| With the rapid development of science and technology,China’s urbanization process continues to accelerate, urban population size and ownership of the family car also increase constantly. The atmospheric environment from human influence is more and more serious. Chemical composition of atmospheric wet deposition is an important means to understand the level of air pollution, the transportation of atmospheric pollutants of a country or region, and analyze harm to environment. So study of precipitation chemistry has attracted highly human attention, and is one of the hotspot and highlight of the current study. Shanghai, located in the southeast Yangtze River Delta and as China’s financial, economic, shipping and trade center as well as East China’s largest industrial city, have large economic activity intensity and population density; At the same time, Shanghai is still a typical East Asian monsoon region, where there exits existence seasonal transition for southeast summer monsoon and northwest winter monsoon, the interaction of southeast monsoon and southwest monsoon in summer monsoon, causing the seasonal variations in water vapor sources; Shanghai is at the border belt of ground and sea, so the composition of aerosol component changes observably. Not only sea salt aerosols from the sea, as well as atmospheric particles and dust particles from human activities are generated inside the continent. In view of this, analysis of chemical composition of atmospheric wet deposition and its characteristics in Shanghai are very important on sources of air pollution and transportation process of atmospheric pollutants throughout the East China region, and the whole East Asian region.To study the chemical composition of atmospheric wet deposition characteristics and water vapor sources in Shanghai, We collected East China Normal University campus as the wet deposition sampling points from September 2014 to August 2015. 73 collected samples of wet deposition were measured including the pH value, the major ions, other metal element content and oxygen stable isotope. We analyze the seasonal variation of chemical composition in precipitation, contamination by heavy metals, the cause of acidification type and composition characteristics of wet deposition of hydrogen and oxygen isotopes. And we use principal component analysis, correlation analysis, the enrichment factor analysis method to track and tracer isotope δ18O and other methods to judge the stream of atmospheric wet deposition of major ions sources and water vapor sources.The conclusions were as follows:1. The annual average pH value of wet deposition in Shanghai is only 4.81, lower than the criteria of acid rain (pH= 5.6), but higher than the average pH value of wet deposition from 2009 to 2010 (pH= 4.35). This shows that acid rain pollution is also serious, but has improved compared with previous years. In addition, the Shanghai atmospheric wet deposition of pH values have significant seasonal variation, with summer and autumn high, winter and spring low.2.The concentrations of major soluble ion in the precipitation of Shanghai were in the order of Ca2+> SO42> Cl-> NH4+> NO3-> Na+> Mg+> F> K+> Br-.Ca2+ and SO42-were the main soluble ion and the 45.73% of the total number of ions; SO42- and NO3-were the main soluble anion and the 31.28% of the total number of ions; Temporal variation pattern of anion SO42- and NO3- were winter>autumn>summer>spring; the concentration of Ca2+ and Mg2+ in the autumn and winter were the highest; NH4+ was autumn>summer>spring> winter.3.During 2014 to 2015, the equivalence ratio of SO42-/NO3- in Shanghai was 2.0, far less than in Shanghai from 1992 to 2014,indicating that the acid rain in Shanghai was acid and nitric acid compound; the ratio of SO42-/NO3- of precipitation in Shanghai has been a downward, indicating that the atmospheric nitrogen oxide pollution in Shanghai was more severe than sulfur dioxide pollution; the equivalent ratio of (Ca2++ NH4+)/(SO42-+NO3-) of atmospheric wet deposition in Shanghai emerged first a trend of increased and then decreased in recent 22 years, indicating that the atmospheric wet deposition in Shanghai in recent years to increase the degree of acidity in the atmosphere may the reduction of alkaline substances in the urban atmosphere of Shanghai brought about the severe acid precipitation.4.The correlation between SO42- and NO3- is higher, and the correlations between Ca2+ and SO42-、NO3- is relatively more higher than the correlations between NH4+ and SO42-、NO3-, indicating that the main neutralization factor were Ca2+ and NH4+. The contribution of NH4+ to neutralize the acidity is smaller than Ca2+.5.The concentrations of heavy metals in the precipitation of Shanghai were in the order of Al>Fe>Zn>Pb> Mn> Sr>Cu>Ni>Cd. The maximum concentrations of Pb and Zn in precipitation were above the National standard of Earth’s surface.Compared to other countries, the main pollution elements were Al、Cd、Pb、Mn and Ni. Al, Fe, Mn and Zn has the same seasonal variation, that relatively low concentrations in the summer season, but higher concentration in the winter season. Seasonal variation of Cd and Ni concentration were same.6.Isotope δ18O ranges from -15.90%o to -0.21‰ with season and annual average was 5.93‰; Isotope 5D ranges from-116.36‰ to -0.17‰ with season and annual average was -37.03‰; These data are comparable to those reported for other areas; Isotope δ18O and 5D variation characters were higher at spring and winter, lower at summer and autumn.7.The local meteoric water line (LMWL) equation of Shanghai was established by δD=7.62δ18O+8.34, and the correlation coefficient R2=0.93. The intercept and slope of precipitation line in Shanghai were less than those of the global meteoric water line and those reported for other areas. Moreover, The intercept and slope of precipitation line in summer and autumn were closer to those of the global meteoric water line, indicating the atmospheric wet deposition in summer and autumn was mainly from low-latitude ocean evaporation of moisture, winter was more complex; The average d values for precipitation in Shanghai was 10.47‰, close to the value (10‰) in most parts of the world.8.1sotope δ18O was negatively correlated with temperature, the line was fited as δ18 O=-0.17T-2.32, which is opposite to effect of temperature. This is because Shanghai is a typical East Asian monsoon region, wet deposition process of re-evaporation of raindrops cause δ18O enrichment was not obvious in summer monsoon; Linear relationship between δ18 O and rainfall in Shanghai was:δ18O=-0.07H-4.73, the precipitation effect of δ18O in the precipitation was very pronounced.Isotope δ18O and Na+ concentration of Wet deposition in Shanghai were controlled prominently vapour source and transportation ways especially migration distance at same time. Research shows that Na+ concentration affected by precipitation before the time when the wet deposition happen on Shanghai in different wet deposition that have same vapour source and transportation ways. Isotope δ18O and Na+ concentration reduced when much precipitation happened before the cloud reached Shanghai.9.The source identification with several different methods suggested that Fe、Al、 Mn and Pb mainly derived from the direct emissions of coal-fired emissions or industrial activities; Cd、Cu、Ni、Sr and Zn mainly derived from soil dust; Ca2+、K+ and Mg2+ mainly derived from cruatal source contributions、street dust and soil dust; Na+ and Cl- were mainly marine source contribution; Ions source contribution analysis showed that the industrial gas emissions in Wujing chemical industry on wet deposition Cl- concentration contribution can not be ignored; the ion concentrations in wet deposition with the continental air mass origin, suggesting that the chemistry of wet deposition in Shanghai was not only under the influence of local pollution, but also the long and moderate range transport of pollutants from northwest and southwest direction over the continent. |
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