The exchanges of NOx between snow and air have significant impact on the atmospheric components and photochemical processes in the overlying boundary layer. Such exchanges increase the oxidizing capacity of the atmosphere and may have a crucial impact on the air signals that are retrieved from ice cores. In the recent years, sunlit snow and ice have been demonstrated to be important NOx sources in the polar atmospheric boundary layer. This paper makes a thorough review on the release of NOx from snow and ice, including field observations and experimental evidences, release mechanisms and influential parameters that affect such a release process, polar NOx concentrations and fluxes, and environmental impacts of the chemical processes of NOx in the polar atmospheric boundary layer. In the Tibetan Plateau, the released NOx observed recently in the sunlit snow/ice-cover is 1-order magnitude more than that in polar regions, but further scientific research is still needed to reveal its impact on the atmospheric oxidizing capacity.
SO 2 measurements made in recent years at sites in Beijing and its surrounding areas are performed to study the variations and trends of surface SO 2 at different types of sites in Northern China.The overall average concentrations of SO 2 are （16.8±13.1） ppb,（14.8±9.4） ppb,and （7.5±4.0） ppb at China Meteorological Administration （CMA,Beijing urban area）,Gucheng （GCH,relatively polluted rural area,110 km to the southwest of Beijing urban area）,and Shangdianzi （SDZ,clean background area,100 km to the northeast of Beijing urban area）,respectively.The SO 2 levels in winter （heating season） are 4–6 folds higher than those in summer.There are highly significant correlations among the daily means of SO 2 at different sites,indicating regional characteristics of SO 2 pollution.Diurnal patterns of surface SO 2 at all sites have a common feature with a daytime peak,which is probably caused by the downward mixing and/or the advection transport of SO 2 -richer air over the North China Plain.The concentrations of SO 2 at CMA and GCH show highly significant downward trends （–4.4 ppb/yr for CMA and –2.4 ppb/yr for GCH）,while a less significant trend （–0.3 ppb/yr） is identified in the data from SDZ,reflecting the character of SDZ as a regional atmospheric background site in North China.The SO 2 concentrations of all three sites show a significant decrease from period before to after the control measures for the 2008 Olympic Games,suggesting that the SO 2 pollution control has long-term effectiveness and benefits.In the post-Olympics period,the mean concentrations of SO 2 at CMA,GCH,and SDZ are （14.3±11.0） ppb,（12.1±7.7） ppb,and （7.5±4.0） ppb,respectively,with reductions of 26%,36%,and 13%,respectively,compared to the levels before.Detailed analysis shows that the differences of temperature,relative humidity,wind speed,and wind direction were not the dominant factors for the significant differences of SO 2 between the pre-Olympics and post-Olympics periods.By extractin
Weili LinXiaobin XUZhiqiang MaHuarong ZhaoXiwen LiuYing Wang
