| As one of the most serious environment problems, acid rain also called "killer of air" not only deteriorate air qualities thus has great impact on human health but also affect ecological systems. Our country is the third pollution area of acid rain after Europe and South America.Especially Shanghai, as the economic center of our country, is the most serious area of precipitation pollution in the world.In this study, based on the long period history data of acid rain in Baoshan meteorological station, the climate characteristics were analyzed, the inter-annual and monthly variation of pH and conductance EC was discussed. The great discrepancy was also found between Baoshan and Shanghai general values. Through comparing with the data of Pudong, we also found the different pollution environment of acid rain.To tell the reason, we analyzed the correlation of pH and atmospheric pollutants, and discussed the influence of meteorological factors on the pH and EC. For better understand the space distribution of acid rain, we expand the study area. With the help of Shanghai meteorological bureau and Fudan University,706samples came from ten sites located at Jiading, Baoshan, Xujiahui, Minhang, Qingpu, Songjiang, Jinshan, Nanhui, Fengxian and Pudong, from May2008to April2009, were colleted, and water-soluble ions (WSI) were quantitatively measured. We evaluated the average pH and concentration of ions, discussed the temporal and space distribution of pH and the ions, then compared and analyzed the chemical constituents and the reason of this disprepancy. At last, we reported the long tendency of Nitrogen wet deposition in Shanghai and the characteristic of space and seasonal distribution.The major results presented below:1. The long-term tendency, monthly characteristic and reason analysis of acid rain in Shanghai Baoshan district:the volume-weighted average pH value was5.21, weakly acidic. The frequency of acid rain was24%, lower than Shanghai average level. The pH ranged from3.03to8.44, focus on6.0-7.0. The annual variation was not apparent, which is different from the tendency of the average pH over Shanghai area. While EC (electrical conductivity) increased gradually by the rate of2.0μS cm-1yr-1. The pH was lower in cold season compared with that in warm season. But the season variation of EC was opposite to that of pH. Compared the data of acid rain in Baoshan and Pudong, the pollution in Pudong is more serious. The pH in both sites was negatively correlated with PM10. SO2, NO2, while the EC was opposite. The pH decreased with increasing of precipitation and the pH value towered up when there was little amount of precipitation; EC decreased with amount of precipitation increasing, which revealed the dilution effect of aerosol precipitation in air. The frequency of acid rain was lower when the amount of precipitation was little, which showed the neutralization effect of particulate matters in air. When the wind direction was northwest, the pH was higher and when the wind direction was northeast to southeast, the pH was lower in Baoshan,While the acidity of precipitation was higher when the wind direction was north or northwest and lower when the direction was South in Pudong.2. The values and the space distribution characteristics of the volume-weighted average pH and the concentration of soluble ions in atmospheric precipitation from May2008to April2009at Shanghai area:the pH was4.51, weakly acidic, and the frequency of acid rain was65%. Shanghai was one of the most serious polluted cities in the world. The pH was not evenly distributed, the lower pH was at Songjiang, Jinshan and Qingpu, west and southwest area of Shanghai, the higher was at Baoshan and Jiading, located at north of Shanghai. The pH had significantly seasonal variation. The lowest was at spring in8of the9sites, and the highest was during summer or autumn, this means the acidity of rain was higher in warm season than cold season, which was in accordance with the variation of SO2and NO2.The concentrations of major ions were in the order of:Ca2+> SO42-> NH4+> Cl-> Na+> NO3-> Mg2+> F-> K+. The ratio of SO42-to NO3-was3.24, which means that Sulfate dominant in precipitation of Shanghai. The sum of ions was730.08μeq/L and can be viewed as the most serious pollution event.3. Chemical analysis of precipitation in Baoshan and Pudong:the average the volume-weighted average pH value of Baoshan and Pudong were5.59and4.19, respectively and the frequency of acid rain in the two sites were9.1%and92.3%, which means the pollution condition of precipitation in Pudong is more serious than that in Baoshan, the total water soluble ions(TWSI) in precipitation of Baoshan was1306.56μeq/L, heavy pollution level, while the precipitated TWSI in Pudong was749.39μeq/L, which was serious pollution level. The common characteristics were that SO42-and NO3-were the main acidic components, and Ca2+and NH4+were the dominant basic components. The acid neutralization capacity of Ca2+at Baoshan was three orders of magnitude that of Pudong, which was the most important reason for the different precipitation pollution. We also found that sulfate and nitrate were mainly formed by photochemical reaction in Pudong, relatively clearer region. While the formation process in Baoshan was different from that of Pudong, where the sulfate and nitrate were formed mainly through heterogeneous reaction. Percentage of fine particulate matters was lager in Pudong than in Baoshan, which aggravated the acidity of precipitation. On the contrary, the air of Baoshan, in which alkali coarse particulate matters had larger proportion, alleviated the acidity of rain. The air of the two sites had different oxidation ability and neutralization capacity, perhaps makes for another reason for the different acidity of rain. The factor analysis of the ions confirmed the mineral dust, secondary aerosol as the main sources.4. The long-term tendency of the concentration and flux of nitrogen wet deposition from1990to2009:The concentration of NO3-N ranged from1.34mg/L (1991) to3.41mg/L (2009), rising by0.05mg L-1/yr. The average concentration of N03"-N during the past twenty years was2.39mg/L The concentration of NH4+-N ranged from1.70mg/L to2.62mg/L, decreasing by0.03mg·L-1/yr. The average concentration was2.07mg/L The concentration of total nitrogen (TN) was3.04-5.75mg/L, and the average concentration was4.46mg/L. The inter-annual variation of nitrogen flux was obvious. The average year wet deposition flux of NO3-N was27.72kg hm-2yr-1. And there was poor correlation between the amount of precipitation (r2=0.215) and wet deposition flux. The average annual wet deposition flux of NH4+-N was27.72kg hm-2yr-1. The year wet deposition flux of NH4+-N showed a better correlation with the amount of precipitation (r2=0.315).the year wet deposition of TN was obvious correlated with amount of precipitation (r2=0.402). The overload of nitrogen deposition had great effects on the ecosystem over Shanghai.5. Spatial and seasonal variation characteristic from May2008to April2009. NO3-N, NH4+-N and TN had obvious but not accordant seasonal characteristic. The spatial distribution of NO3-N, NH4+-N and TN had distinct discrepancy. The highest average concentration of N03-N appeared at city centre like Xujiahui, which was3.78mg/L while the lowest at suburban region like Fengxian, was1.42mg/L. The highest average concentration of NH4+-N was2.42mg/L at Fengxian, the lowest at Nanhui, was1.06mg/L. The concentration of TN at city centre was also higher than suburbs. The highest deposition flux of NO3-N was in summer at seven sites out of nine which had positive correlation with rainfall amount.The highest deposition flux of NH4+-N was also in summer at seven sites out of nine but the sites different from the sites with highest flux of NO3-N. The highest flux occurred also in summer at all sites except Songjiang and Qingpu, which had the highest at spring. The flux of NO3-N, NH4+-N and TN was higher at city centre than at suburbs. In all sites, the flux of NH4-N was smaller than that of NO3-N, which illustrated that contribution of emission from fossil fuel burning was greater than that coming from agriculture and human and poultry excrement. |
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