Intercomparison of Tropospheric NO2 Retrievals from the OMI AND GOME-2 Sensors Over China
YU, Huan1; Van Roozendael, Michel1; WANG, Pucai2; De Smedt, Isabelle1; van Gent, Jeroen1; van der A, Ronald3; Boersma, Folkert3; Valks, Pieter4; Richter, Andreas5; Stavrakou, Trissevgeni1; Muller, Jean-Francois1
1BIRA-IASB, BELGIUM; 2IAP/CAS, CHINA; 3KNMI, NETHERLANDS; 4IMF/DLR, GERMANY; 5IUP/University of Bremen, GERMANY
Satellite-borne UV-Vis spectrometers (such as GOME, SCIAMACHY, OMI, GOME-2), have been widely used for the retrieval of atmospheric trace gas concentrations (like O3, NO2, BrO, HCHO, SO2, CHOCHO, IO and OClO) on a global scale. In particular, the retrieval of tropospheric NO2 column density has been demonstrated in several papers, and these measurements have been used in many studies on NO2 burdens, emissions and their change over time, in the past decade.
The main approach of the current tropospheric NO2 retrievals developed for the satellite sensors is the well known DOAS technique, composed of: a spectral fit to determine the slant column density (SCD), and the application of an air mass factor (AMF) to convert the tropospheric slant column into a vertical column. However, these retrievals use different settings (such as spectral fitting, stratospheric correction and a priori information in AMF calculations), which may lead to significantly different results in some situations.
In this study, we present intercomparison results for several available tropospheric NO2 products (OMI: KNMI and NASA; GOME-2: KNMI, DLR and IUP-Bremen) and a reference product generated at BIRA-IASB using common settings for both sensors. We focus on the heavily polluted region of North China. Step-by-step comparisons are presented with a special emphasis on the AMF calculation, which is the key parameter for NO2 retrieval under polluted conditions. Sensitivity studies are discussed in terms of dependence on surface reflectance, a priori profile shape, and cloud treatment.