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Ground-based measurements of scattered sunlight by the Multi AxisDifferential Optical Absorption Spectroscopy (MAX-DOAS) have been carried outat an urban site (39.95° N, 116.32° E) in Beijing megacitysince 6 August 2008. In this study, we retrieved the tropospheric NO2vertical column densities (VCDs) over Beijing from these MAX-DOASobservations from August 2008 to September 2011. Over this period, thedaytime (08:00–17:00 Beijing Time (BJT, which equals UTC + 8)) mean tropospheric NO2 VCDs variedfrom 0.5 to 13.3 with an average of 3.6 during summertime, and from 0.2 to16.8 with an average of 5.8 during wintertime, all in units of 1016 molecules cm−2.The average diurnal variation patterns of troposphericNO2 over Beijing appeared to be rather different from one season toanother, indicating differences in the emission strength and atmosphericlifetime. In contrast to previous studies, we find a small weekly cycle ofthe tropospheric NO2 VCD over Beijing. The NO2 VCD in the lateafternoon was the largest on Saturday and the lowest on Sunday, and in themorning it reached a clear maximum on Wednesday. We also find a post-OlympicGames effect, with 39–54% decrease in the tropospheric NO2 VCD overBeijing estimated for August of 2008, compared to the following years. Thetropospheric NO2 VCDs derived by our ground MAX-DOAS measurements show agood correlation with SCIAMACHY and OMI satellite data. However, comparedwith the MAX-DOAS measurements, the satellite observations underestimate thetropospheric NO2 VCDs over Beijing systematically, by 43% forSCIAMACHY and 26–38% for OMI (DOMINO v2.0 and DOMINO v1.02). Based onradiative transfer simulations, we show that the aerosol shielding effect canexplain this underestimation, while the gradient smoothing effect caused bythe coarse spatial resolution of the satellite observations could play anadditional role

atmospheric chemistry and physics

Tropospheric NO&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt; vertical column densities over Beijing: results of the first three years of ground-based MAX-DOAS measurements (2008–2011) and satellite validation