dióxido de nitrógeno (NO2) realizadas mediante la técnica de espectroscopia óptica de absorción diferencial (DOAS, por sus siglas en inglés) con mediciones del instrumento satelital para la medición de ozono (OMI, por sus siglas en inglés) realizadas de 2006 a 2011. Las mediciones realizadas desde la superficie presentaron grandes variaciones diarias y fueron en promedio tres veces más altas que las columnas medidas desde el espacio. La diferencia se atribuye a una fuerte heterogeneidad horizontal presente en la capa inferior de las columnas de NO2, las cuales fueron muestreadas por el instrumento satelital a partir de un área extensa; de igual manera, esta discrepancia se atribuye a la sensibilidad reducida del satélite cerca de la superficie, donde se encuentran las mayores concentraciones. A partir de los datos del OMI analizados se reconstruyeron mapas de distribución de NO2 sobre el centro de México, y se identificaron tres áreas principales de interés: la zona metropolitana de la Ciudad de México, que fue el área predominante; la zona altamente industrializada de Tula, al norte, y el valle de Cuernavaca, al sur. En este análisis se detectaron de igual forma variaciones estacionales de columnas de NO2 sobre el centro de México: se encontraron columnas más altas durante la estación fría y seca, seguidas por las de la estación caliente y seca; las columnas más bajas se encontraron durante la época de lluvias. Este conjunto de datos evidencia el transporte de contaminación de este gas desde Tula hasta la Ciudad de México, así como al Valle de Cuernavaca." ] "en" => array:2 [ "titulo" => "Abstract" "resumen" => "The use of satellite data in combination with ground-based measurements can provide valuable information about atmospheric chemistry and air quality. In this study, ground-based Differential Optical Absorption Spectroscopy (DOAS) measurements of nitrogen dioxide (NO2) conducted in central Mexico are compared with the space-borne Ozone Monitoring Instrument (OMI) dataset of 2006-2011. Ground-based measurements exhibited large day-to-day variations and were on average three times higher than the space-borne derived average over the observation site. This difference is attributed to strong horizontal inhomogeneity of the lower layer of the measured NO2 columns, sampled over a large footprint from the satellite instrument. Also, a reduced sensitivity of the satellite observation near the surface, where the largest concentrations are expected, could be responsible for this large discrepancy. From the analyzed OMI dataset, distribution maps of NO2 above central Mexico were reconstructed, allowing to identify three main areas with increased NO2 column densities: The dominating metropolitan area of Mexico City, the heavily industrialized region of Tula<a name="p402"></a> to the north and the Cuernavaca valley to the south. In this analysis, seasonal variability of NO2 columns over central Mexico was detected, finding higher NO2 columns during the dry and cold season, followed by the dry and warm period, and finally the lowest NO2 columns were found during the rainy season. Pollution transport of this gas from Tula into Mexico City, as well as towards the Cuernavaca valley, is evident from this dataset." ] ] "multimedia" => array:9 [ 0 => array:7 [ "identificador" => "fig0005" "etiqueta" => "Fig. 1" "tipo" => "MULTIMEDIAFIGURA" "mostrarFloat" => true "mostrarDisplay" => false "figura" => array:1 [ 0 => array:4 [ "imagen" => "gr1.jpeg" "Alto" => 1181 "Ancho" => 1931 "Tamanyo" => 268619 ] ] "descripcion" => array:1 [ "en" => "Differential NO2 columns measured at UNAM between (a) August-December 2010, (b) February-June 2011, and (c) August-December 2011. The vertical gray lines represent the columns evaluation error. Only measurements conducted between 14:00 and 15:00 LT are shown. The data points showed in this figure are zenith measurements." ] ] 1 => array:7 [ "identificador" => "fig0010" "etiqueta" => "Fig. 2" "tipo" => "MULTIMEDIAFIGURA" "mostrarFloat" => true "mostrarDisplay" => false "figura" => array:1 [ 0 => array:4 [ "imagen" => "gr2.jpeg" "Alto" => 604 "Ancho" => 932 "Tamanyo" => 61565 ] ] "descripcion" => array:1 [ "en" => "Averaged hourly differential NO2 columns measured at UNAM over the whole time period (August 2010 to December 2011). The vertical grey lines represent the column standard deviation." ] ] 2 => array:7 [ "identificador" => "fig0015" "etiqueta" => "Fig. 3" "tipo" => "MULTIMEDIAFIGURA" "mostrarFloat" => true "mostrarDisplay" => false "figura" => array:1 [ 0 => array:4 [ "imagen" => "gr3.jpeg" "Alto" => 790 "Ancho" => 942 "Tamanyo" => 167616 ] ] "descripcion" => array:1 [ "en" => "Averaged NO2 total columns distribution derived from OMI measurements conducted between June 2006 and December 2011 over the Mexico basin. Scale from (0 to 2.2)×1016 molecules/cm2." ] ] 3 => array:7 [ "identificador" => "fig0020" "etiqueta" => "Fig. 4" "tipo" => "MULTIMEDIAFIGURA" "mostrarFloat" => true "mostrarDisplay" => false "figura" => array:1 [ 0 => array:4 [ "imagen" => "gr4.jpeg" "Alto" => 789 "Ancho" => 941 "Tamanyo" => 181907 ] ] "descripcion" => array:1 [ "en" => "NO2 total columns distribution reconstructed from OMI measurements (see section 2.2.1) conducted between June 2006 and December 2011. Same scale as <a class="elsevierStyleCrossRef" href="#fig0015">Figure 3</a>." ] ] 4 => array:7 [ "identificador" => "fig0025" "etiqueta" => "Fig. 5" "tipo" => "MULTIMEDIAFIGURA" "mostrarFloat" => true "mostrarDisplay" => false "figura" => array:1 [ 0 => array:4 [ "imagen" => "gr5.jpeg" "Alto" => 1919 "Ancho" => 1883 "Tamanyo" => 481370 ] ] "descripcion" => array:1 [ "en" => "NO2 total columns distribution over different areas of central Mexico reconstructed from OMI measurements conducted between June 2006 and December 2011. (a) The Mexico City metropolitan area; (b) the heavily industrialized region around Tula, Hidalgo; and (c) the Cuernavaca valley. Note that different scales have been used in the plots." ] ] 5 => array:7 [ "identificador" => "fig0030" "etiqueta" => "Fig. 6" "tipo" => "MULTIMEDIAFIGURA" "mostrarFloat" => true "mostrarDisplay" => false "figura" => array:1 [ 0 => array:4 [ "imagen" => "gr6.jpeg" "Alto" => 1684 "Ancho" => 1758 "Tamanyo" => 505497 ] ] "descripcion" => array:1 [ "en" => "NO2 total columns distribution over central Mexico reconstructed from OMI measurements conducted between 2006-2011 on: (a) November, December. January and February (dry and cold); (b) March, April and May (dry and warm); (c) June, July, August, September and October (rainy season). Scale from 0 to 1.8×1016 molecules/cm2." ] ] 6 => array:7 [ "identificador" => "fig0035" "etiqueta" => "Fig. 7" "tipo" => "MULTIMEDIAFIGURA" "mostrarFloat" => true "mostrarDisplay" => false "figura" => array:1 [ 0 => array:4 [ "imagen" => "gr7.jpeg" "Alto" => 820 "Ancho" => 933 "Tamanyo" => 92077 ] ] "descripcion" => array:1 [ "en" => "Monthly average comparison between ground-based, OMI and RAMA (calculated) NO2 columns." ] ] 7 => array:7 [ "identificador" => "fig0040" "etiqueta" => "Fig. 8" "tipo" => "MULTIMEDIAFIGURA" "mostrarFloat" => true "mostrarDisplay" => false "figura" => array:1 [ 0 => array:4 [ "imagen" => "gr8.jpeg" "Alto" => 767 "Ancho" => 1915 "Tamanyo" => 193287 ] ] "descripcion" => array:1 [ "en" => "(a) Frequency distribution of the air masses 2.25 h before measured at UN AM computed from wind data taken at RAMA stations; (b) distribution pattern of the averaged NO2 total columns 2.25 h before were measured at UNAM" ] ] 8 => array:7 [ "identificador" => "tbl0005" "etiqueta" => "Table I" "tipo" => "MULTIMEDIATABLA" "mostrarFloat" => true "mostrarDisplay" => false "tabla" => array:1 [ "tablatextoimagen" => array:1 [ 0 => array:2 [ "tabla" => array:1 [ 0 => """ <table border="0" frame="\n \t\t\t\t\tvoid\n \t\t\t\t" class=""><thead title="thead"><tr title="table-row"><th class="td" title="table-head " align="center" valign="middle" scope="col" style="border-bottom: 2px solid black">Radius (km) \t\t\t\t\t\t\n \t\t\t\t</th><th class="td" title="table-head " align="center" valign="middle" scope="col" style="border-bottom: 2px solid black">Number of coincident days \t\t\t\t\t\t\n \t\t\t\t</th><th class="td" title="table-head " align="center" valign="middle" scope="col" style="border-bottom: 2px solid black">UNAM average column (molecules/cm2) \t\t\t\t\t\t\n \t\t\t\t</th><th class="td" title="table-head " align="center" valign="middle" scope="col" style="border-bottom: 2px solid black">UNAM stdev column (molecules/cm2) \t\t\t\t\t\t\n \t\t\t\t</th><th class="td" title="table-head " align="center" valign="middle" scope="col" style="border-bottom: 2px solid black">OMI average column (molecules/cm2) \t\t\t\t\t\t\n \t\t\t\t</th><th class="td" title="table-head " align="center" valign="middle" scope="col" style="border-bottom: 2px solid black">OMI stdev column (molecules/cm2) \t\t\t\t\t\t\n \t\t\t\t</th><th class="td" title="table-head " align="center" valign="middle" scope="col" style="border-bottom: 2px solid black">Correlation coefficient \t\t\t\t\t\t\n \t\t\t\t</th><th class="td" title="table-head " align="center" valign="middle" scope="col" style="border-bottom: 2px solid black">Linear fit offset \t\t\t\t\t\t\n \t\t\t\t</th><th class="td" title="table-head " align="center" valign="middle" scope="col" style="border-bottom: 2px solid black">Linear fit slope \t\t\t\t\t\t\n \t\t\t\t</th></tr></thead><tbody title="tbody"><tr title="table-row"><td class="td" title="table-entry " align="char" valign="middle">9 \t\t\t\t\t\t\n \t\t\t\t</td><td class="td" title="table-entry " align="center" valign="middle">4 \t\t\t\t\t\t\n \t\t\t\t</td><td class="td" title="table-entry " align="char" valign="middle">2.48E + 016 \t\t\t\t\t\t\n \t\t\t\t</td><td class="td" title="table-entry " align="char" valign="middle">9.86E + 015 \t\t\t\t\t\t\n \t\t\t\t</td><td class="td" title="table-entry " align="char" valign="middle">7.20E + 015 \t\t\t\t\t\t\n \t\t\t\t</td><td class="td" title="table-entry " align="char" valign="middle">2.96E + 015 \t\t\t\t\t\t\n \t\t\t\t</td><td class="td" title="table-entry " align="char" valign="middle">−0.059 \t\t\t\t\t\t\n \t\t\t\t</td><td class="td" title="table-entry " align="char" valign="middle">7.64E + 015 \t\t\t\t\t\t\n \t\t\t\t</td><td class="td" title="table-entry " align="char" valign="middle">−0.018 \t\t\t\t\t\t\n \t\t\t\t</td></tr><tr title="table-row"><td class="td" title="table-entry " align="char" valign="middle">10 \t\t\t\t\t\t\n \t\t\t\t</td><td class="td" title="table-entry " align="char" valign="middle">5 \t\t\t\t\t\t\n \t\t\t\t</td><td class="td" title="table-entry " align="char" valign="middle">2.91E + 016 \t\t\t\t\t\t\n \t\t\t\t</td><td class="td" title="table-entry " align="char" valign="middle">1.28E + 016 \t\t\t\t\t\t\n \t\t\t\t</td><td class="td" title="table-entry " align="char" valign="middle">8.12E + 015 \t\t\t\t\t\t\n \t\t\t\t</td><td class="td" title="table-entry " align="char" valign="middle">3.31E + 015 \t\t\t\t\t\t\n \t\t\t\t</td><td class="td" title="table-entry " align="char" valign="middle">0.439 \t\t\t\t\t\t\n \t\t\t\t</td><td class="td" title="table-entry " align="char" valign="middle">4.83E + 015 \t\t\t\t\t\t\n \t\t\t\t</td><td class="td" title="table-entry " align="char" valign="middle">0.113 \t\t\t\t\t\t\n \t\t\t\t</td></tr><tr title="table-row"><td class="td" title="table-entry " align="char" valign="middle">15 \t\t\t\t\t\t\n \t\t\t\t</td><td class="td" title="table-entry " align="center" valign="middle">6 \t\t\t\t\t\t\n \t\t\t\t</td><td class="td" title="table-entry " align="char" valign="middle">3.52E + 016 \t\t\t\t\t\t\n \t\t\t\t</td><td class="td" title="table-entry " align="char" valign="middle">1.87E + 016 \t\t\t\t\t\t\n \t\t\t\t</td><td class="td" title="table-entry " align="char" valign="middle">1.27E + 016 \t\t\t\t\t\t\n \t\t\t\t</td><td class="td" title="table-entry " align="char" valign="middle">1.22E + 016 \t\t\t\t\t\t\n \t\t\t\t</td><td class="td" title="table-entry " align="char" valign="middle">0.781 \t\t\t\t\t\t\n \t\t\t\t</td><td class="td" title="table-entry " align="char" valign="middle">5.16E + 015 \t\t\t\t\t\t\n \t\t\t\t</td><td class="td" title="table-entry " align="char" valign="middle">0.509 \t\t\t\t\t\t\n \t\t\t\t</td></tr><tr title="table-row"><td class="td" title="table-entry " align="char" valign="middle">20 \t\t\t\t\t\t\n \t\t\t\t</td><td class="td" title="table-entry " align="center" valign="middle">6 \t\t\t\t\t\t\n \t\t\t\t</td><td class="td" title="table-entry " align="char" valign="middle">3.52E + 016 \t\t\t\t\t\t\n \t\t\t\t</td><td class="td" title="table-entry " align="char" valign="middle">1.87E + 016 \t\t\t\t\t\t\n \t\t\t\t</td><td class="td" title="table-entry " align="char" valign="middle">1.11E + 016 \t\t\t\t\t\t\n \t\t\t\t</td><td class="td" title="table-entry " align="char" valign="middle">1.00E + 016 \t\t\t\t\t\t\n \t\t\t\t</td><td class="td" title="table-entry " align="char" valign="middle">0.807 \t\t\t\t\t\t\n \t\t\t\t</td><td class="td" title="table-entry " align="char" valign="middle">4.12E + 015 \t\t\t\t\t\t\n \t\t\t\t</td><td class="td" title="table-entry " align="char" valign="middle">0.433 \t\t\t\t\t\t\n \t\t\t\t</td></tr><tr title="table-row"><td class="td" title="table-entry " align="char" valign="middle">25 \t\t\t\t\t\t\n \t\t\t\t</td><td class="td" title="table-entry " align="center" valign="middle">6 \t\t\t\t\t\t\n \t\t\t\t</td><td class="td" title="table-entry " align="char" valign="middle">3.52E + 016 \t\t\t\t\t\t\n \t\t\t\t</td><td class="td" title="table-entry " align="char" valign="middle">1.87E + 016 \t\t\t\t\t\t\n \t\t\t\t</td><td class="td" title="table-entry " align="char" valign="middle">1.05E + 016 \t\t\t\t\t\t\n \t\t\t\t</td><td class="td" title="table-entry " align="char" valign="middle">9.24E + 015 \t\t\t\t\t\t\n \t\t\t\t</td><td class="td" title="table-entry " align="char" valign="middle">0.772 \t\t\t\t\t\t\n \t\t\t\t</td><td class="td" title="table-entry " align="char" valign="middle">2.88E + 015 \t\t\t\t\t\t\n \t\t\t\t</td><td class="td" title="table-entry " align="char" valign="middle">0.381 \t\t\t\t\t\t\n \t\t\t\t</td></tr><tr title="table-row"><td class="td" title="table-entry " align="char" valign="middle">30 \t\t\t\t\t\t\n \t\t\t\t</td><td class="td" title="table-entry " align="center" valign="middle">6 \t\t\t\t\t\t\n \t\t\t\t</td><td class="td" title="table-entry " align="char" valign="middle">3.52E + 016 \t\t\t\t\t\t\n \t\t\t\t</td><td class="td" title="table-entry " align="char" valign="middle">1.87E + 016 \t\t\t\t\t\t\n \t\t\t\t</td><td class="td" title="table-entry " align="char" valign="middle">1.01E + 016 \t\t\t\t\t\t\n \t\t\t\t</td><td class="td" title="table-entry " align="char" valign="middle">8.30E + 015 \t\t\t\t\t\t\n \t\t\t\t</td><td class="td" title="table-entry " align="char" valign="middle">0.764 \t\t\t\t\t\t\n \t\t\t\t</td><td class="td" title="table-entry " align="char" valign="middle">1.86E + 015 \t\t\t\t\t\t\n \t\t\t\t</td><td class="td" title="table-entry " align="char" valign="middle">0.339 \t\t\t\t\t\t\n \t\t\t\t</td></tr><tr title="table-row"><td class="td" title="table-entry " align="char" valign="middle">40 \t\t\t\t\t\t\n \t\t\t\t</td><td class="td" title="table-entry " align="center" valign="middle">6 \t\t\t\t\t\t\n \t\t\t\t</td><td class="td" title="table-entry " align="char" valign="middle">3.52E + 016 \t\t\t\t\t\t\n \t\t\t\t</td><td class="td" title="table-entry " align="char" valign="middle">1.87E + 016 \t\t\t\t\t\t\n \t\t\t\t</td><td class="td" title="table-entry " align="char" valign="middle">8.70E + 015 \t\t\t\t\t\t\n \t\t\t\t</td><td class="td" title="table-entry " align="char" valign="middle">5.88E + 015 \t\t\t\t\t\t\n \t\t\t\t</td><td class="td" title="table-entry " align="char" valign="middle">0.734 \t\t\t\t\t\t\n \t\t\t\t</td><td class="td" title="table-entry " align="char" valign="middle">5.87E + 014 \t\t\t\t\t\t\n \t\t\t\t</td><td class="td" title="table-entry " align="char" valign="middle">0.231 \t\t\t\t\t\t\n \t\t\t\t</td></tr><tr title="table-row"><td class="td" title="table-entry " align="char" valign="middle">50 \t\t\t\t\t\t\n \t\t\t\t</td><td class="td" title="table-entry " align="center" valign="middle">6 \t\t\t\t\t\t\n \t\t\t\t</td><td class="td" title="table-entry " align="char" valign="middle">3.52E + 016 \t\t\t\t\t\t\n \t\t\t\t</td><td class="td" title="table-entry " align="char" valign="middle">1.87E + 016 \t\t\t\t\t\t\n \t\t\t\t</td><td class="td" title="table-entry " align="char" valign="middle">7.86E + 015 \t\t\t\t\t\t\n \t\t\t\t</td><td class="td" title="table-entry " align="char" valign="middle">4.05E + 015 \t\t\t\t\t\t\n \t\t\t\t</td><td class="td" title="table-entry " align="char" valign="middle">0.703 \t\t\t\t\t\t\n \t\t\t\t</td><td class="td" title="table-entry " align="char" valign="middle">2.51E + 015 \t\t\t\t\t\t\n \t\t\t\t</td><td class="td" title="table-entry " align="char" valign="middle">0.152 \t\t\t\t\t\t\n \t\t\t\t</td></tr><tr title="table-row"><td class="td" title="table-entry " align="char" valign="middle">60 \t\t\t\t\t\t\n \t\t\t\t</td><td class="td" title="table-entry " align="center" valign="middle">7 \t\t\t\t\t\t\n \t\t\t\t</td><td class="td" title="table-entry " align="char" valign="middle">3.49E + 016 \t\t\t\t\t\t\n \t\t\t\t</td><td class="td" title="table-entry " align="char" valign="middle">1.71E + 016 \t\t\t\t\t\t\n \t\t\t\t</td><td class="td" title="table-entry " align="char" valign="middle">7.21E + 015 \t\t\t\t\t\t\n \t\t\t\t</td><td class="td" title="table-entry " align="char" valign="middle">3.24E + 015 \t\t\t\t\t\t\n \t\t\t\t</td><td class="td" title="table-entry " align="char" valign="middle">0.687 \t\t\t\t\t\t\n \t\t\t\t</td><td class="td" title="table-entry " align="char" valign="middle">2.66E + 015 \t\t\t\t\t\t\n \t\t\t\t</td><td class="td" title="table-entry " align="char" valign="middle">0.130 \t\t\t\t\t\t\n \t\t\t\t</td></tr><tr title="table-row"><td class="td" title="table-entry " align="char" valign="middle">70 \t\t\t\t\t\t\n \t\t\t\t</td><td class="td" title="table-entry " align="center" valign="middle">7 \t\t\t\t\t\t\n \t\t\t\t</td><td class="td" title="table-entry " align="char" valign="middle">3.49E + 016 \t\t\t\t\t\t\n \t\t\t\t</td><td class="td" title="table-entry " align="char" valign="middle">1.71E + 016 \t\t\t\t\t\t\n \t\t\t\t</td><td class="td" title="table-entry " align="char" valign="middle">6.89E + 015 \t\t\t\t\t\t\n \t\t\t\t</td><td class="td" title="table-entry " align="char" valign="middle">2.76E + 015 \t\t\t\t\t\t\n \t\t\t\t</td><td class="td" title="table-entry " align="char" valign="middle">0.583 \t\t\t\t\t\t\n \t\t\t\t</td><td class="td" title="table-entry " align="char" valign="middle">3.61E + 015 \t\t\t\t\t\t\n \t\t\t\t</td><td class="td" title="table-entry " align="char" valign="middle">0.094 \t\t\t\t\t\t\n \t\t\t\t</td></tr><tr title="table-row"><td class="td" title="table-entry " align="char" valign="middle">80 \t\t\t\t\t\t\n \t\t\t\t</td><td class="td" title="table-entry " align="center" valign="middle">7 \t\t\t\t\t\t\n \t\t\t\t</td><td class="td" title="table-entry " align="char" valign="middle">3.49E + 016 \t\t\t\t\t\t\n \t\t\t\t</td><td class="td" title="table-entry " align="char" valign="middle">1.71E + 016 \t\t\t\t\t\t\n \t\t\t\t</td><td class="td" title="table-entry " align="char" valign="middle">6.67E + 015 \t\t\t\t\t\t\n \t\t\t\t</td><td class="td" title="table-entry " align="char" valign="middle">2.28E + 015 \t\t\t\t\t\t\n \t\t\t\t</td><td class="td" title="table-entry " align="char" valign="middle">0.512 \t\t\t\t\t\t\n \t\t\t\t</td><td class="td" title="table-entry " align="char" valign="middle">4.29E + 015 \t\t\t\t\t\t\n \t\t\t\t</td><td class="td" title="table-entry " align="char" valign="middle">0.068 \t\t\t\t\t\t\n \t\t\t\t</td></tr><tr title="table-row"><td class="td" title="table-entry " align="char" valign="middle">90 \t\t\t\t\t\t\n \t\t\t\t</td><td class="td" title="table-entry " align="center" valign="middle">7 \t\t\t\t\t\t\n \t\t\t\t</td><td class="td" title="table-entry " align="char" valign="middle">3.49E + 016 \t\t\t\t\t\t\n \t\t\t\t</td><td class="td" title="table-entry " align="char" valign="middle">1.71E + 016 \t\t\t\t\t\t\n \t\t\t\t</td><td class="td" title="table-entry " align="char" valign="middle">6.50E + 015 \t\t\t\t\t\t\n \t\t\t\t</td><td class="td" title="table-entry " align="char" valign="middle">2.10E + 015 \t\t\t\t\t\t\n \t\t\t\t</td><td class="td" title="table-entry " align="char" valign="middle">0.422 \t\t\t\t\t\t\n \t\t\t\t</td><td class="td" title="table-entry " align="char" valign="middle">4.70E + 015 \t\t\t\t\t\t\n \t\t\t\t</td><td class="td" title="table-entry " align="char" valign="middle">0.052 \t\t\t\t\t\t\n \t\t\t\t</td></tr><tr title="table-row"><td class="td" title="table-entry " align="char" valign="middle">100 \t\t\t\t\t\t\n \t\t\t\t</td><td class="td" title="table-entry " align="center" valign="middle">8 \t\t\t\t\t\t\n \t\t\t\t</td><td class="td" title="table-entry " align="char" valign="middle">3.46E + 016 \t\t\t\t\t\t\n \t\t\t\t</td><td class="td" title="table-entry " align="char" valign="middle">1.58E + 016 \t\t\t\t\t\t\n \t\t\t\t</td><td class="td" title="table-entry " align="char" valign="middle">5.78E + 015 \t\t\t\t\t\t\n \t\t\t\t</td><td class="td" title="table-entry " align="char" valign="middle">2.17E + 015 \t\t\t\t\t\t\n \t\t\t\t</td><td class="td" title="table-entry " align="char" valign="middle">0.334 \t\t\t\t\t\t\n \t\t\t\t</td><td class="td" title="table-entry " align="char" valign="middle">4.19E + 015 \t\t\t\t\t\t\n \t\t\t\t</td><td class="td" title="table-entry " align="char" valign="middle">0.046 \t\t\t\t\t\t\n \t\t\t\t</td></tr></tbody></table> """ ] "imagenFichero" => array:1 [ 0 => "xTab796668.png" ] ] ] ] "descripcion" => array:1 [ "en" => "Comparison of NO2 ground-based columns measured at UNAM and NO2 columns measured by OMI at overpass time 14:21 LT ± 20 min." ] ] ] "bibliografia" => array:2 [ "titulo" => "References" "seccion" => array:1 [ 0 => array:2 [ "identificador" => "bibs0005" "bibliografiaReferencia" => array:48 [ 0 => array:3 [ "identificador" => "bib0005" "etiqueta" => "Beirle et al., 2011" "referencia" => array:1 [ 0 => array:2 [ "contribucion" => array:1 [ 0 => array:2 [ "titulo" => "Megacity emissions and lifetimes of nitrogen oxides probed from space" "autores" => array:1 [ 0 => array:2 [ "etal" => false "autores" => array:5 [ 0 => "Beirle S." 1 => "K.F. 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SIMAT and the SMA-GDF are thanked for providing the in situ measurements and the Pro-grama de Estaciones Meteorológicas del Bachillerato Universitario (PEMBU) network is acknowledged for the meteorological data. Alfredo Rodríguez from the Centro de Ciencias de la Atmósfera workshop is acknowledged for providing technical support on the installation of the instrumentation used to conduct the ground-based measurements." "vista" => "all" ] ] ] "idiomaDefecto" => "en" "url" => "/01876236/0000002600000003/v2_201505081749/S0187623613710853/v2_201505081749/en/main.assets" "Apartado" => null "PDF" => "https://static.elsevier.es/multimedia/01876236/0000002600000003/v2_201505081749/S0187623613710853/v2_201505081749/en/main.pdf?idApp=UINPBA00004N&text.app=https://www.elsevier.es/" "EPUB" => "https://multimedia.elsevier.es/PublicationsMultimediaV1/item/epub/S0187623613710853?idApp=UINPBA00004N"]

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C. Rivera

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claudia.rivera@atmosfera.unam.mx

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Facultad de Ciencias, Universidad Nacional Autónoma de México, Circuito Exterior s/n, Ciudad Universitaria, 04510 México, D.F.

W. Stremme, M. Grutter

Centro de Ciencias de la Atmósfera, Universidad Nacional Autónoma de México, Circuito de la Investigación Científica s/n, Ciudad Universitaria, 04510 México, D.F.

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          "en" => "<p id="spar0055" class="elsevierStyleSimplePara elsevierViewall">&#40;a&#41; Frequency distribution of the air masses 2&#46;25 h before measured at UN AM computed from wind data taken at RAMA stations&#59; &#40;b&#41; distribution pattern of the averaged NO<span class="elsevierStyleInf">2</span> total columns 2&#46;25 h before were measured at UNAM</p>"
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    "textoCompleto" => "<span class="elsevierStyleSections"><span id="sec0005" class="elsevierStyleSection elsevierViewall"><span class="elsevierStyleLabel">1</span><span class="elsevierStyleSectionTitle" id="sect0020">Introduction</span><p id="par0005" class="elsevierStylePara elsevierViewall">Mexico City is the third largest city of the world&#44; with more than 20 million inhabitants &#40;<a class="elsevierStyleCrossRef" href="#bib0215">UN&#44; 2012</a>&#41;&#46; It is located at 19&#46;4&#176; N latitude and approximately at an elevation of 2200 m above sea level &#40;masl&#41;&#46; For several years Mexico City has experienced severe air quality problems&#44; which have been addressed by authorities trough the implementation of various programs&#46; While the countermeasures and technological improvements have effectively decreased the amount of some pollutants&#44; the city continues to experience pollution episodes resulting on the frequent violation of air quality standards&#46;</p><p id="par0010" class="elsevierStylePara elsevierViewall">Nitrogen dioxide &#40;NO<span class="elsevierStyleInf">2</span>&#41; plays a major role in tropospheric and stratospheric chemistry &#40;<a class="elsevierStyleCrossRef" href="#bib0080">Crutzen&#44; 1979</a>&#41;&#46; In polluted regions such as Mexico City&#44; tropospheric NO<span class="elsevierStyleInf">2</span> concentrations can be highly variable in time and space and are influenced by both natural and anthropogenic emissions&#46; In Mexico City&#44; most of the NO<span class="elsevierStyleInf">2</span> present in the atmosphere comes from fossil fuel combustion from transportation and partly from industrial activities&#46; NO<span class="elsevierStyleInf">2</span> levels in Mexico City have decreased by 30&#37; between 1990 and 2008&#44; and the average surface concentration registered in 2008 by four stations around the Universidad Nacional Aut&#243;noma de M&#233;xico &#40;UNAM&#41; campus in Mexico City&#8212;where the ground-based measurements presented in this study were conducted&#8212;was 56&#46;8 ppb &#40;<a class="elsevierStyleCrossRef" href="#bib0200">SMA-GDF&#44; 2010</a>&#41;&#46;</p><p id="par0015" class="elsevierStylePara elsevierViewall">Satellites provide global observations&#44; allowing to retrieve information from nearly every region in the globe &#40;<a class="elsevierStyleCrossRef" href="#bib0190">Rees&#44; 2001</a>&#41;&#46; There are a number of instruments measuring from space&#44; covering a wide range of wavelengths&#44; with different spatial resolution and viewing geometries&#46; In this work we have focused on the Ozone Monitoring Instrument &#40;OMI&#41; &#40;<a class="elsevierStyleCrossRef" href="#bib0155">Levelt <span class="elsevierStyleItalic">et al</span>&#46; 2006a</a>&#44; <a class="elsevierStyleCrossRef" href="#bib0160">b</a>&#41;&#44; a UV-VIS spectrometer on board NASA&#8217;s Earth Observing System-Aura satellite&#44; able to provide a daily global coverage&#46;</p><p id="par0020" class="elsevierStylePara elsevierViewall">Data from the OMI instrument have been used in an extensive number of studies involving different atmospheric gases as NO<span class="elsevierStyleInf">2</span>&#44; O<span class="elsevierStyleInf">3</span>&#44; SO<span class="elsevierStyleInf">2</span>&#44; HCHO&#44; among others &#40;<a class="elsevierStyleCrossRef" href="#bib0010">Bhartia&#44; 2002</a>&#59; <a class="elsevierStyleCrossRef" href="#bib0060">Chance&#44; 2002</a>&#41;&#46; The NO2 data products provide information about total and tropospheric NO<span class="elsevierStyleInf">2</span> columns&#46; The NO<span class="elsevierStyleInf">2</span> dataset in particular&#44; has been validated in different studies&#46; <a class="elsevierStyleCrossRef" href="#bib0050">Celarier <span class="elsevierStyleItalic">et al</span>&#46; &#40;2006&#41;</a> compared NO<span class="elsevierStyleInf">2</span> columns from OMI with NO<span class="elsevierStyleInf">2</span> columns measured by the Syst&#233;me d&#8217;Analyse par Observation Z&#233;nithale &#40;SAOZ&#41; network&#44; finding good agreement between the two of them although the measurements were not done at the same time&#46; As a continuation&#44; the different measurements and validation activities for OMI NO<span class="elsevierStyleInf">2</span> stratospheric&#44; tropospheric and total columns using ground- and aircraft-based measurements were summarized by <a class="elsevierStyleCrossRef" href="#bib0055">Celarier <span class="elsevierStyleItalic">et al</span>&#46; &#40;2008&#41;</a>&#46; Good agreement was reported between OMI and nearby instruments&#44; finding correlations higher than 0&#46;6 with ground-based instruments&#44; whereas OMI NO<span class="elsevierStyleInf">2</span> stratospheric and total columns were found to be underestimated by 14 and 15-30&#37;&#44; respectively&#46;</p><p id="par0025" class="elsevierStylePara elsevierViewall">During the Dutch Aerosol and Nitrogen Dioxide Experiments for Validation of OMI and SCIA-MACHY &#40;DANDELIONS&#41; 2005 and 2006 field experiments in the Netherlands&#44; <a class="elsevierStyleCrossRef" href="#bib0025">Brinksma <span class="elsevierStyleItalic">et al</span>&#46; &#40;2008&#41;</a> found good agreement between tropospheric from OMI and MAX-DOAS &#40;Multi-AXis&#41;&#44; as well as between total NO<span class="elsevierStyleInf">2</span> from OMI and direct sun observations&#46; <a class="elsevierStyleCrossRef" href="#bib0015">Boersma <span class="elsevierStyleItalic">et al</span>&#46; &#40;2008&#41;</a> compared OMI NO<span class="elsevierStyleInf">2</span> tropospheric columns with <span class="elsevierStyleItalic">in situ</span> aircraft measurements during the INTEX-B campaign&#44; the results showed good correlation with no significant bias between the two data sets&#46; <a class="elsevierStyleCrossRef" href="#bib0140">Irie <span class="elsevierStyleItalic">et al</span>&#46; &#40;2008&#41;</a> used MAX-DOAS measurements to validate OMI NO<span class="elsevierStyleInf">2</span> columns during the Mount Tai Experiment 2006 finding that OMI data may have a positive bias of 20&#37; over the North China Plain&#46; <a class="elsevierStyleCrossRef" href="#bib0110">Gruzdev and Elokhov &#40;2010&#41;</a> found that tropospheric OMI NO<span class="elsevierStyleInf">2</span> columns measured over Zvenigorod&#44; Russia were approximately 40&#37; lower than ground-based twilight measurements&#46; <a class="elsevierStyleCrossRef" href="#bib0230">Vlemmix <span class="elsevierStyleItalic">et al</span>&#46; &#40;2010&#41;</a> compared tropospheric ground-based measured NO<span class="elsevierStyleInf">2</span> columns with OMI-satellite tropospheric data finding no significant difference and a correlation of 0&#46;88&#46;<a name="p403"></a><a class="elsevierStyleCrossRef" href="#bib0135">Ionov <span class="elsevierStyleItalic">et al</span>&#46; &#40;2008&#41;</a> conducted a geophysical validation of OMI NO<span class="elsevierStyleInf">2</span> vertical column measurements with collocated ground-based observations at two stations in central Asia and Europe finding that mid-latitude OMI stratospheric NO<span class="elsevierStyleInf">2</span> column data underestimates ground-based measurements&#46;</p><p id="par0030" class="elsevierStylePara elsevierViewall">Direct-Sun mode measurements have been conducted as well in order to validate OMI NO<span class="elsevierStyleInf">2</span> columns&#46; <a class="elsevierStyleCrossRef" href="#bib0240">Wenig <span class="elsevierStyleItalic">et al</span>&#46; &#40;2008&#41;</a> used direct-Sun mode Brewer measurements to conduct a validation study with OMI tropospheric NO<span class="elsevierStyleInf">2</span> columns concluding that monthly averages of coinciding measurements yielded a correlation of 0&#46;9&#44; however OMI data was 25&#37; lower than the ground-based Brewer measurements&#46; <a class="elsevierStyleCrossRef" href="#bib0115">Herman <span class="elsevierStyleItalic">et al</span>&#46; &#40;2009&#41;</a> conducted intercomparisons of NO<span class="elsevierStyleInf">2</span> columns measured with ground-based direct solar irradiance measurements to OMI NO<span class="elsevierStyleInf">2</span> columns finding good agreement &#40;correlation of 0&#46;73&#41;&#46;</p><p id="par0035" class="elsevierStylePara elsevierViewall">Regional and global models have also been used to compare and validate OMI NO<span class="elsevierStyleInf">2</span> tropospheric columns &#40;<a class="elsevierStyleCrossRef" href="#bib0145">Kim <span class="elsevierStyleItalic">et al</span>&#44; 2009</a>&#59; <a class="elsevierStyleCrossRef" href="#bib0125">Herron-Thorpe <span class="elsevierStyleItalic">et al</span>&#46;&#44; 2010</a>&#59; <a class="elsevierStyleCrossRef" href="#bib0130">Huijnen <span class="elsevierStyleItalic">et al</span>&#44; 2010</a>&#59; <a class="elsevierStyleCrossRef" href="#bib0095">Dirksen <span class="elsevierStyleItalic">et al</span>&#44; 2011</a>&#41; and to estimate NOx emissions from different sources &#40;<a class="elsevierStyleCrossRef" href="#bib0165">Lin&#44; 2012</a>&#41;&#46; Recently&#44; <a class="elsevierStyleCrossRef" href="#bib0005">Beirle <span class="elsevierStyleItalic">et al</span>&#46; &#40;2011&#41;</a> studied NOx emissions and lifetimes from megacities and power plants&#44; while <a class="elsevierStyleCrossRef" href="#bib0220">Valin <span class="elsevierStyleItalic">et al</span>&#46; &#40;2011&#41;</a> used the super-zoom mode of OMI to observe NO<span class="elsevierStyleInf">2</span>slant columns from point sources and two cities&#44; capturing significant spatial variability at fine scales&#46;</p><p id="par0040" class="elsevierStylePara elsevierViewall">In this work&#44; the OMI NO<span class="elsevierStyleInf">2</span> data product &#40;OMI&#47; Aura nitrogen dioxide total and tropospheric column 1-orbit L2 swath 13<span class="elsevierStyleHsp" style=""></span>&#215;<span class="elsevierStyleHsp" style=""></span>24 km&#44; data set version 003&#41; has been analyzed and compared to ground-based measurements conducted at UNAM campus in Mexico City&#46; NO<span class="elsevierStyleInf">2</span> column maps were reconstructed from the OMI data product using an algorithm based on the optimal estimation approach with a weighted assignment of the measurement onto a fine grid&#44; in order to identify the spatial features of its distribution with higher resolution&#46; Focus has been given to the study of NO<span class="elsevierStyleInf">2</span> mainly to its role as a catalyst in the production of tropospheric ozone &#40;O<span class="elsevierStyleInf">3</span>&#41;&#44; which is responsible for one of the most severe air quality problems in the Mexico City metropolitan area &#40;MCMA&#41;&#46;</p></span><span id="sec0010" class="elsevierStyleSection elsevierViewall"><span class="elsevierStyleLabel">2</span><span class="elsevierStyleSectionTitle" id="sect0025">Data set</span><span id="sec0015" class="elsevierStyleSection elsevierViewall"><span class="elsevierStyleLabel">2&#46;1</span><span class="elsevierStyleSectionTitle" id="sect0030">OMI</span><p id="par0045" class="elsevierStylePara elsevierViewall">OMI is a nadir viewing spectrometer covering a spectral region of 264-504 nm&#44; conducting atmospheric chemistry measurements with a resolution of 0&#46;42-0&#46;63 nm since 2004 on board NASA&#8217;s Earth Observation System &#40;EOS&#41;-Aura satellite&#46; Aura follows a sun-synchronous polar orbit &#40;705 km altitude&#41;&#44; with an ascending local equator crossing time at 13&#58;45&#46; OMIs observations provide complete global coverage in one day with a nominal ground footprint of 13<span class="elsevierStyleHsp" style=""></span>&#215;<span class="elsevierStyleHsp" style=""></span>24 km<span class="elsevierStyleSup">2</span> at nadir &#40;<a class="elsevierStyleCrossRef" href="#bib0180">OMI Team&#44; 2009</a>&#41;&#46; It measures a number of air quality components such as NO<span class="elsevierStyleInf">2</span>&#44; SO<span class="elsevierStyleInf">2</span>&#44; BrO&#44; HCHO&#44; and aerosols &#40;<a class="elsevierStyleCrossRef" href="#bib0155">Levelt <span class="elsevierStyleItalic">et al</span>&#46; 2006a</a>&#44; <a class="elsevierStyleCrossRef" href="#bib0160">b</a>&#41;&#46;</p><p id="par0050" class="elsevierStylePara elsevierViewall">The OMI operational algorithm is described in detail by <a class="elsevierStyleCrossRef" href="#bib0030">Bucsela <span class="elsevierStyleItalic">et al</span>&#46; &#40;2006&#41;</a>&#46; For the evaluation of spectra recorded by OMI&#44; a similar DOAS method &#40;<a class="elsevierStyleCrossRef" href="#bib0185">Platt and Stutz&#44; 2008</a>&#41; is used as for the ground instruments&#46; In this method a spectral fit is applied to all measured spectra&#46; The evaluation of spectra results on slant column densities &#40;SCDs&#41; for each viewing pixel&#46; The computed SCDs are then converted to vertical column densities &#40;VCDs&#41; using an air mass factor &#40;AMF&#41;&#46; The AMF is the ratio between the retrieved SCD and the VCD&#46; The AMF concept was introduced by <a class="elsevierStyleCrossRef" href="#bib0205">Solomon <span class="elsevierStyleItalic">et al</span>&#46; &#40;1987&#41;</a> and is dependent on clouds and aerosols&#44; as well as viewing geometries and surface albedo&#46;</p><p id="par0055" class="elsevierStylePara elsevierViewall">In this work&#44; the OMI&#47;Aura nitrogen dioxide total and tropospheric column 1-orbit L2 swath 13<span class="elsevierStyleHsp" style=""></span>&#215;<span class="elsevierStyleHsp" style=""></span>24 km&#44; data set version 003&#44; level 2&#44; algorithm version 1&#46;1&#46;4&#46;4 data product was downloaded from NASA&#8217;s Goddard Earth Sciences Data and Information Services Center &#40;GESDISC&#41; using the MIRADOR 1&#46;42 tool &#40;<a href="http://disc.sci.gsfc.nasa.gov/Aura/data-holdings/OMI">http&#58;&#47;&#47;disc&#46;sci&#46;gsfc&#46;nasa&#46;gov&#47;Aura&#47;data-holdings&#47;OMI</a>&#41;&#46; We have analyzed the dataset and only considered total N<span class="elsevierStyleInf">O2</span> columns from cloud fraction below 20&#37; over the period 2006-2011&#46; This dataset has been reprocessed and released on December 2011 &#40;<a class="elsevierStyleCrossRef" href="#bib0070">Claas&#44; 2012</a>&#41;&#46;</p><span id="sec0020" class="elsevierStyleSection elsevierViewall"><span class="elsevierStyleLabel">2&#46;1&#46;1</span><span class="elsevierStyleSectionTitle" id="sect0035">Reconstruction of the mean column NO<span class="elsevierStyleInf">2</span> distribution</span><p id="par0060" class="elsevierStylePara elsevierViewall">The mean column NO<span class="elsevierStyleInf">2</span> distribution over a fine grid &#40;of the order of about 2 km&#41; can be reconstructed from measurements done on a larger footprint &#40;here 13<span class="elsevierStyleHsp" style=""></span>&#215;<span class="elsevierStyleHsp" style=""></span>24 km<span class="elsevierStyleSup">2</span> for OMI&#41;&#44; solving the mathematically ill-posed problem <span class="elsevierStyleBold">Y &#61; Kx &#43; &#949;</span>&#46; In this equation&#44; <span class="elsevierStyleBold">Y</span> represents the vector containing all OMI NO<span class="elsevierStyleInf">2</span> measurements&#44; <span class="elsevierStyleBold">x</span> is the solution vector which contains the estimation of the mean column NO<span class="elsevierStyleInf">2</span> distribution on the chosen grid&#44; <span class="elsevierStyleBold">&#949;</span> represents the deviation between a column measured&#47;retrieved from the space<a name="p404"></a> measurement and the estimated mean value&#46; <span class="elsevierStyleBold">&#949;</span> is mainly dependent on the day-to-day variance&#44; but also on the seasonal variance and measurement precision&#46; The matrix <span class="elsevierStyleBold">K</span> describes how the measurements are related with the mean gas horizontal distribution&#46; It represents a forward model that reproduces mainly the footprint of the OMI instrument &#40;13<span class="elsevierStyleHsp" style=""></span>&#215;<span class="elsevierStyleHsp" style=""></span>24 km<span class="elsevierStyleSup">2</span> at nadir&#41;&#46; The reconstruction uses a Tihkonov-smoothing-constraint&#44; similar as the one used for profile retrievals or in image restoration&#46; Further details of this method can be found in <a class="elsevierStyleCrossRef" href="#bib0210">Stremme <span class="elsevierStyleItalic">et al</span>&#46; &#40;2013&#41;</a>&#46;</p></span></span><span id="sec0025" class="elsevierStyleSection elsevierViewall"><span class="elsevierStyleLabel">2&#46;2</span><span class="elsevierStyleSectionTitle" id="sect0040">Zenith scattered sunlight DOAS</span><p id="par0065" class="elsevierStylePara elsevierViewall">Ground-based DOAS measurements were conducted at the UNAM campus in Mexico City &#40;19&#46;33&#176; N&#44; 99&#46;18&#176; W&#41; between August 2010 and December 2011&#46; Scattered light was collected by means of a telescope and transferred into an Ocean Optics TR2000 spectrometer with a spectral resolution of ~0&#46;6 nm and wavelength coverage of 291-501 nm&#46; Spectra were taken every minute using the DOASIS software &#40;<a class="elsevierStyleCrossRef" href="#bib0150">Kraus&#44; 2006</a>&#41; and were evaluated using the QDOAS &#40;version 1&#46;00&#41; software &#40;<a class="elsevierStyleCrossRef" href="#bib0105">Fayt <span class="elsevierStyleItalic">et al</span>&#46;&#44; 2011</a>&#41;&#46; Differential cross-sections of NO<span class="elsevierStyleInf">2</span> at 298 K &#40;<a class="elsevierStyleCrossRef" href="#bib0225">Vandaele <span class="elsevierStyleItalic">et al</span>&#46;&#44; 1998</a>&#41;&#44; O<span class="elsevierStyleInf">3</span> at 221 K and 241 K &#40;<a class="elsevierStyleCrossRef" href="#bib0035">Burrows <span class="elsevierStyleItalic">et al</span>&#44; 1999</a>&#41;&#44; the oxygen dimer O<span class="elsevierStyleInf">4</span> &#40;<a class="elsevierStyleCrossRef" href="#bib0120">Hermans <span class="elsevierStyleItalic">et al</span>&#44; 1999</a>&#41; and a Ring spectrum&#44; generated at 273 K from a high resolution Kurucz file using the QDOAS software &#40;<a class="elsevierStyleCrossRef" href="#bib0105">Fayt <span class="elsevierStyleItalic">et al</span>&#46;&#44; 2011</a>&#41; were included in the analysis&#46; The oxygen dimer O<span class="elsevierStyleInf">4</span> was included in the evaluation procedure in order to detect enhancement on the light path of the measurements since it is expected to have a constant concentration in the atmosphere&#46; Evaluations were conducted in the 405 and 465 nm wavelength ranges with a fixed reference spectrum measured on 21 December 2011 at 14&#58;21 local time &#40;LT&#41; under clear sky and high ventilation conditions for the whole time series&#46; Geometrical AMFs were calculated and used to convert SCDs to VCDs for further comparison with OMI results&#46; For this it was considered that the light path through a trace gas layer varies with 1&#47;cos &#952;&#44; where &#952; is the solar zenith angle &#40;<a class="elsevierStyleCrossRef" href="#bib0065">Chen <span class="elsevierStyleItalic">et al</span>&#44; 2009</a>&#41;&#46; It should be noted that this simplified approach without the use of a full radiative transfer calculation that effectively incorporates scattering processes&#44; reflection on the earth&#8217;s surface&#44; refraction&#44; the curvature of the earth&#44; the vertical distribution of trace gases and the influence of aerosols leads to additional errors discussed in detail by <a class="elsevierStyleCrossRef" href="#bib0185">Platt and Stutz &#40;2008&#41;</a>&#46;</p></span><span id="sec0030" class="elsevierStyleSection elsevierViewall"><span class="elsevierStyleLabel">2&#46;3</span><span class="elsevierStyleSectionTitle" id="sect0045">Surface measurements</span><p id="par0070" class="elsevierStylePara elsevierViewall">Since 1986&#44; a network of 34 stations run by the local government &#40;Red Autom&#225;tica de Monitoreo Atmosf&#233;rico&#44; RAMA&#41; reports continuous and permanent measurements of concentrations of O<span class="elsevierStyleInf">3</span>&#44; sulfur dioxide &#40;SO<span class="elsevierStyleInf">2</span>&#41;&#44; nitrogen oxides &#40;NOx&#41;&#44; carbon monoxide &#40;CO&#41; and particulate matter PM10 and PM2&#46;5 to evaluate air quality in the metropolitan area&#46; All data generated by RAMA are freely available online at <a href="http://www.calidadaire.df.gob.mx/">http&#58;&#47;&#47;www&#46;calidadaire&#46;df&#46;gob&#46;mx&#47;</a>&#46; NO<span class="elsevierStyleInf">2</span> is determined using a nitrogen oxides analyzer based on the chemilumi-nescence measurement technique&#46; For this study NO<span class="elsevierStyleInf">2</span> concentration results generated during 2010 and 2011 by Pedregal &#40;19&#46;32&#176; N&#44; 99&#46;20&#176; W&#41;&#44; Tlalpan &#40;19&#46;25 N&#44; 99&#46;18 W&#41;&#44; Santa &#218;rsula &#40;19&#46;31&#176;N&#44; 99&#46;14&#176; W&#41;&#44; Merced &#40;19&#46;42&#176; N&#44; 99&#46;11&#176; W&#41; and FES Acatl&#225;n &#40;19&#46;48&#176; N&#44; 99&#46;24&#176; W&#41; stations situated close to the study site were used to calculate NO<span class="elsevierStyleInf">2</span> columns&#44; using mixing layer heights &#40;<a class="elsevierStyleCrossRef" href="#sec0035">section 2&#46;4</a>&#41; as well as pressure and temperature data &#40;<a class="elsevierStyleCrossRef" href="#sec0040">section 2&#46;5</a>&#41;&#46;</p></span><span id="sec0035" class="elsevierStyleSection elsevierViewall"><span class="elsevierStyleLabel">2&#46;4</span><span class="elsevierStyleSectionTitle" id="sect0050">LIDAR</span><p id="par0075" class="elsevierStylePara elsevierViewall">A Vaisala CL31 ceilometer&#44; an active instrument based on the LIDAR &#40;light detection and ranging&#41; measurement technique&#44; was used to measure back-scattered aerosol density at the UNAM campus&#44; allowing the estimation of mixing layer heights&#46; The measurement principle involves the transmission of pulsed laser radiation into the atmosphere&#44; which is further backscattered and detected with a certain time delay&#46; The measurement range of the instrument used in this field study was from 0 up to 7500 m above the ground with a 10 m resolution&#46; The LIDAR wavelength of 905 nm was provided by an InGaAs MOCVD laser diode&#44; 110 ns&#44; 1&#46;2 &#956;J per pulse&#44; and repetition rate of 8192 Hz&#46; The mixing height was retrieved using the CL31 MLH software based on the gradient method&#44; which is focused on selecting the maximum of the negative gradient of the backscatter coefficient &#40;<span class="elsevierStyleItalic">&#946;</span>&#41; to be the top of the mixed layer &#40;-d<span class="elsevierStyleItalic">&#946;</span>&#47;d<span class="elsevierStyleItalic">x&#41;</span>&#46; A more detailed description of this methodology can be found in <a class="elsevierStyleCrossRef" href="#bib0175">M&#252;nkel <span class="elsevierStyleItalic">et al</span>&#46; &#40;2007&#41;</a>&#46;</p></span><span id="sec0040" class="elsevierStyleSection elsevierViewall"><span class="elsevierStyleLabel">2&#46;5</span><span class="elsevierStyleSectionTitle" id="sect0055">Meteorological parameters</span><p id="par0080" class="elsevierStylePara elsevierViewall">Meteorological parameters were recorded by a Davis Vantage Pro 2 station installed at the UNAM campus&#44; forming part of the Programa de Estaciones Meteorol&#243;gicas del Bachillerato Universitario &#40;PEMBU&#41; network&#46; From the dataset recorded by<a name="p405"></a> the meteorological station&#44; pressure and temperature were used for the calculation of ground-based NO<span class="elsevierStyleInf">2</span> columns based on NO<span class="elsevierStyleInf">2</span> concentrations measured by the RAMA network&#46;</p></span></span><span id="sec0045" class="elsevierStyleSection elsevierViewall"><span class="elsevierStyleLabel">3</span><span class="elsevierStyleSectionTitle" id="sect0060">Results and discussion</span><span id="sec0050" class="elsevierStyleSection elsevierViewall"><span class="elsevierStyleLabel">3&#46;1</span><span class="elsevierStyleSectionTitle" id="sect0065">Ground-based NO<span class="elsevierStyleInf">2</span> columns</span><p id="par0085" class="elsevierStylePara elsevierViewall"><a class="elsevierStyleCrossRef" href="#fig0005">Figure 1</a> shows differential <span class="elsevierStyleItalic">NO<span class="elsevierStyleInf">2</span></span>columns measured at UNAM between August 2010 and December 2011&#46; The gaps in the graph represent periods where no measurements were conducted&#44; either due to experimental problems&#44; or periods of cloudiness or rain identified through the analysis of the data itself using evaluation results of the oxygen dimer&#46; Only measurements conducted between 14&#58;00 and 15&#58;00 LT are plotted in order to depict columns coincident with OMI overpasses &#40;~14&#58;21 LT&#41; over Mexico City&#46; Large variability on day-to-day NO<span class="elsevierStyleInf">2</span>columns was measured&#44; illustrating the dynamics of fresh emissions and rapid conversion of pollutants occurring in the study area&#46; The ground-based average NO<span class="elsevierStyleInf">2</span> column values and standard deviation measured from August 2010 to December 2011 between 14&#58;00 and 15&#58;00 LT was &#40;5&#46;61 &#177; 3&#46;32&#41; &#215;10<span class="elsevierStyleSup">16</span> molecules&#47;cm<span class="elsevierStyleSup">2</span>&#46;</p><elsevierMultimedia ident="fig0005"></elsevierMultimedia><p id="par0090" class="elsevierStylePara elsevierViewall">The average diurnal behavior of differential NO<span class="elsevierStyleInf">2</span> columns measured at UNAM on the same measurement period are presented in <a class="elsevierStyleCrossRef" href="#fig0010">Figure 2</a>&#46; The maximum of the measured NO<span class="elsevierStyleInf">2</span> differential columns is found around noon&#44; which is consistent with the reported hourly NOx emissions for the MCMA by the official inventory &#40;<a class="elsevierStyleCrossRef" href="#bib0200">SMA-GDF&#44; 2010</a>&#41;&#46;</p><elsevierMultimedia ident="fig0010"></elsevierMultimedia></span><span id="sec0055" class="elsevierStyleSection elsevierViewall"><span class="elsevierStyleLabel">3&#46;2</span><span class="elsevierStyleSectionTitle" id="sect0070">NO<span class="elsevierStyleInf">2</span> spatial distribution</span><p id="par0095" class="elsevierStylePara elsevierViewall">The average distribution of NO<span class="elsevierStyleInf">2</span> total columns derived from OMI measurements conducted between<a name="p406"></a> June 2006 and December 2011 over the Mexico basin is depicted in <a class="elsevierStyleCrossRef" href="#fig0015">Figure 3</a>&#46; Higher NO<span class="elsevierStyleInf">2</span> total columns are observed in the downtown area of Mexico City&#44; which gradually decrease with distance from the city center&#46; The average and standard deviation of all OMI NO<span class="elsevierStyleInf">2</span> columns measured over UNAM during 2010-2011 at its overpass time&#44; ~14&#58;21&#44; is &#40;1&#46;18 &#177; 1&#46;1&#41;<span class="elsevierStyleHsp" style=""></span>&#215;<span class="elsevierStyleHsp" style=""></span>10<span class="elsevierStyleSup">16</span> molecules&#47;cm<span class="elsevierStyleSup">2</span>&#46; While <a class="elsevierStyleCrossRef" href="#fig0015">Figure 3</a> is simply the average of the columns falling on each grid point&#44; <a class="elsevierStyleCrossRef" href="#fig0020">Figure 4</a> shows a reconstruction of the mean column gas distribution according to the methodology described in <a class="elsevierStyleCrossRef" href="#sec0020">section 2&#46;1&#46;1</a>&#46; The reconstructed value of the average over UNAM for the same time period is 1&#46;7<span class="elsevierStyleHsp" style=""></span>&#215;<span class="elsevierStyleHsp" style=""></span>10<span class="elsevierStyleSup">16</span> molecules&#47;cm<span class="elsevierStyleSup">2</span>&#46; A clear difference can be observed between the average and reconstructed column gas distributions with the average column being smaller over UNAM&#46; This bias is present during all seasons&#44; it might originate from the sensitivity of the space-borne measurements to pollution near the surface and is discussed in more detail in <a class="elsevierStyleCrossRef" href="#sec0070">section 3&#46;3</a>&#46;</p><elsevierMultimedia ident="fig0015"></elsevierMultimedia><elsevierMultimedia ident="fig0020"></elsevierMultimedia><span id="sec0060" class="elsevierStyleSection elsevierViewall"><span class="elsevierStyleLabel">3&#46;2&#46;1</span><span class="elsevierStyleSectionTitle" id="sect0075">Three main areas of interest in central Mexico</span><p id="par0100" class="elsevierStylePara elsevierViewall">From <a class="elsevierStyleCrossRef" href="#fig0015">Figures 3</a> and <a class="elsevierStyleCrossRef" href="#fig0020">4</a>&#44; three main areas of interest in central Mexico can be identified&#58; the Tula industrial area to the north&#44; a dominating blur centered around Mexico City&#44; and the Cuernavaca valley to the south&#46; However&#44; the reconstructed map in <a class="elsevierStyleCrossRef" href="#fig0020">Figure 4</a> allows to better appreciate the finely defined areas where either NO<span class="elsevierStyleInf">2</span> columns are enhanced or diminished over the studied area&#46; Lacustrian and mountainous areas inside the MCMA are&#44; for instance&#44; more clearly observed in <a class="elsevierStyleCrossRef" href="#fig0020">Figure 4</a> than in <a class="elsevierStyleCrossRef" href="#fig0015">Figure 3</a>&#46; Also&#44; the industrial and more densely populated areas are better defined as well&#46;</p><p id="par0105" class="elsevierStylePara elsevierViewall">Unique distribution structures of NO<span class="elsevierStyleInf">2</span> columns above the MCMA were identified through the reconstruction of mean NO<span class="elsevierStyleInf">2</span> column distribution in <a class="elsevierStyleCrossRef" href="#fig0025">Figure 5a</a>&#46; NO<span class="elsevierStyleInf">2</span> column enhancements can be observed on the northern and central parts of the city&#44; which are characterized by intense and heavy traffic &#40;central and northern parts&#44; respectively&#41; and industrial activities &#40;northern part&#41;&#46; On the southern&#44; eastern and western sides&#44; the N<span class="elsevierStyleInf">O2</span> columns tend to decrease with distance of the city center&#44; while on the northern part&#44; the enhancement of NO<span class="elsevierStyleInf">2</span> columns is even extended towards the Tula industrial area &#40;<a class="elsevierStyleCrossRef" href="#fig0025">Fig&#46; 5b</a>&#41;&#44; from which a clear footprint was observed and differentiated from the MCMA&#46;</p><elsevierMultimedia ident="fig0025"></elsevierMultimedia><p id="par0110" class="elsevierStylePara elsevierViewall">The region around Tula shown in <a class="elsevierStyleCrossRef" href="#fig0025">Figure 5b</a>&#44; is located in the state of Hidalgo&#44; ~50 km northwest of Mexico City&#46; It is characterized by the presence of a power plant&#44; a refinery&#44; several cement works and other manufacturing industries&#46; The OMI footprint clearly shows higher NO<span class="elsevierStyleInf">2</span> column values close to the industries&#46; As in the case of the MCMA&#44; the NO<span class="elsevierStyleInf">2</span> column enhancement gradually decreases with distance of the pollution sources with an increase of NO<span class="elsevierStyleInf">2</span> columns towards Mexico City&#46; From <a class="elsevierStyleCrossRef" href="#fig0025">Figure 5b</a>&#44; an empiric dispersion pattern of the plume of the Tula industrial complex towards the southeast can be inferred&#46; This result is consistent with previous studies using<a name="p407"></a> dispersion models addressing the transport of the emissions from the Tula industrial complex towards the MCMA &#40;<a class="elsevierStyleCrossRef" href="#bib0045">CCA-UNAM&#44; 2008</a>&#59; <a class="elsevierStyleCrossRef" href="#bib0085">de Foy <span class="elsevierStyleItalic">et al</span>&#46;&#44; 2009</a>&#41;&#46;</p><p id="par0115" class="elsevierStylePara elsevierViewall">The third identified area in central Mexico where a clear&#44; though less intense NO<span class="elsevierStyleInf">2</span> column enhancement is observed&#44; is the Cuernavaca valley shown in <a class="elsevierStyleCrossRef" href="#fig0025">Figure 5c</a>&#46; This result was somewhat unexpected when compared to the relatively small column enhancements observed above the cities of Puebla and Toluca&#44; which are similar to Cuernavaca in terms of population and industrial activity&#46; Cuernavaca valley&#44; however&#44; is much lower in altitude &#40;&#60; 1500 masl&#41;&#44; host of a large number of sugar cane plantations&#46; During the industrial processing and harvesting intense biomass burning is practiced&#44; which could explain the increased NO<span class="elsevierStyleInf">2</span> column values observed with the OMI instrument&#46; Another possible explanation is the impact of emissions by the surrounding states&#46; Recently <a class="elsevierStyleCrossRef" href="#bib0195">Salcedo <span class="elsevierStyleItalic">et al</span>&#46; &#40;2012&#41;</a> reported an efficient transport of ozone and its precursors to this area by the surrounding Mexico City and Puebla state&#44; while <a class="elsevierStyleCrossRef" href="#bib0170">Melamed <span class="elsevierStyleItalic">et al</span>&#46; &#40;2009&#41;</a> detected during a study in 2006&#44; NO<span class="elsevierStyleInf">2</span> transport events southeast of Mexico City from and towards the Cuautla region&#46;<a name="p408"></a></p></span><span id="sec0065" class="elsevierStyleSection elsevierViewall"><span class="elsevierStyleLabel">3&#46;2&#46;2</span><span class="elsevierStyleSectionTitle" id="sect0080">Seasonal variability</span><p id="par0120" class="elsevierStylePara elsevierViewall">An additional approach was used analyzing OMI data into three different seasons&#58; &#40;1&#41; dry and cold covering the months of November&#44; December&#44; January and February&#59; &#40;2&#41; dry and warm for the months of March&#44; April and May&#59; and &#40;3&#41; the rainy season including the months of June&#44; July&#44; August&#44; September and October&#46; <a class="elsevierStyleCrossRef" href="#fig0030">Figure 6</a> depicts NO<span class="elsevierStyleInf">2</span> total column distributions reconstructed from OMI measurements conducted between June 2006 and December 2011 for the dry and cold &#40;<a class="elsevierStyleCrossRef" href="#fig0030">Fig&#46; 6a</a>&#41;&#44; dry and warm &#40;<a class="elsevierStyleCrossRef" href="#fig0030">Fig&#46; 6b</a>&#41; and rainy seasons &#40;<a class="elsevierStyleCrossRef" href="#fig0030">Fig&#46; 6c</a>&#41;&#46; The same scale is used in all three plots for comparison&#46; Higher NO<span class="elsevierStyleInf">2</span> columns were measured during the dry and cold season&#44; followed by the dry and warm period and finally the lowest NO<span class="elsevierStyleInf">2</span> columns were found during the raining season&#46; For the dry-cold and dry-warm seasons the pattern is confirmed by the ground-based measurements at UNAM &#40;<a class="elsevierStyleCrossRef" href="#fig0035">Fig&#46; 7</a>&#41;&#46; <a class="elsevierStyleCrossRef" href="#bib0235">Wallace and Kanaroglou &#40;2009&#41;</a> have reported sensitivity of OMI to subtle temperature changes&#44; while <a class="elsevierStyleCrossRef" href="#bib0020">Boersma <span class="elsevierStyleItalic">et al</span>&#46; &#40;2009&#41;</a> observed NO<span class="elsevierStyleInf">2</span> columns 1&#46;6-2&#46;7 times higher in winter than in summer in Israeli cities&#46; The atmospheric stability characteristic of the dry and cold season promotes a larger number of molecules to be present in the studied area&#44; while during the raining season&#44; water is mainly responsible for the removal of pollutants in the atmosphere&#46;</p><elsevierMultimedia ident="fig0030"></elsevierMultimedia><elsevierMultimedia ident="fig0035"></elsevierMultimedia><p id="par0125" class="elsevierStylePara elsevierViewall">The NO<span class="elsevierStyleInf">2</span> total column distribution reconstructed during the rainy season &#40;<a class="elsevierStyleCrossRef" href="#fig0030">Fig&#46; 6c</a>&#41; allows to better identify the presence of areas with NO<span class="elsevierStyleInf">2</span> column enhancements over the cities of Toluca and Puebla&#44; whose contribution is somewhat lost by the large<a name="p409"></a> column densities measured over the MCMA&#44; the Tula industrial area or even the Cuernavaca valley during other months of the year&#46; Also&#44; a transport between the MCMA and Cuernavaca is visible during the dry seasons both through the Cuautla and Cuernavaca sides&#44; where the highway connecting both cities runs&#46; This confirms the pollution transport highlighted in the previous section and addressed by previous studies &#40;<a class="elsevierStyleCrossRef" href="#bib0170">Melamed <span class="elsevierStyleItalic">et&#46; al</span>&#44; 2009</a>&#59; <a class="elsevierStyleCrossRef" href="#bib0195">Salcedo <span class="elsevierStyleItalic">et al</span>&#44; 2012</a>&#41;&#46;</p></span></span><span id="sec0070" class="elsevierStyleSection elsevierViewall"><span class="elsevierStyleLabel">3&#46;3</span><span class="elsevierStyleSectionTitle" id="sect0085">Comparison of ground-based and space-borne measurements</span><p id="par0130" class="elsevierStylePara elsevierViewall">A comparison of direct coincident measurements was done for NO<span class="elsevierStyleInf">2</span> ground-based and OMI measured columns&#46; All ground-based measurements conducted during the overpass time of OMI over UNAM &#40;~14&#58;21 LT&#41; were used with a tolerance of &#177; 20 min&#46; OMI measurements having a pixel center within a certain radius are used&#46; The number of coincident days for the direct comparison is increasing when using larger radius&#46; Ground-based and OMI measurements conducted on the coincident days and times were averaged and are reported in <a class="elsevierStyleCrossRef" href="#tbl0005">Table I</a> along with its standard deviation&#44; correlation coefficient&#44; linear-fit offset and linear-fit slope&#46; In general&#44; ground-based measured NO<span class="elsevierStyleInf">2</span> columns were 2&#46;7-6 times higher than OMI measured columns&#44; which could be explained by the spatial discrepancy between the ground-based measurements &#40;basically local&#41; and the satellite averaged measurements&#46; The<a name="p410"></a> highest correlation coefficient &#40;0&#46;807&#41; was found when comparing OMI data with a pixel center falling within a 20 km radius around UNAM&#59; however&#44; good correlation was also observed for data with pixel centers of 15 to 50 km radius around the ground-based site&#46;</p><elsevierMultimedia ident="tbl0005"></elsevierMultimedia><p id="par0135" class="elsevierStylePara elsevierViewall">The averages from OMI NO<span class="elsevierStyleInf">2</span> columns shown in <a class="elsevierStyleCrossRef" href="#tbl0005">Table I</a>&#44; however&#44; are underestimated since the instrument&#8217;s footprint of 13<span class="elsevierStyleHsp" style=""></span>&#215;<span class="elsevierStyleHsp" style=""></span>24 km<span class="elsevierStyleSup">2</span> at nadir covers a large area and the UNAM site is within a region with large gradients in its horizontal distribution&#46; A better representation of the fine structure is obtained by the reconstruction shown in <a class="elsevierStyleCrossRef" href="#fig0020">Figure 4</a> according to the methodology described in <a class="elsevierStyleCrossRef" href="#sec0020">section 2&#46;1&#46;1</a>&#46; The reconstructed value of the average over UNAM for the entire time period is 1&#46;7<span class="elsevierStyleHsp" style=""></span>&#215;<span class="elsevierStyleHsp" style=""></span>10<span class="elsevierStyleSup">16</span> molecules&#47;cm<span class="elsevierStyleSup">2</span>&#46; This value is approximately three times lower than the average ground-based NO<span class="elsevierStyleInf">2</span> column measured over UNAM&#46;</p><p id="par0140" class="elsevierStylePara elsevierViewall">Furthermore&#44; the satellite observation might not be sensitive enough close to the surface&#44; where most of the pollution is present&#46; A sensitivity of 0&#46;5 would actually explain the difference by a factor of 2 between the mean values of the ground-based and space-borne DOAS measurements&#46; The surface albedo of Mexico City in the 450 nm spectral region was measured by <a class="elsevierStyleCrossRef" href="#bib0040">Castro <span class="elsevierStyleItalic">et al</span>&#46; &#40;2001&#41;</a> and recently during the MILAGRO field experiment by <a class="elsevierStyleCrossRef" href="#bib0075">Coddington <span class="elsevierStyleItalic">et al</span>&#46; &#40;2008&#41;</a> and found to be in average around 0&#46;05 and 0&#46;07 respectively&#46; Therefore&#44; the estimated sensitivity of space-borne DOAS retrievals on surface near NO<span class="elsevierStyleInf">2</span> anomalies &#40;local pollution in the mixing layer&#41; should range between 0&#46;2 and 0&#46;7&#44; which correspond to the two examples for surface albedo&#44; 0&#46;02 and 0&#46;15&#44; showed by <a class="elsevierStyleCrossRef" href="#bib0100">Eskes and Boersma &#40;2003&#41;</a>&#46;</p><p id="par0145" class="elsevierStylePara elsevierViewall">Monthly average comparisons were also made between ground-based measurements&#44; OMI and RAMA calculated NO<span class="elsevierStyleInf">2</span> columns &#40;<a class="elsevierStyleCrossRef" href="#fig0035">Fig&#46; 7</a>&#41;&#46; Again&#44; only ground-based measurements within &#177; 20 min of the OMI overpass time &#40;~14&#58;21 LT&#41; above UNAM are kept&#46; OMI averages were restricted to columns with pixel centers within &#177;0&#46;1 degrees latitude&#47; longitude of UNAM&#46; Ground-based reported RAMA NO<span class="elsevierStyleInf">2</span> columns were calculated from the average concentrations measured by five stations located near UNAM&#44; multiplied by the measured mixing layer height &#40;<a class="elsevierStyleCrossRef" href="#sec0035">section 2&#46;4</a>&#41; and correcting for pressure and temperature &#40;<a class="elsevierStyleCrossRef" href="#sec0040">section 2&#46;5</a>&#41;&#46; Amonthly cycle was clearly identified&#44; however&#44; it would seem to differ slightly depending on the origin of the data&#46; The calculated columns from surface RAMA data were the highest&#44; followed by the ground-based DOAS measurements and finally the OMI data yielded the lowest values&#46; This result was expected since the ground-based RAMA stations sample NO<span class="elsevierStyleInf">2</span> at a single sampling point very close to the sources and&#47; or when it has just been recently converted&#46; The DOAS ground-based measurements are conducted close to the sources as well&#44; however&#44; its location and vegetation around the UNAM campus could partly dampen the detection of NO<span class="elsevierStyleInf">2</span> at the current sampling point&#46; Furthermore&#44; while the ground-based DOAS instrument is sampling at a single point&#44; the observed columns by OMI are effectively averaged over the instrument&#8217;s field of view &#40;13<span class="elsevierStyleHsp" style=""></span>&#215;<span class="elsevierStyleHsp" style=""></span>24 km<span class="elsevierStyleSup">2</span> at nadir&#41; yielding lower NO<span class="elsevierStyleInf">2</span> columns than the other two instruments probably because of the averaging of polluted and unpolluted areas&#46;</p></span><span id="sec0075" class="elsevierStyleSection elsevierViewall"><span class="elsevierStyleLabel">3&#46;4</span><span class="elsevierStyleSectionTitle" id="sect0090">Spatial distribution of NO<span class="elsevierStyleInf">2</span> columns reconstructed from UNAM measurements</span><p id="par0150" class="elsevierStylePara elsevierViewall">An additional analysis was made in the interest of gaining more information regarding the possible origin of the NO<span class="elsevierStyleInf">2</span> columns measured over UNAM&#44; especially due to the inhomogeneous distribution of N<span class="elsevierStyleInf">O2</span> columns in the city&#44; attested by OMI measurements&#46; The averaged surface wind speed and wind direction measured at five stations near UNAM were used to compute the statistical wind propagation pattern that indicates the probability that a column measured at UNAM is originated or transported from a certain area of the city&#46; Wind directions were analyzed on 30&#176; intervals&#44; and a probability was assigned to each sector according to the relative frequencies&#46; Each sector was differentiated into three regions according to the intervals of certain probability &#40;15&#46;85&#44;68&#46;3&#44; and 15&#46;85&#37;&#41; related to different intervals of confidence of a log-normal distribution&#46; Accordingly&#44; wind speeds were expected to follow a log-normal distribution as well&#44; described by the median and the log-normal standard deviation &#40;<a class="elsevierStyleCrossRef" href="#fig0040">Fig&#46; 8a</a>&#41;&#46;</p><elsevierMultimedia ident="fig0040"></elsevierMultimedia><p id="par0155" class="elsevierStylePara elsevierViewall">The false color of each region in <a class="elsevierStyleCrossRef" href="#fig0040">Figure 8a</a> indicates the probability that an airmass will be measured at UNAM with the certain time delay of 2&#46;25 h&#46; This time delay used for the visualization &#40;<a class="elsevierStyleCrossRef" href="#fig0040">Fig&#46; 8</a>&#41; is the estimated lifetime of NO<span class="elsevierStyleInf">2</span> in the mixing layer with a typical temperature of the Mexico City boundary layer and according to <a class="elsevierStyleCrossRef" href="#bib0090">Dils &#40;2008&#41;</a>&#46; The measured NO<span class="elsevierStyleInf">2</span> columns were then distributed to the corresponding<a name="p411"></a> sectors using wind direction and wind speed at the time when measurements were conducted&#46; The distribution pattern of the averaged NO<span class="elsevierStyleInf">2</span> total columns reconstructed from the measurements at UNAM and wind measurements is presented in <a class="elsevierStyleCrossRef" href="#fig0040">Figure 8b</a>&#46; The result of this approach indicates a tendency to measure higher NO<span class="elsevierStyleInf">2</span> columns when the wind direction is predominant from the north-northwest&#44; coinciding with the most industrialized and populated area of the city&#46;</p></span></span><span id="sec0080" class="elsevierStyleSection elsevierViewall"><span class="elsevierStyleLabel">4</span><span class="elsevierStyleSectionTitle" id="sect0095">Conclusions</span><p id="par0160" class="elsevierStylePara elsevierViewall">NO<span class="elsevierStyleInf">2</span> columns were measured at UNAM in Mexico City and compared to space-borne observations conducted by the OMI instrument&#46; Space-borne NO<span class="elsevierStyleInf">2</span> columns were lower than ground-based measurements&#44; which presented large day-to-day variations&#46; The difference between space-borne and ground-based columns could be explained by the strong horizontal inhomogeneity of the lower layer of the measured NO<span class="elsevierStyleInf">2</span> columns and by the difference in horizontal sensitivity of the two techniques &#40;local versus averaging over 13<span class="elsevierStyleHsp" style=""></span>&#215;<span class="elsevierStyleHsp" style=""></span>24 km<span class="elsevierStyleSup">2</span>&#41;&#46; More instruments located in and around the MCMA would allow to better evaluate the spatial distribution obtained from satellite measurements&#46; A network of several MAX-DOAS has already been installed starting the fall of 2012&#44; which will result in better ground-based coverage and accuracy of the reported columns&#46;</p><p id="par0165" class="elsevierStylePara elsevierViewall">A reconstructed distribution of weighted NO<span class="elsevierStyleInf">2</span> columns over a fine grid was made over a large region in central Mexico&#44; allowing to successfully define three main areas with enhanced pollution&#58; the dominating metropolitan area of Mexico City&#44; the heavily industrialized Tula region and the Cuernavaca valley&#46; Other mayor urban areas such as Toluca and Puebla showed in average relatively lower column densities than Cuernavaca&#46; The considerably higher altitude of Toluca probably favors the photolytic destruction of NO<span class="elsevierStyleInf">2</span>&#44; and this area does not appear to receive as much pollution from Mexico City&#44; as the Cuernavaca valley&#46; The importance of considering changes in albedo&#44; aerosols load and their refractions indexes in NO<span class="elsevierStyleInf">2</span> photolysis rates should also be noted&#46; The unanticipated enhancement of NO<span class="elsevierStyleInf">2</span> columns above the Cuernavaca valley&#44; corroborates results from previous studies conducted by <a class="elsevierStyleCrossRef" href="#bib0170">Melamed <span class="elsevierStyleItalic">et al</span>&#46; &#40;2009&#41;</a> and <a class="elsevierStyleCrossRef" href="#bib0195">Salcedo <span class="elsevierStyleItalic">et al</span>&#46; &#40;2012&#41;</a>&#44; where a possible transport of pollutants from the MCMA have been discussed&#46; A plausible source arising from the biomass burning activities in that region is suggested for the high NO<span class="elsevierStyleInf">2</span> columns detected&#46;</p><p id="par0170" class="elsevierStylePara elsevierViewall">From the reconstructed maps it is also possible to observe an important interconnection between the MCMA and the Tula industrial region&#44; where emissions from one can evidently affect the other and the transport of pollutants between them most probably occurs&#46; In addition&#44; distinct seasonal variability of NO<span class="elsevierStyleInf">2</span> columns over central Mexico was found&#44; concurring with previous studies&#44; with higher NO<span class="elsevierStyleInf">2</span> columns measured during the dry and cold season&#44; followed by the dry and warm period and finally the lowest NO<span class="elsevierStyleInf">2</span> columns were found during the rainy<a name="p412"></a> season&#46; Furthermore&#44; the spatial distribution of NO<span class="elsevierStyleInf">2</span> columns reconstructed from measurements conducted at UNAM indicate a tendency to measure higher NO<span class="elsevierStyleInf">2</span> columns when the wind direction is predominant from the north-northwest&#44; coinciding with the most industrialized area of the city&#46;</p></span></span>"
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          "titulo" => "Resumen"
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          "identificador" => "xres499033"
          "titulo" => "Abstract"
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              "identificador" => "abst0010"
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        2 => array:2 [
          "identificador" => "xpalclavsec520466"
          "titulo" => "Keywords"
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          "identificador" => "sec0005"
          "titulo" => "Introduction"
        ]
        4 => array:3 [
          "identificador" => "sec0010"
          "titulo" => "Data set"
          "secciones" => array:5 [
            0 => array:3 [
              "identificador" => "sec0015"
              "titulo" => "OMI"
              "secciones" => array:1 [
                0 => array:2 [
                  "identificador" => "sec0020"
                  "titulo" => "Reconstruction of the mean column NO distribution"
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              ]
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            1 => array:2 [
              "identificador" => "sec0025"
              "titulo" => "Zenith scattered sunlight DOAS"
            ]
            2 => array:2 [
              "identificador" => "sec0030"
              "titulo" => "Surface measurements"
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            3 => array:2 [
              "identificador" => "sec0035"
              "titulo" => "LIDAR"
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            4 => array:2 [
              "identificador" => "sec0040"
              "titulo" => "Meteorological parameters"
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          "titulo" => "Results and discussion"
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            0 => array:2 [
              "identificador" => "sec0050"
              "titulo" => "Ground-based NO columns"
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            1 => array:3 [
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              "titulo" => "NO spatial distribution"
              "secciones" => array:2 [
                0 => array:2 [
                  "identificador" => "sec0060"
                  "titulo" => "Three main areas of interest in central Mexico"
                ]
                1 => array:2 [
                  "identificador" => "sec0065"
                  "titulo" => "Seasonal variability"
                ]
              ]
            ]
            2 => array:2 [
              "identificador" => "sec0070"
              "titulo" => "Comparison of ground-based and space-borne measurements"
            ]
            3 => array:2 [
              "identificador" => "sec0075"
              "titulo" => "Spatial distribution of NO columns reconstructed from UNAM measurements"
            ]
          ]
        ]
        6 => array:2 [
          "identificador" => "sec0080"
          "titulo" => "Conclusions"
        ]
        7 => array:2 [
          "identificador" => "xack161229"
          "titulo" => "Acknowledgements"
        ]
        8 => array:1 [
          "titulo" => "References"
        ]
      ]
    ]
    "pdfFichero" => "main.pdf"
    "tienePdf" => true
    "fechaRecibido" => "2012-08-30"
    "fechaAceptado" => "2013-02-22"
    "PalabrasClave" => array:1 [
      "en" => array:1 [
        0 => array:4 [
          "clase" => "keyword"
          "titulo" => "Keywords"
          "identificador" => "xpalclavsec520466"
          "palabras" => array:6 [
            0 => "DOAS"
            1 => "OMI"
            2 => "nitrogen dioxide"
            3 => "ground-based"
            4 => "space-borne"
            5 => "central Mexico"
          ]
        ]
      ]
    ]
    "tieneResumen" => true
    "resumen" => array:2 [
      "es" => array:2 [
        "titulo" => "Resumen"
        "resumen" => "<span id="abst0005" class="elsevierStyleSection elsevierViewall"><p id="spar0005" class="elsevierStyleSimplePara elsevierViewall">El uso de datos satelitales en combinaci&#243;n con mediciones realizadas en superficie puede proporcionar informaci&#243;n valiosa acerca de la qu&#237;mica atmosf&#233;rica y la calidad del aire&#46; En este estudio se comparan mediciones en superficie de di&#243;xido de nitr&#243;geno &#40;NO<span class="elsevierStyleInf">2</span>&#41; realizadas mediante la t&#233;cnica de espectroscopia &#243;ptica de absorci&#243;n diferencial &#40;DOAS&#44; por sus siglas en ingl&#233;s&#41; con mediciones del instrumento satelital para la medici&#243;n de ozono &#40;OMI&#44; por sus siglas en ingl&#233;s&#41; realizadas de 2006 a 2011&#46; Las mediciones realizadas desde la superficie presentaron grandes variaciones diarias y fueron en promedio tres veces m&#225;s altas que las columnas medidas desde el espacio&#46; La diferencia se atribuye a una fuerte heterogeneidad horizontal presente en la capa inferior de las columnas de NO<span class="elsevierStyleInf">2</span>&#44; las cuales fueron muestreadas por el instrumento satelital a partir de un &#225;rea extensa&#59; de igual manera&#44; esta discrepancia se atribuye a la sensibilidad reducida del sat&#233;lite cerca de la superficie&#44; donde se encuentran las mayores concentraciones&#46; A partir de los datos del OMI analizados se reconstruyeron mapas de distribuci&#243;n de NO<span class="elsevierStyleInf">2</span> sobre el centro de M&#233;xico&#44; y se identificaron tres &#225;reas principales de inter&#233;s&#58; la zona metropolitana de la Ciudad de M&#233;xico&#44; que fue el &#225;rea predominante&#59; la zona altamente industrializada de Tula&#44; al norte&#44; y el valle de Cuernavaca&#44; al sur&#46; En este an&#225;lisis se detectaron de igual forma variaciones estacionales de columnas de NO<span class="elsevierStyleInf">2</span> sobre el centro de M&#233;xico&#58; se encontraron columnas m&#225;s altas durante la estaci&#243;n fr&#237;a y seca&#44; seguidas por las de la estaci&#243;n caliente y seca&#59; las columnas m&#225;s bajas se encontraron durante la &#233;poca de lluvias&#46; Este conjunto de datos evidencia el transporte de contaminaci&#243;n de este gas desde Tula hasta la Ciudad de M&#233;xico&#44; as&#237; como al Valle de Cuernavaca&#46;</p></span>"
      ]
      "en" => array:2 [
        "titulo" => "Abstract"
        "resumen" => "<span id="abst0010" class="elsevierStyleSection elsevierViewall"><p id="spar0010" class="elsevierStyleSimplePara elsevierViewall">The use of satellite data in combination with ground-based measurements can provide valuable information about atmospheric chemistry and air quality&#46; In this study&#44; ground-based Differential Optical Absorption Spectroscopy &#40;DOAS&#41; measurements of nitrogen dioxide &#40;NO<span class="elsevierStyleInf">2</span>&#41; conducted in central Mexico are compared with the space-borne Ozone Monitoring Instrument &#40;OMI&#41; dataset of 2006-2011&#46; Ground-based measurements exhibited large day-to-day variations and were on average three times higher than the space-borne derived average over the observation site&#46; This difference is attributed to strong horizontal inhomogeneity of the lower layer of the measured NO<span class="elsevierStyleInf">2</span> columns&#44; sampled over a large footprint from the satellite instrument&#46; Also&#44; a reduced sensitivity of the satellite observation near the surface&#44; where the largest concentrations are expected&#44; could be responsible for this large discrepancy&#46; From the analyzed OMI dataset&#44; distribution maps of NO<span class="elsevierStyleInf">2</span> above central Mexico were reconstructed&#44; allowing to identify three main areas with increased NO<span class="elsevierStyleInf">2</span> column densities&#58; The dominating metropolitan area of Mexico City&#44; the heavily industrialized region of Tula<a name="p402"></a> to the north and the Cuernavaca valley to the south&#46; In this analysis&#44; seasonal variability of NO<span class="elsevierStyleInf">2</span> columns over central Mexico was detected&#44; finding higher NO<span class="elsevierStyleInf">2</span> columns during the dry and cold season&#44; followed by the dry and warm period&#44; and finally the lowest NO<span class="elsevierStyleInf">2</span> columns were found during the rainy season&#46; Pollution transport of this gas from Tula into Mexico City&#44; as well as towards the Cuernavaca valley&#44; is evident from this dataset&#46;</p></span>"
      ]
    ]
    "multimedia" => array:9 [
      0 => array:7 [
        "identificador" => "fig0005"
        "etiqueta" => "Fig&#46; 1"
        "tipo" => "MULTIMEDIAFIGURA"
        "mostrarFloat" => true
        "mostrarDisplay" => false
        "figura" => array:1 [
          0 => array:4 [
            "imagen" => "gr1.jpeg"
            "Alto" => 1181
            "Ancho" => 1931
            "Tamanyo" => 268619
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        ]
        "descripcion" => array:1 [
          "en" => "<p id="spar0015" class="elsevierStyleSimplePara elsevierViewall">Differential NO<span class="elsevierStyleInf">2</span> columns measured at UNAM between &#40;a&#41; August-December 2010&#44; &#40;b&#41; February-June 2011&#44; and &#40;c&#41; August-December 2011&#46; The vertical gray lines represent the columns evaluation error&#46; Only measurements conducted between 14&#58;00 and 15&#58;00 LT are shown&#46; The data points showed in this figure are zenith measurements&#46;</p>"
        ]
      ]
      1 => array:7 [
        "identificador" => "fig0010"
        "etiqueta" => "Fig&#46; 2"
        "tipo" => "MULTIMEDIAFIGURA"
        "mostrarFloat" => true
        "mostrarDisplay" => false
        "figura" => array:1 [
          0 => array:4 [
            "imagen" => "gr2.jpeg"
            "Alto" => 604
            "Ancho" => 932
            "Tamanyo" => 61565
          ]
        ]
        "descripcion" => array:1 [
          "en" => "<p id="spar0020" class="elsevierStyleSimplePara elsevierViewall">Averaged hourly differential NO<span class="elsevierStyleInf">2</span> columns measured at UNAM over the whole time period &#40;August 2010 to December 2011&#41;&#46; The vertical grey lines represent the column standard deviation&#46;</p>"
        ]
      ]
      2 => array:7 [
        "identificador" => "fig0015"
        "etiqueta" => "Fig&#46; 3"
        "tipo" => "MULTIMEDIAFIGURA"
        "mostrarFloat" => true
        "mostrarDisplay" => false
        "figura" => array:1 [
          0 => array:4 [
            "imagen" => "gr3.jpeg"
            "Alto" => 790
            "Ancho" => 942
            "Tamanyo" => 167616
          ]
        ]
        "descripcion" => array:1 [
          "en" => "<p id="spar0025" class="elsevierStyleSimplePara elsevierViewall">Averaged NO<span class="elsevierStyleInf">2</span> total columns distribution derived from OMI measurements conducted between June 2006 and December 2011 over the Mexico basin&#46; Scale from &#40;0 to 2&#46;2&#41;<span class="elsevierStyleHsp" style=""></span>&#215;<span class="elsevierStyleHsp" style=""></span>10<span class="elsevierStyleSup">16</span> molecules&#47;cm<span class="elsevierStyleSup">2</span>&#46;</p>"
        ]
      ]
      3 => array:7 [
        "identificador" => "fig0020"
        "etiqueta" => "Fig&#46; 4"
        "tipo" => "MULTIMEDIAFIGURA"
        "mostrarFloat" => true
        "mostrarDisplay" => false
        "figura" => array:1 [
          0 => array:4 [
            "imagen" => "gr4.jpeg"
            "Alto" => 789
            "Ancho" => 941
            "Tamanyo" => 181907
          ]
        ]
        "descripcion" => array:1 [
          "en" => "<p id="spar0030" class="elsevierStyleSimplePara elsevierViewall">NO<span class="elsevierStyleInf">2</span> total columns distribution reconstructed from OMI measurements &#40;see section 2&#46;2&#46;1&#41; conducted between June 2006 and December 2011&#46; Same scale as <a class="elsevierStyleCrossRef" href="#fig0015">Figure 3</a>&#46;</p>"
        ]
      ]
      4 => array:7 [
        "identificador" => "fig0025"
        "etiqueta" => "Fig&#46; 5"
        "tipo" => "MULTIMEDIAFIGURA"
        "mostrarFloat" => true
        "mostrarDisplay" => false
        "figura" => array:1 [
          0 => array:4 [
            "imagen" => "gr5.jpeg"
            "Alto" => 1919
            "Ancho" => 1883
            "Tamanyo" => 481370
          ]
        ]
        "descripcion" => array:1 [
          "en" => "<p id="spar0035" class="elsevierStyleSimplePara elsevierViewall">NO<span class="elsevierStyleInf">2</span> total columns distribution over different areas of central Mexico reconstructed from OMI measurements conducted between June 2006 and December 2011&#46; &#40;a&#41; The Mexico City metropolitan area&#59; &#40;b&#41; the heavily industrialized region around Tula&#44; Hidalgo&#59; and &#40;c&#41; the Cuernavaca valley&#46; Note that different scales have been used in the plots&#46;</p>"
        ]
      ]
      5 => array:7 [
        "identificador" => "fig0030"
        "etiqueta" => "Fig&#46; 6"
        "tipo" => "MULTIMEDIAFIGURA"
        "mostrarFloat" => true
        "mostrarDisplay" => false
        "figura" => array:1 [
          0 => array:4 [
            "imagen" => "gr6.jpeg"
            "Alto" => 1684
            "Ancho" => 1758
            "Tamanyo" => 505497
          ]
        ]
        "descripcion" => array:1 [
          "en" => "<p id="spar0040" class="elsevierStyleSimplePara elsevierViewall">NO<span class="elsevierStyleInf">2</span> total columns distribution over central Mexico reconstructed from OMI measurements conducted between 2006-2011 on&#58; &#40;a&#41; November&#44; December&#46; January and February &#40;dry and cold&#41;&#59; &#40;b&#41; March&#44; April and May &#40;dry and warm&#41;&#59; &#40;c&#41; June&#44; July&#44; August&#44; September and October &#40;rainy season&#41;&#46; Scale from 0 to 1&#46;8<span class="elsevierStyleHsp" style=""></span>&#215;<span class="elsevierStyleHsp" style=""></span>10<span class="elsevierStyleSup">16</span> molecules&#47;cm<span class="elsevierStyleSup">2</span>&#46;</p>"
        ]
      ]
      6 => array:7 [
        "identificador" => "fig0035"
        "etiqueta" => "Fig&#46; 7"
        "tipo" => "MULTIMEDIAFIGURA"
        "mostrarFloat" => true
        "mostrarDisplay" => false
        "figura" => array:1 [
          0 => array:4 [
            "imagen" => "gr7.jpeg"
            "Alto" => 820
            "Ancho" => 933
            "Tamanyo" => 92077
          ]
        ]
        "descripcion" => array:1 [
          "en" => "<p id="spar0045" class="elsevierStyleSimplePara elsevierViewall">Monthly average comparison between ground-based&#44; OMI and RAMA &#40;calculated&#41; NO<span class="elsevierStyleInf">2</span> columns&#46;</p>"
        ]
      ]
      7 => array:7 [
        "identificador" => "fig0040"
        "etiqueta" => "Fig&#46; 8"
        "tipo" => "MULTIMEDIAFIGURA"
        "mostrarFloat" => true
        "mostrarDisplay" => false
        "figura" => array:1 [
          0 => array:4 [
            "imagen" => "gr8.jpeg"
            "Alto" => 767
            "Ancho" => 1915
            "Tamanyo" => 193287
          ]
        ]
        "descripcion" => array:1 [
          "en" => "<p id="spar0055" class="elsevierStyleSimplePara elsevierViewall">&#40;a&#41; Frequency distribution of the air masses 2&#46;25 h before measured at UN AM computed from wind data taken at RAMA stations&#59; &#40;b&#41; distribution pattern of the averaged NO<span class="elsevierStyleInf">2</span> total columns 2&#46;25 h before were measured at UNAM</p>"
        ]
      ]
      8 => array:7 [
        "identificador" => "tbl0005"
        "etiqueta" => "Table I"
        "tipo" => "MULTIMEDIATABLA"
        "mostrarFloat" => true
        "mostrarDisplay" => false
        "tabla" => array:1 [
          "tablatextoimagen" => array:1 [
            0 => array:2 [
              "tabla" => array:1 [
                0 => """
                  <table border="0" frame="\n
                  \t\t\t\t\tvoid\n
                  \t\t\t\t" class=""><thead title="thead"><tr title="table-row"><th class="td" title="table-head  " align="center" valign="middle" scope="col" style="border-bottom: 2px solid black">Radius &#40;km&#41;&nbsp;\t\t\t\t\t\t\n
                  \t\t\t\t</th><th class="td" title="table-head  " align="center" valign="middle" scope="col" style="border-bottom: 2px solid black">Number of coincident days&nbsp;\t\t\t\t\t\t\n
                  \t\t\t\t</th><th class="td" title="table-head  " align="center" valign="middle" scope="col" style="border-bottom: 2px solid black">UNAM average column &#40;molecules&#47;cm<span class="elsevierStyleSup">2</span>&#41;&nbsp;\t\t\t\t\t\t\n
                  \t\t\t\t</th><th class="td" title="table-head  " align="center" valign="middle" scope="col" style="border-bottom: 2px solid black">UNAM stdev column &#40;molecules&#47;cm<span class="elsevierStyleSup">2</span>&#41;&nbsp;\t\t\t\t\t\t\n
                  \t\t\t\t</th><th class="td" title="table-head  " align="center" valign="middle" scope="col" style="border-bottom: 2px solid black">OMI average column &#40;molecules&#47;cm<span class="elsevierStyleSup">2</span>&#41;&nbsp;\t\t\t\t\t\t\n
                  \t\t\t\t</th><th class="td" title="table-head  " align="center" valign="middle" scope="col" style="border-bottom: 2px solid black">OMI stdev column &#40;molecules&#47;cm<span class="elsevierStyleSup">2</span>&#41;&nbsp;\t\t\t\t\t\t\n
                  \t\t\t\t</th><th class="td" title="table-head  " align="center" valign="middle" scope="col" style="border-bottom: 2px solid black">Correlation coefficient&nbsp;\t\t\t\t\t\t\n
                  \t\t\t\t</th><th class="td" title="table-head  " align="center" valign="middle" scope="col" style="border-bottom: 2px solid black">Linear fit offset&nbsp;\t\t\t\t\t\t\n
                  \t\t\t\t</th><th class="td" title="table-head  " align="center" valign="middle" scope="col" style="border-bottom: 2px solid black">Linear fit slope&nbsp;\t\t\t\t\t\t\n
                  \t\t\t\t</th></tr></thead><tbody title="tbody"><tr title="table-row"><td class="td" title="table-entry  " align="char" valign="middle">9&nbsp;\t\t\t\t\t\t\n
                  \t\t\t\t</td><td class="td" title="table-entry  " align="center" valign="middle">4&nbsp;\t\t\t\t\t\t\n
                  \t\t\t\t</td><td class="td" title="table-entry  " align="char" valign="middle">2&#46;48E &#43; 016&nbsp;\t\t\t\t\t\t\n
                  \t\t\t\t</td><td class="td" title="table-entry  " align="char" valign="middle">9&#46;86E &#43; 015&nbsp;\t\t\t\t\t\t\n
                  \t\t\t\t</td><td class="td" title="table-entry  " align="char" valign="middle">7&#46;20E &#43; 015&nbsp;\t\t\t\t\t\t\n
                  \t\t\t\t</td><td class="td" title="table-entry  " align="char" valign="middle">2&#46;96E &#43; 015&nbsp;\t\t\t\t\t\t\n
                  \t\t\t\t</td><td class="td" title="table-entry  " align="char" valign="middle">&#8722;0&#46;059&nbsp;\t\t\t\t\t\t\n
                  \t\t\t\t</td><td class="td" title="table-entry  " align="char" valign="middle">7&#46;64E &#43; 015&nbsp;\t\t\t\t\t\t\n
                  \t\t\t\t</td><td class="td" title="table-entry  " align="char" valign="middle">&#8722;0&#46;018&nbsp;\t\t\t\t\t\t\n
                  \t\t\t\t</td></tr><tr title="table-row"><td class="td" title="table-entry  " align="char" valign="middle">10&nbsp;\t\t\t\t\t\t\n
                  \t\t\t\t</td><td class="td" title="table-entry  " align="char" valign="middle">5&nbsp;\t\t\t\t\t\t\n
                  \t\t\t\t</td><td class="td" title="table-entry  " align="char" valign="middle">2&#46;91E &#43; 016&nbsp;\t\t\t\t\t\t\n
                  \t\t\t\t</td><td class="td" title="table-entry  " align="char" valign="middle">1&#46;28E &#43; 016&nbsp;\t\t\t\t\t\t\n
                  \t\t\t\t</td><td class="td" title="table-entry  " align="char" valign="middle">8&#46;12E &#43; 015&nbsp;\t\t\t\t\t\t\n
                  \t\t\t\t</td><td class="td" title="table-entry  " align="char" valign="middle">3&#46;31E &#43; 015&nbsp;\t\t\t\t\t\t\n
                  \t\t\t\t</td><td class="td" title="table-entry  " align="char" valign="middle">0&#46;439&nbsp;\t\t\t\t\t\t\n
                  \t\t\t\t</td><td class="td" title="table-entry  " align="char" valign="middle">4&#46;83E &#43; 015&nbsp;\t\t\t\t\t\t\n
                  \t\t\t\t</td><td class="td" title="table-entry  " align="char" valign="middle">0&#46;113&nbsp;\t\t\t\t\t\t\n
                  \t\t\t\t</td></tr><tr title="table-row"><td class="td" title="table-entry  " align="char" valign="middle">15&nbsp;\t\t\t\t\t\t\n
                  \t\t\t\t</td><td class="td" title="table-entry  " align="center" valign="middle">6&nbsp;\t\t\t\t\t\t\n
                  \t\t\t\t</td><td class="td" title="table-entry  " align="char" valign="middle">3&#46;52E &#43; 016&nbsp;\t\t\t\t\t\t\n
                  \t\t\t\t</td><td class="td" title="table-entry  " align="char" valign="middle">1&#46;87E &#43; 016&nbsp;\t\t\t\t\t\t\n
                  \t\t\t\t</td><td class="td" title="table-entry  " align="char" valign="middle">1&#46;27E &#43; 016&nbsp;\t\t\t\t\t\t\n
                  \t\t\t\t</td><td class="td" title="table-entry  " align="char" valign="middle">1&#46;22E &#43; 016&nbsp;\t\t\t\t\t\t\n
                  \t\t\t\t</td><td class="td" title="table-entry  " align="char" valign="middle">0&#46;781&nbsp;\t\t\t\t\t\t\n
                  \t\t\t\t</td><td class="td" title="table-entry  " align="char" valign="middle">5&#46;16E &#43; 015&nbsp;\t\t\t\t\t\t\n
                  \t\t\t\t</td><td class="td" title="table-entry  " align="char" valign="middle">0&#46;509&nbsp;\t\t\t\t\t\t\n
                  \t\t\t\t</td></tr><tr title="table-row"><td class="td" title="table-entry  " align="char" valign="middle">20&nbsp;\t\t\t\t\t\t\n
                  \t\t\t\t</td><td class="td" title="table-entry  " align="center" valign="middle">6&nbsp;\t\t\t\t\t\t\n
                  \t\t\t\t</td><td class="td" title="table-entry  " align="char" valign="middle">3&#46;52E &#43; 016&nbsp;\t\t\t\t\t\t\n
                  \t\t\t\t</td><td class="td" title="table-entry  " align="char" valign="middle">1&#46;87E &#43; 016&nbsp;\t\t\t\t\t\t\n
                  \t\t\t\t</td><td class="td" title="table-entry  " align="char" valign="middle">1&#46;11E &#43; 016&nbsp;\t\t\t\t\t\t\n
                  \t\t\t\t</td><td class="td" title="table-entry  " align="char" valign="middle">1&#46;00E &#43; 016&nbsp;\t\t\t\t\t\t\n
                  \t\t\t\t</td><td class="td" title="table-entry  " align="char" valign="middle">0&#46;807&nbsp;\t\t\t\t\t\t\n
                  \t\t\t\t</td><td class="td" title="table-entry  " align="char" valign="middle">4&#46;12E &#43; 015&nbsp;\t\t\t\t\t\t\n
                  \t\t\t\t</td><td class="td" title="table-entry  " align="char" valign="middle">0&#46;433&nbsp;\t\t\t\t\t\t\n
                  \t\t\t\t</td></tr><tr title="table-row"><td class="td" title="table-entry  " align="char" valign="middle">25&nbsp;\t\t\t\t\t\t\n
                  \t\t\t\t</td><td class="td" title="table-entry  " align="center" valign="middle">6&nbsp;\t\t\t\t\t\t\n
                  \t\t\t\t</td><td class="td" title="table-entry  " align="char" valign="middle">3&#46;52E &#43; 016&nbsp;\t\t\t\t\t\t\n
                  \t\t\t\t</td><td class="td" title="table-entry  " align="char" valign="middle">1&#46;87E &#43; 016&nbsp;\t\t\t\t\t\t\n
                  \t\t\t\t</td><td class="td" title="table-entry  " align="char" valign="middle">1&#46;05E &#43; 016&nbsp;\t\t\t\t\t\t\n
                  \t\t\t\t</td><td class="td" title="table-entry  " align="char" valign="middle">9&#46;24E &#43; 015&nbsp;\t\t\t\t\t\t\n
                  \t\t\t\t</td><td class="td" title="table-entry  " align="char" valign="middle">0&#46;772&nbsp;\t\t\t\t\t\t\n
                  \t\t\t\t</td><td class="td" title="table-entry  " align="char" valign="middle">2&#46;88E &#43; 015&nbsp;\t\t\t\t\t\t\n
                  \t\t\t\t</td><td class="td" title="table-entry  " align="char" valign="middle">0&#46;381&nbsp;\t\t\t\t\t\t\n
                  \t\t\t\t</td></tr><tr title="table-row"><td class="td" title="table-entry  " align="char" valign="middle">30&nbsp;\t\t\t\t\t\t\n
                  \t\t\t\t</td><td class="td" title="table-entry  " align="center" valign="middle">6&nbsp;\t\t\t\t\t\t\n
                  \t\t\t\t</td><td class="td" title="table-entry  " align="char" valign="middle">3&#46;52E &#43; 016&nbsp;\t\t\t\t\t\t\n
                  \t\t\t\t</td><td class="td" title="table-entry  " align="char" valign="middle">1&#46;87E &#43; 016&nbsp;\t\t\t\t\t\t\n
                  \t\t\t\t</td><td class="td" title="table-entry  " align="char" valign="middle">1&#46;01E &#43; 016&nbsp;\t\t\t\t\t\t\n
                  \t\t\t\t</td><td class="td" title="table-entry  " align="char" valign="middle">8&#46;30E &#43; 015&nbsp;\t\t\t\t\t\t\n
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          "en" => "<p id="spar0050" class="elsevierStyleSimplePara elsevierViewall">Comparison of NO<span class="elsevierStyleInf">2</span> ground-based columns measured at UNAM and NO<span class="elsevierStyleInf">2</span> columns measured by OMI at overpass time 14&#58;21 LT &#177; 20 min&#46;</p>"
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        0 => array:2 [
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                        0 => array:2 [
                          "etal" => false
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                            2 => "U&#46; Platt"
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                            4 => "T&#46; Wagner"
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                          "autores" => array:1 [
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                            2 => "M&#46;O&#46; Wenig"
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                        "tituloSerie" => "J&#46; Geophys&#46; Res&#46;"
                        "fecha" => "2008"
                        "volumen" => "113"
                        "paginaInicial" => "D16S45"
                      ]
                    ]
                  ]
                ]
              ]
            ]
          ]
        ]
      ]
    ]
    "agradecimientos" => array:1 [
      0 => array:4 [
        "identificador" => "xack161229"
        "titulo" => "Acknowledgements"
        "texto" => "<p id="par0175" class="elsevierStylePara elsevierViewall">The authors gratefully acknowledge financial support provided by DGAPA-UNAM through project number IN119310 and a DGAPA-UNAM postdoctoral research scholarship&#46; The OMI project is managed by the Netherlands Agency for Aerospace Programs &#40;NIVR&#41; and the Royal Netherlands Meteorological Institute &#40;KNMI&#41;&#46; The authors acknowledge the free use of NO<span class="elsevierStyleInf">2</span> total column data from the OMI sensor accessed via the NASA Aura Validation Data Center &#40;AVDC&#41; and Goddard Earth Science &#40;GES&#41; Data and Information Services Center &#40;DISC&#41;&#46; Caroline Fayt and Michel van Roozendael from the Belgian Institute for Space Aeronomie are acknowledged for the free use of the QDOAS software used for the evaluation of the ground-based data presented in this work&#46; SIMAT and the SMA-GDF are thanked for providing the <span class="elsevierStyleItalic">in situ</span> measurements and the Pro-grama de Estaciones Meteorol&#243;gicas del Bachillerato Universitario &#40;PEMBU&#41; network is acknowledged for the meteorological data&#46; Alfredo Rodr&#237;guez from the Centro de Ciencias de la Atm&#243;sfera workshop is acknowledged for providing technical support on the installation of the instrumentation used to conduct the ground-based measurements&#46;</p>"
        "vista" => "all"
      ]
    ]
  ]
  "idiomaDefecto" => "en"
  "url" => "/01876236/0000002600000003/v2_201505081749/S0187623613710853/v2_201505081749/en/main.assets"
  "Apartado" => null
  "PDF" => "https://static.elsevier.es/multimedia/01876236/0000002600000003/v2_201505081749/S0187623613710853/v2_201505081749/en/main.pdf?idApp=UINPBA00004N&text.app=https://www.elsevier.es/"
  "EPUB" => "https://multimedia.elsevier.es/PublicationsMultimediaV1/item/epub/S0187623613710853?idApp=UINPBA00004N"
]

Article information

ISSN: 01876236
Original language: English

DOI:10.1016/S0187-6236(13)71085-3

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Year/Month	Html	Pdf	Total

2024 October	24	1	25
2024 September	19	3	22
2024 August	28	1	29
2024 July	23	5	28
2024 June	34	11	45
2024 May	14	7	21
2024 April	37	5	42
2024 March	39	14	53
2024 February	21	4	25
2024 January	31	13	44
2023 December	23	13	36
2023 November	50	14	64
2023 October	75	13	88
2023 September	45	13	58
2023 August	43	2	45
2023 July	19	8	27
2023 June	35	9	44
2023 May	61	8	69
2023 April	54	9	63
2023 March	34	5	39
2023 February	16	9	25
2023 January	14	11	25
2022 December	29	14	43
2022 November	42	8	50
2022 October	23	11	34
2022 September	30	10	40
2022 August	39	10	49
2022 July	30	20	50
2022 June	17	5	22
2022 May	28	7	35
2022 April	18	10	28
2022 March	22	9	31
2022 February	38	13	51
2022 January	56	9	65
2021 December	34	20	54
2021 November	28	14	42
2021 October	18	23	41
2021 September	35	15	50
2021 August	22	16	38
2021 July	10	29	39
2021 June	15	15	30
2021 May	31	13	44
2021 April	39	27	66
2021 March	22	7	29
2021 February	10	6	16
2021 January	13	6	19
2020 December	27	6	33
2020 November	10	6	16
2020 October	9	5	14
2020 September	8	6	14
2020 August	10	5	15
2020 July	6	1	7
2020 June	4	4	8
2020 May	9	5	14
2020 April	10	0	10
2020 March	6	5	11
2020 February	6	9	15
2020 January	9	3	12
2019 December	9	10	19
2019 November	10	6	16
2019 October	11	5	16
2019 September	14	9	23
2019 August	9	10	19
2019 July	9	4	13
2019 June	25	6	31
2019 May	92	24	116
2019 April	54	10	64
2019 March	8	3	11
2019 February	9	6	15
2019 January	4	1	5
2018 December	5	7	12
2018 November	2	2	4
2018 October	5	9	14
2018 September	22	9	31
2018 August	15	2	17
2018 July	4	1	5
2018 June	25	3	28
2018 May	14	2	16
2018 April	23	5	28
2018 March	5	0	5
2018 February	3	1	4
2018 January	7	2	9
2017 December	11	0	11
2017 November	7	1	8
2017 October	11	3	14
2017 September	3	1	4
2017 August	6	0	6
2017 July	8	2	10
2017 June	10	6	16
2017 May	12	0	12
2017 April	12	0	12
2017 March	3	5	8
2017 February	5	6	11
2017 January	9	4	13
2016 December	10	1	11
2016 November	6	1	7
2016 October	9	2	11
2016 September	6	1	7
2016 August	6	1	7
2016 July	6	1	7
2016 June	1	2	3
2016 May	3	18	21
2016 April	3	0	3
2016 March	1	1	2
2016 February	4	5	9
2016 January	3	4	7
2015 December	4	2	6
2015 November	4	2	6
2015 October	5	4	9
2015 September	3	4	7
2015 August	2	2	4
2015 July	2	1	3
2015 June	0	1	1
2015 May	1	2	3
2015 April	1	5	6
2015 March	1	3	4
2015 February	0	1	1

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