| Great attention has been paid on serious ecological damage and economic losses caused by acid precipitation. Study on precipitation and chemical characteristics is always one of the highlight of atmospheric wet deposition over the world. Shanghai has been a heavy air-polluted city over the world. In order to understand the regional pollution characteristics of rainwater over Shanghai, one hundred and nine rain samples were collected from May 2009 to April 2010. The two sites located at Fudan University and Chongming Dongtan wetland, standing for the urban and background station. pH value, electrical conductivity, major water-soluble ions (SO42-,NO3-,F-Cl-,NH4+,Na+,K+,Ca2+,Mg2+) and organic acid (CH3COO-,HCOO-,C2O42-) were quantitatively measure of the rainwater. The precipitation's acidity, constituents, concentration level, temporal and spatial variation, pollution feature were analyzed comparatively between the two sites. Inter-relationship, enrichment factors, different sources and backward trajectory analysis were used to identify the potential sources of Inorganic ions.The results showed that the volume-weighted pH value of rainwater in Fudan was 4.54, with the range of 3.92-6.82. The pH value of rainwater in Chongming was 4.54, with the range of 3.92-6.82. The frequency of acid rain was 80.6% and 50% respectively. The volume-weighted electrical conductivity was 31.37 andl7.49μS/cm of rainwater in Fudan and Chongming, respectively, which indicated good air quality and low pollution in Chongming. Negative correlation was showed between electrical conductivity and total ion concentrations.The annual volume-weighted total ion concentrations of rain were 434.11 and 283.88μeq/L in Fudan and Chongming respectively. SO42-,NO3, Ca2+and NH4+ were major anions and cations, respectively. The annual volume-weighted concentrations of SO42-, NO3, Ca2+and NH4+in rain were SO42-120.10/52.27μeq/L, NO350.62/21.39μeq/L, Ca2+80.61/27.13μeq/L, NH4+63.66/33.36μeq/L, and about 72.56%,42.26% in total ion concentrations in Fudan and Chongming's rain. The high coal/fuel consumption and the rapid increase of vehicles resulted in the high emission of SO2 and NOx caused high concentration of SO42- and NO3, which were the main reason of the severe acid rain in Shanghai. Temporal variation of Mg2+, K+, Cl- and Na+showed the seasonal trends of high concentrations in winter/autumn, while SO42-, NO3-, NH4+and Ca2+peaked in winter and lowest in autumn. The volume-weighted ratio of SO42-/NO3- was 2.62 and 2.44 of rain in Fudan and Chongming respectively, which were than the average lever,6.24, in China. This indicated the style of rain pollution was from the type of sulphur to mixture type of vitriol and nitric acid. The enrichment factors of NH4+, nss-Ca2+and nss-Mg2+of rain in Fudan and Chongming were NH4+0.43/0.50, nss-Ca2+0.50/0.45, nss-Mg2+ 0.09/0.10, respectivrly. NH4+and Ca2+were the major alkaline cations, and showed the dominant neutralization effect over Mg2+. Correlation analysis showed the main forms of the atmospheric particles in Chongming were (NH4)SO4, NH4HSO4 and NH4NO3, while SO42-and NO3- were mainly in the form of CaSO4 and Ca(NO3)2 in the particles in Fudan.The total concentrations of organic acids (mainly included formate, acetic acid and oxalic acid) were 3.01 and 3.64μeq/L of rain in Fudan and Chongming, respectively. Formate and acetic acid mainly from plant emission, while oxalic acid may from the secondary organic aerosol.Source identification indicated that SO42-, NO3-, NF4+and most Ca2+derived from anthropogenic sources, Na+and most Cl- from the sea. Mg2+mainly originated from crust and sea in Fudan and Chongming, respectively. K+ originated from mineral and anthropogenic sources in Fudan, while derived from crust in Chongming. Of all the three sources of the rain in Fudan, the anthropogenic source dominated over the total ion concentrations followed by the marine and the crustal source, while the rain in Chongming was affected by anthropogenic source and sea.Back trajectory analysis implied that the precipitation in Chongming was mostly affected by air masses from sea, in which the air masses from Northwest, Southwest China and Yangtze River Delta might bring large amounts of anthropogenic pollutions through long- and moderate-range transport, which resulted the acid rain events at Chongming Dongtan.
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