Abstract
Tropospheric vertical column densities (VCDs) of NO2, SO2 and
HCHO derived from the Ozone Monitoring Instrument (OMI) on AURA and the
Global Ozone Monitoring Experiment 2 aboard METOP-A (GOME-2A) and METOP-B
(GOME-2B) are widely used to characterize the global distributions, trends
and dominating sources of these trace gases. They are also useful for the
comparison with chemical transport models (CTMs). 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 (daily and bi-monthly-averaged data) derived from OMI and GOME-2A/B
by different scientific teams. Prior to the comparison, the spatial and
temporal coincidence criteria for MAX-DOAS and satellite data are determined
by a sensitivity study using different spatial and temporal averaging
conditions. Cloud effects on both MAX-DOAS and satellite observations are
also investigated. Our results indicate that the discrepancies between
satellite and MAX-DOAS results increase with increasing effective cloud
fraction and are dominated by the effects of clouds on the satellite
products. In comparison with MAX-DOAS, we found 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 %), but good
consistency with the DOMINO version 2 NO2 product. To better understand
the reasons for these differences, we evaluated the a priori profile shapes
used in the OMI retrievals (derived from CTM) by comparison with those
derived from the MAX-DOAS observations. Significant differences are found
for the SO2 and HCHO profile shapes derived from the IMAGES model,
whereas on average good agreement is found for the NO2 profile shapes
derived from the TM4 model. We also applied the MAX-DOAS profile shapes to
the satellite retrievals and found that these modified satellite VCDs agree
better with the MAX-DOAS VCDs than the VCDs from the original data sets by
up to 10, 47 and 35 % for NO2, SO2 and HCHO,
respectively. Furthermore, we investigated the effect of aerosols on the
satellite retrievals. For OMI observations of NO2, a systematic
underestimation is found for large AOD, which is mainly attributed to effect
of the aerosols on the cloud retrieval and the subsequent application of a
cloud correction scheme (implicit aerosol correction). In contrast, the
effect of aerosols on the clear-sky air mass factor (explicit aerosol correction) has a
smaller effect. For SO2 and HCHO observations selected in the same way,
no clear aerosol effect is found, probably because for the considered data
sets no cloud correction is applied (and also because of the larger
scatter). From our findings we conclude that for satellite observations with
cloud top pressure (CTP) > 900 hPa and effective cloud fraction
(eCF) < 10 % the application of a clear-sky air mass factor might be a good
option if accurate aerosol information is not available. Another finding of
our study is that the ratio of morning-to-afternoon NO2 VCDs can be
considerably overestimated if results from different sensors and/or
retrievals (e.g. OMI and GOME-2) are used, whereas fewer deviations for HCHO
and SO2 VCDs are found.
Citation
ID:
12597
Ref Key:
wang2017validationatmospheric