The Mixing And Interaction Of Asian Dust With The Typical Atmospheric Pollutants During The Long-range Transport And Its Impact On Urban Air Quality

Atmospheric aerosol could change the radiation balance and water cycle in Land-Hydrosphere-Atmosphere System by influencing the radiation forcing, atmospheric chemistry, the formation of cloud and precipitation, therefore, could impact significantly on global climatic and environmental change. Along the elevation of the industrialiaztion, urbanization, and motorization degree of China, pollution emission by human activities, including fossil fuel burning, motor vehicles, and so on, increases sharply, and the amtopshere is more and more polluted. Furthermore, the pollutant could mix and interact with transported Asian dust, and then be carried far away, resulting in significant impact on the air quality of downstream areas and the ocean ecosystem. Asian dust, high pollution emission by human activities, and the mixing between dust and pollutants challenge the environmental pollution control over China. In this study, the characteristics and distribution of Asian dust, and the mixing between dust aerosol and pollutant during the long-range transport and its impact on urban air quality were studied. Also, the sources and formation mechanisms of sulfate and nitrate in the aerosol over dust source areas and megacities were studied.1. Aerosol samples were collected in Shanghai during the super Asian dust storm from March19to March252010to study the impact of the transported dust aerosol on the air quality over the downstream areas. The transport pathways of dust aerosol were identified based on the air mass trajectories and the lidar observation. Dust aerosol was transported over the areas with high emission of anthropogenic pollutants. Pollution gases and particles in the atmosphere over Shanghai accumulated before the invading of dust and the concentrations of pollutants increased when the dust invaded Shanghai, revealing that a lot of pollutants was transported with dust aerosol. During this dust storm, two dust periods were identified in Shanghai, during which the mixing extents of dust aerosol and pollutants were different. The concentrations of pollution elements Zn, S, Cu, Pb, Cd, Mo, Se and water-soluble components SO42-, NO3-in the aerosol were higher during the dust period when dust aerosol was transported over more areas with high emission of pollutants. Furthermore, the nitrogen oxidation ratio (NOR) and sulfur oxidation ratio (SOR) were higher in the dust periods than non dust periods, revealing that the formation of SO42-and NO3-could be promoted by dust particles. This study revealed that the mixing between dust aerosol and pollutants during the transport would significantly influence the regional and global atmosphere.2. Both PM2.5and TSP were monitored in the spring from2006to2008at Yulin site, the north edge of Loess Plateau to investigate the mixing of dust aerosol with pollution on the pathway of the long-range transport of Asian dust. Elements As, Cd, Pb, Zn, and S and water-soluble ions NO3-and NH4+in the aerosol over Yulin were mainly from local pollution emissions. During dust periods, the contents of pollution elements As, Cd, Pb, Zn, and S in the aerosol were much higher than their abundance in the crust, and the contents of NO3-in PM2.5and TSP were0.8%and0.3%, higher than that in the aerosol over Taklimakan desert, revealing the mixing of dust with pollutant over Yulin in dust event day was ubiquitous. The mixing extent of dust aerosol and pollutants could be expressed by the ratio of NO3-/Al in the aerosol. The mixing extent was higher over Yulin than Tazhong and Duolun, two other sites in Asian dust sources, based on the comparison of the ratio of NO3-/Al in the aerosol. The ratio of Ca/Al was used as a tracer to study the dust source. The comparison of the ratios of Ca/Al together with back trajectory analysis indicated that the sources of the dust aerosol invaded Yulin could be from northwestern desert in China and Mongolia Gobi, and had a contribution of16～100%to the dust aerosol over Yulin.3. Both PM2.5and TSP over Yulin, the north edge of Loess Plateau, were collected, and the seasonal variations and formations of particle SO42-and NO3-were studied. SO42-had a distinct seasonal variation with highest concentration in summer in both PM2.5and TSP. NO3-in PM2.5was lowest in summer, while in TSP increased as fall<summer<spring/winter. SO42-and NO3-in the aerosol over Yulin was mainly from pollution emissions included fossil fuel burning, the emission of industry, fireworks burning, biomass burning, and transportation. The ambient temperature and relative humidity were two important factors that influenced the formation of SO42-and NO3-. High temperature in summer would increase the gas phase formation of SO42-, but reduce the gas phase formation of NO3-in PM2.5, while in other seasons in cloud/fog process should be responsible for the formations of SO42-and NO3-. Sulfate and nitrate over Yulin could not be completely neutralized by Ammonia and mineral dust could play an important role in the formation of SO42-and NO3-. Therefore, it is probably that mineral dust over Yulin, which could be transported by northly/northwestly wind, would have great impact on the chemistry in the regional atmosphere.4. A synergy of satellite observation, lidar measurement and ground-based aerosol sampling was conducted to monitor the air quality over Shanghai during the2010Expo. The ratio of PM2.5/PM10is higher than50%over Shanghai during the Expo, and the ratio of NO3-/SO42-in the aerosol was higher than1, revealing that fine particle and the pollutants from transportation had significant contributions to the air pollution. Based on the online measurement of PM1, PM2.5and PM10, the transport of Asian dust, secondary formation, and the accumulation of primary pollutants were found to be the major causes of air pollution during the study period. During the accumulation periods of primary pollutants, the concentrations of particle Cl-、K+that mainly from biomass burning and particle Ca2+that mainly from constrution significantly increased as well as the enrichment factors of pollution elements As、Zn、S、Cu、Pb、Cd、Cr, revealing that biomass burning, industry, and construstion were major emission sources for the air pollution. Though the emission of pollutants was significantly controlled for Shanghai Expo, Secondary components SO42-, NO3-, NH4+were still the major components in the aerosol. Particle SO42-and NO3-in summer could be completely neutralized by NH3, while in spring and autumn, CaCO3in aerosol should be involved in the formation of SO42-and NO3-.5. Atmospheric aerosol samples were collected at four characteristic sites, Tazhong, Yulin, Beijing, Shanghai. The spatial variations, sources, and formations of water-soluble components SO42-and NO3-were analyzed to study the air pollution over China and to estimate the effect of emission control of SO2and NOx on the aerosol. Particle SO42-over Tazhong, the center of Taklimakan Desert, was mainly from crust, particle SO42-over Yulin could be from both crust and anthropogenic pollution sources, while over Beijing and Shanghai was mainly from anthropogenic pollution sources. Particle NO3-were mainly from anthropogenic pollution sources over all the four sites. Along the elevation of the industrialization and motorization degree of the sites, the contributions of SO42-、 NH4+、 NO3-to particles increased, revealing the significant impact of anthropogenic pollution on the air pollution. In winter, the reaction between acid NOx and CaCO3in aerosol was the main formation mechanism of particle NO3-over Tazhong, while particle SO42-and NO3-over Yulin, Beijing, and Shanghai were formed mainly by in-cloud process and the reactions on preexiting partiles invovled by NH3and CaCO3. This study revealed that Asian dust and the mixing of dust aerosol and pollutant would have great contributions to global particle sulfate and nitrate. Forthermore, NO2and particle NO3-over Yulin increased from2006to2008, so did the ratio of NO3-/SO42-in the aerosol over Beijing and Shanghai from2005to2008. As the increase of industrialization, urbanization, and transportation over China, the contribution of pollution NOx to the air pollution will be more and more significant.