Abstract. Ground-based measurements of scattered sunlight by the Multi Axis Differential Optical Absorption Spectroscopy (MAX-DOAS) have been carried out at an urban site (39.95° N, 116.32° E) in Beijing megacity since 6 August 2008. In this study, we retrieved the tropospheric NO2 vertical column densities (VCDs) over Beijing from these MAX-DOAS observations from August 2008 to September 2011. Over this period, the daytime (08:00–17:00 Beijing Time (BJT, which equals UTC + 8)) mean tropospheric NO2 VCDs varied from 0.5 to 13.3 with an average of 3.6 during summertime, and from 0.2 to 16.8 with an average of 5.8 during wintertime, all in units of 1016 molecules cm−2. The average diurnal variation patterns of tropospheric NO2 over Beijing appeared to be rather different from one season to another, indicating differences in the emission strength and atmospheric lifetime. In contrast to previous studies, we find a small weekly cycle of the tropospheric NO2 VCD over Beijing. The NO2 VCD in the late afternoon was the largest on Saturday and the lowest on Sunday, and in the morning it reached a clear maximum on Wednesday. We also find a post-Olympic Games effect, with 39–54% decrease in the tropospheric NO2 VCD over Beijing estimated for August of 2008, compared to the following years. The tropospheric NO2 VCDs derived by our ground MAX-DOAS measurements show a good correlation with SCIAMACHY and OMI satellite data. However, compared with the MAX-DOAS measurements, the satellite observations underestimate the tropospheric NO2 VCDs over Beijing systematically, by 43% for SCIAMACHY and 26–38% for OMI (DOMINO v2.0 and DOMINO v1.02). Based on radiative transfer simulations, we show that the aerosol shielding effect can explain this underestimation, while the gradient smoothing effect caused by the coarse spatial resolution of the satellite observations could play an additional role.

Abstract. Information about the long-term trends of surface and tropospheric ozone is important for assessing the impact of ozone on human health, vegetation, and climate. Long-term measurements from East Asia, especially China's eastern provinces, are urgently needed to evaluate potential changes of tropospheric ozone over this economically rapid developing region. In this paper, surface ozone data from the Linan Regional Background Station in eastern China are analyzed and results about the long-term trends of surface ozone at the station are presented. Surface ozone data were collected at Linan during 6 periods between August 1991 and July 2006. The seasonality and the long-term changes of surface ozone at the site are discussed, with focus on changes in the diurnal variations, the extreme values, and the ozone distribution. Some long-term trends of surface ozone, e.g. decrease in the average concentration, increase in the daily amplitude of the relative diurnal variations, increase in the monthly highest 5% of the ozone concentration, decrease in the monthly lowest 5% of the ozone concentration, increase in the frequencies at the high and low ends of the ozone distribution have been uncovered by the analysis. All the trends indicate that the variability of surface ozone has been enhanced. Possible causes for the observed trends are discussed. The most likely cause is believed to be the increase of NO x concentration.

Abstract. North China Plain (NCP) is one of the most densely populated regions in China and has experienced enormous economic growth in the past decades. Its regional trace gas pollution has also become one of the top environmental concerns in China. Measurements of surface trace gases, including O 3 , NO x , SO 2 and CO were carried out within the HaChi (Haze in China) project at Wuqing Meteorology Station, located between 2 mega-cities (Beijing and Tianjin) in the NCP, from 9 July 2009 to 21 January 2010. Detailed statistical analyses were made in order to provide information on the levels of the measured air pollutants and their characteristics. Gaseous air pollutant concentrations were also studied together with meteorological data and satellite data to help us better understand the causes of the observed variations in the trace gases during the field campaign. In comparison to measurements from other rural and background stations in the NCP, relatively high concentrations were detected in Wuqing, presumably due to regional mixing and transport of pollutants. Local meteorology had deterministic impacts on air pollution levels, which have to be accounted for when evaluating other effects on pollutant concentrations. Trace gas concentrations showed strong dependence on wind, providing information on regional pollution characteristics. O 3 mixing ratio also showed clear dependencies on temperature and relative humidity.