Abstract
Nitrogen dioxide (NO2) is a gas pollutant that can be measured from space and several operational products are now available from instruments on-board of satellite-based platforms. There are still, however, many unknowns about the accuracy of these products under different viewing and surface conditions since ground-based observations are generally scarce. This is particularly the case of high-altitude sub-tropical megacities such as the Mexico City Metropolitan Area (MCMA). In this study, we use more than five years of data from four ground-based MAX-DOAS instruments distributed within the MCMA in order to evaluate the DOMINO product from the Ozone Monitoring Instrument (OMI) on board the Aura satellite. We compare OMI against each MAX-DOAS site independently using the vertical column densities (VCDs) reported by each instrument. The VCDs are also compared after smoothing the MAX-DOAS profiles with the a priori and the Averaging Kernels of the satellite product. We obtain an overall correlation coefficient (R) of 0.6 that does not improve significantly after the smoothing is applied. However, the slopes in the linear regressions for the individual sites improve when applying the smoothing from 0.36 to 0.62 at UNAM, from 0.26 to 0.49 at Acatlán, from 0.78 to 1.23 at Vallejo, and from 0.50 to 0.97 at the Cuautitlán station. The large differences observed between the OMI and MAX-DOAS VCDs are attributed to a reduced sensitivity of the satellite product near the surface and the large aerosol loading typically present within the mixed layer of the MCMA. This may also contribute to a slight overestimation of the VCDs from the MAX-DOAS measurements that presents a total error (random + systematic) of about 20%. As a result of this comparison, we find that OMI retrievals are on average 56% lower than the MAX-DOAS without any correction. The near-surface concentrations are estimated from the lowest layers of the MAX-DOAS retrievals and these compare well with surface measurements from in situ analyzers operated at the co-located air quality monitoring stations. The diurnal variability for each station is analyzed and discussed in relation to their location within the city.
Highlights
IntroductionNitrogen dioxide (NO2 ) in the atmosphere has both natural and anthropogenic sources
The NO2 column data collected from the four MAX-DOAS stations, placed such as to achieve a representative coverage of the Mexico City Metropolitan Area (MCMA), were used to evaluate the Dutch OMI NO2 (DOMINO)
We evaluate the Ozone Monitoring Instrument (OMI)-DOMINO NO2 satellite product with a rich set of ground-based measurements that consist of four MAX-DOAS instruments co-located at surface air quality monitoring stations
Summary
Nitrogen dioxide (NO2 ) in the atmosphere has both natural and anthropogenic sources. Only 1% is formed naturally by lightning, volcanoes, water, and bacterial activity on soil and plants [1], it can be emitted through the burning of biomass. NO2 originates mostly from the combustion of fossil fuels from a large variety of activities, such as electricity generation, manufacturing industries, food processing, transportation, among others [2] This toxic gas participates in the formation of photochemical smog and the production of tropospheric ozone through its fast dissociation in the presence of solar radiation, and forms nitric acid through its oxidation with OH radicals, acidifying rain and soil. It has a lifetime of several hours, posing a high risk to human
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