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Abstract:
Tropospheric vertical column densities (VCDs) of NO2, SO2 and HCHO derived from Ozone Monitoring Instrument (OMI) on AURA and Global Ozone Monitoring Experiment 2 aboard METOP-A (GOME-2A) and METOP-B (GOME-2B) are widely used to characterize the global distributions, trends, dominating sources of the trace gases and for comparisons with chemical transport models (CTM). We use tropospheric VCDs and vertical profiles of NO2, SO2 and HCHO derived from MAX-DOAS measurements from 2011 to 2014 in Wuxi, China, to validate the corresponding products derived from OMI, GOME-2A/B by different scientific teams (daily and bimonthly averaged data). Prior to the comparison we investigate the effects of the spatial and temporal coincidence criteria for MAX-DOAS and satellite data on the comparison results. We find that the distance of satellite data from the location of the MAX-DOAS station is the dominating effect, and we make suggestions for the spatial (20 km for OMI NO2 and SO2 products and 50 km for OMI HCHO and all GOME-2A/B products) and temporal averaging (2 hours around satellite overpass time). We also investigate the effect of clouds on both MAX-DOAS and satellite observations. Our results indicate that the discrepancies between satellite and MAX-DOAS results increase with increasing effective cloud fractions and are dominated by the cloud effect on the satellite products. Our comparison results indicate a systematic underestimation of all SO2 (40 % to 57 %) and HCHO products (about 20 %) and an overestimation of the GOME-2A/B NO2 products (about 30 %) (DOMINO NO2 product is only slightly underestimated by 1 %). To better understand the reasons for the differences, we recalculated the AMFs for satellite observations based on the shape factors (SFs) derived from MAX-DOAS. The recalculated satellite VCDs agree better with the MAX-DOAS VCDs than those from the original products by up to 10 %, 47 % and 35 % for NO2, SO2 and HCHO, respectively. The improvement is strongest for periods with large trace gas VCDs. Finally we investigate the effect of aerosols on the satellite retrievals. We find an increasing underestimation of the OMI NO2, SO2 and HCHO products with increasing AOD by up to 8 %, 12 % and 2 %, respectively. One reason for this finding is that aerosols systematically affect the satellite cloud retrievals and can lead to apparent effective cloud fractions of up to 10 % and apparent cloud top pressures of down to 830 hPa for the typical urban region in Wuxi. We show that in such cases the implicit aerosol correction could cause a strong underestimation of tropospheric VCDs by up to about 45 %, 77 % and 100 % for NO2, SO2 and HCHO, respectively. For such conditions it might be better to apply AMFs for clear sky conditions than AMFs based on the satellite cloud retrievals. We find that the satellites systematically overestimate the magnitude of the diurnal variations of NO2 and HCHO. No significant weekly cycle for all trace gases is found by either the satellites or the MAX-DOAS measurements.
