An evaluation of the CMAQ reproducibility of satellite tropospheric NO₂ column observations at different local times over East Asia

Abstract. Despite the importance of the role of nitrogen dioxide (NO₂) in tropospheric chemistry, the causes leading to the discrepancy between satellite-derived and modeled tropospheric NO₂ vertical column densities (VCDs) over East Asia remain unclear. Here the reproducibility of satellite tropospheric NO₂ VCD data by a regional chemical transport model (CMAQ) with the Regional Emission inventory in ASia (REAS) Version 2 is evaluated from the viewpoint of the diurnal variation of tropospheric NO₂ VCDs, where satellite observations at different local times (SCIAMACHY/ENVISAT, OMI/Aura, and GOME-2/Metop-A) are utilized considering literature validation results. As a case study, we concentrate on June and December 2007 for a detailed evaluation based on various sensitivity simulations, for example with different spatial resolutions (80, 40, 20, and 10 km) for CMAQ. For June, CMAQ generally reproduces absolute values of satellite NO₂ VCDs and their diurnal variations over all 12 selected diagnostic regions in East Asia. In contrast, a difficulty arises in interpreting the significant disagreement between satellite and CMAQ values over most of the diagnostic regions in December. The disagreement cannot be explained by any of the sensitivity simulations performed in this study. To address this, more investigations, including further efforts for satellite validations in wintertime, are needed.