Retrieval of tropospheric NO2 columns from satellite measurements in presence of cirrus: A theoretical sensitivity study using SCIATRAN and prospect application for the A-Train

A theoretical sensitivity study of the influence of cirrus cloud properties on tropospheric NO₂ columns retrieved from the spaceborne Ozone Monitoring Instrument (OMI) measurements is performed. It is conducted within the framework of the synergetic use of A-Train sensors to derive more representative trace gas products. We aim to study the potential effects of cirrus clouds on tropospheric NO₂ retrievals using a retrieval algorithm that, unlike the OMI Standard and DOMINO algorithms, does not correct for the effects of clouds. The sensitivity study is based on the radiative transfer code SCIATRAN that performs both simulations of top of atmosphere (TOA) reflectances as measured by an OMI-like band and tropospheric NO₂ column retrievals based on the differential optical absorption spectroscopy (DOAS) method. The results of the sensitivity study show that if a correction for cirrus clouds is not included in our simple retrieval that does not account for clouds in the first place, the tropospheric column can be underestimated by 55%. This underestimation depends strongly on cirrus parameters as, in order of importance, cloud fraction, cloud optical depth, asymmetry factor of cirrus cloud phase function and cloud top height. The perspective of the synergy between OMI and cloud information obtained from cloud-derived products of the A-Train is evaluated in two parts by applying a simple cloud correction scheme based on the independent pixel approximation (IPA). Firstly, we evaluated the tropospheric NO₂ column retrievals error caused by uncertainties in cirrus cloud properties. Secondly we studied the influence of subpixel cloud optical depth variability on NO₂ retrievals. From our simulations, it is demonstrated that the error will be reduced significantly if the cloud fraction is lower or equal to 0.5. In this case, the cloud fraction and the cloud optical depth must be known within accuracy less than 0.05% and 50%, respectively. The cloud top height and the asymmetry factor must be known within uncertainty of at least 1 km and less than 0.05, respectively. The latter result shows that the uncertainty of the asymmetry factor is a major source of error in the cloud correction for tropospheric NO₂ retrieval in the presence of cirrus.