Study On Atmospheric Chemistry And Long-range Transport At High Altitudes In The Lower Troposphere Over China

The pollution characteristics, atmospheric chemistry and transport at high altitudes in the lower troposphere (i.e.1000～4000 m a.s.l.) are crucial for understanding the influences of anthropogenic forcing on the regional and even global atmosphere. In the present study, several intensive field campaigns were carried out at Mt. Waliguan (100.90°E,36.28°E,3816 m a.s.l.) in western China, Mt. Tai (117.10°E, 36.25°E,1532 m a.s.l.) in northern China and Mt. Heng (112.70°E,27.30°E,1269 m a.s.l.) in southern China. Aircraft studies were also conducted over northeast (Jilin), northwest (Gansu) and central eastern China (Shandong and Jiangsu). During these experiments, large suites of air pollutants were measured. The data was analyzed with the aid of many statistical tools and modeling techniques including a photochemical box model, a backward trajectory model and cluster analysis. The results developed some better understandings of the pollution situation, chemical transformation and transport processes in the elevated lower troposphere over China.Based on the firsthand observational data, this study provides a whole knowledge on the levels and temporal-spatial variations of air pollution at high altitudes in the lower troposphere over typical regions of China. As China’s atmospheric baseline, the concentrations of air pollutants at Mt. Waliguan are generally low. However, it can also be affected to some extent by the long range transport of anthropogenic pollution. Major pollutants showed a well-defined seasonal pattern with higher concentrations in spring and lower levels in summer, and a unique diurnal profile with lower mixing ratios during the day and higher values at night. In comparison, the concentrations of air pollutants at Mt. Tai and Mt. Heng are substantially high, indicating more influences from anthropogenic activities. In particular, the abundances of primary pollutants at Mt. Tai are several to dozens of times higher than those at Mt. Waliguan and some European and American mountainous sites, demonstrating that air pollution at high altitudes over North China are quite serious. Affected by evolution of the boundary layer and mountain-valley breeze, all the major pollutants at Mt. Tai and Mt. Heng presented diurnal variations with the mixing ratios being the highest in the afternoon and the lowest at night.We also found that the cloud water over northeast China (Jilin) had been evidently acidified with the mean pH value of～4.93. The dominant anion ions were SO42- and NO3- with the equivalent ratio of～2.1. Comparison of the cloud sample with the concurrent precipitation at the ground level suggested that the wash-out process can neutralize the acidity of rainfalls over this area. In addition, by comparing the results of several key halocarbons with the earlier PEM-West B (1994) and TRACE-P (2001) aircraft studies, continuous declining trends were derived for methyl chloroform (CH3CCl3), tetrachloromethane (CCl4) and tetrachloroethene (C2Cl4) over the great China-northwestern Pacific region. These results indicate the accomplishment of China in reducing these ozone-depleting compounds under the Montreal protocol.The summertime climatological transport pattern of air masses arriving at Mt. Waliguan was derived by the cluster analysis of backward trajectories. Air parcels originating from central and eastern China dominated the air flow at Mt. Waliguan in summer. The pollution plumes from central and eastern China also showed high ozone production efficiencies (7.7～11.3 ppbv/ppbv). These results suggest strong impacts of anthropogenic forcing on the surface ozone and other atmospheric trace constituents on the Qinghai-Tibetan Plateau. The chemical budget of ozone was estimated using a photochemical box model constrained by the measured time-profiles of trace gases and meteorological variables. In-situ atmospheric photochemistry can lead to net ozone production in both spring and summer, with the mean daytime net production rates of 0.30 and 0.26 ppb/h respectively. Reaction of NO with HO2 dominates the ozone production, while ozone chemical destruction is mainly contributed by its photolysis followed by the reactions with OH and HO2.The partitioning of reactive nitrogen was investigated for the first time in China based on the measurements of total and speciated nitrogen oxides (i.e. NOy, NO, NO2, PAN, HNO3, and particulate NO3-) at Mt. Waliguan, Mt. Tai and Mt. Heng. Distinct NOy budgets were obtained at the above three stations, suggesting different chemical transformation processes in different areas. However, inorganic nitrate (i.e. HNO3+ aerosol NO3-) was the most abundant reactive nitrogen species in all the three sites, indicating the chemical evolution of nitrogen oxides was dominated by the inorganic reactions. The size distributions of particulate NO3- were also gained at Mt. Tai and Mt. Heng during the measurement campaigns. The results showed bimodal distributions of NO3- in both spring and summer at Mt. Tai, while NO3- was mainly presented in coarse particles in spring at Mt. Heng.Backward trajectories were also calculated and categorized to learn about the long-range transport patterns of air masses reaching Mt. Tai and Mt. Heng during the field experiments. In spring 2007, Mt. Tai was mainly affected by regional transport of air pollution from the western and northern parts of north China plains (NCP), while in summer mostly influenced by the regional transport from the southwestern NCP and the eastern Shandong peninsula. In spring 2009, air pollutants at Mt. Heng were mostly transported from eastern China (including NCP and Yangtze River Delta), Pearl River Delta and the northern areas.Vertical distributions of SO2 and aerosol scattering coefficients (Bsp) were determined by aircraft measurements over northeast, northwest and central eastern China. Altitude profiles of many non-methane hydrocarbons and halocarbons were also obtained over northeast China. Most compounds exhibited a typical negative profile of decreasing mixing ratios with increasing altitude, although the gradients differed with different species. These data can be used to validate the chemical transport models and satellite retrievals. The SO2 column concentrations determined from the in-situ aircraft measurements were compared with the Ozone Monitoring Instrument (OMI) SO2 retrievals. The results show that the OMI data could distinguish the varying levels of SO2 pollution in the study regions, but appeared to have underestimated the SO2 column in the highly polluted region of eastern China (e.g. Ji’nan).