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-Francois^1
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.