Abstract
Abstract. Multi-axis differential optical absorption spectroscopy (MAX-DOAS) and direct sun NO2 vertical column network data are used to investigate the accuracy of tropospheric NO2 column measurements of the GOME-2 instrument on the MetOp-A satellite platform and the OMI instrument on Aura. The study is based on 23 MAX-DOAS and 16 direct sun instruments at stations distributed worldwide. A method to quantify and correct for horizontal dilution effects in heterogeneous NO2 field conditions is proposed. After systematic application of this correction to urban sites, satellite measurements are found to present smaller biases compared to ground-based reference data in almost all cases. We investigate the seasonal dependence of the validation results as well as the impact of using different approaches to select satellite ground pixels in coincidence with ground-based data. In optimal comparison conditions (satellite pixels containing the station) the median bias between satellite tropospheric NO2 column measurements and the ensemble of MAX-DOAS and direct sun measurements is found to be significant and equal to −34 % for GOME-2A and −24 % for OMI. These biases are further reduced to −24 % and −18 % respectively, after application of the dilution correction. Comparisons with the QA4ECV satellite product for both GOME-2A and OMI are also performed, showing less scatter but also a slightly larger median tropospheric NO2 column bias with respect to the ensemble of MAX-DOAS and direct sun measurements.
Highlights
Nitrogen dioxide (NO2) is a key species for atmospheric chemistry, present both in the stratosphere and in the troposphere
As can be seen, tropospheric columns selected at GOME2A overpass times are usually larger than those selected at OMI overpass time (13:30±0:90), which is explained by lower OH levels and somewhat higher NOx emissions, leading to slower NO2 chemical loss midmorning (09:30) compared to noon (13:30) (Boersma et al, 2008; Kim et al, 2009)
Note that the median tropospheric column is negative at the mountaintop stations of Izaña and Mauna Loa
Summary
Nitrogen dioxide (NO2) is a key species for atmospheric chemistry, present both in the stratosphere and in the troposphere. We validate GOME-2A and OMI tropospheric NO2 column measurements using data from a large number of MAX-DOAS and direct sun instruments operating in Europe, Asia, North America and Africa under a wide variety of atmospheric conditions and pollution patterns. Some of these datasets have already been used in the past for tropospheric NO2 validation of different satellites and products and participated in the CINDI-1 and/or 2 intercomparison campaigns (Piters et al, 2012; Kreher et al, 2020).
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