| Based on summarizing the acid rain research status abroad and the acid rain research outcomes of our country, this study investigated systemically on the time and space distribution of the precipitation's pH and chemical composition in Chengdu urban area using the particular and integrated precipitation monitoring data of Chengdu urban area during the period from 1989 to 2000. What' more the study analyzed carefully some factors which had effect on the precipitation's pH and chemical composition, combining lots of statistics and correlative data. Analyses consider:The acid rain space distribution shows the acid rain pollution levels are different in every area of Chengdu, the city zone and outskirts are more serious than exurb, and the south of Chengdu is lighter. The annural distribution of acid rain exhibits that the acid rain status was stable in the "ninth five" period, and it weakened in recent years. In 1993 and period from 1996 to 1998, the probability of strongly acid in precipitation was higher. The monthly distribution of precipitation's acidity exhibits that in July, August, September and October the acid rain frequencies were highest in a year, and the frequency in August was highest, reaching 36.7%. In March, April and June the acid rain frequencies were between 11% and 13%. In May the acid rain frequency was lower, about 5.4%. Otherwise there was never acid rain in January and December during statistic 12 years. In smallrain, hard rain and cloudburst days the probability of acid rain occurred was far less than which in moderate rain and heavy rain days. So the precipitation intensity has a special effect on rainfall acidity's forming. When suspended substances in the atmospheric environment add 1.0mg/m3, the acidity of precipitation will reduce about 2.8umol/L.Cantonal precipitation's chemical composition in Chengdu has evident characteristics. Thereinto content of the dominating ions (NH4"\ F, K"\ NO3") are higher evidently than the average level of China, and ions' (SC>42\ Ca2+) are lower. When the ratio of SO^'to NO3" is 2.05, the style of acid rain is the mixing style of sulfuric acid and nitrate. The precipitation's acidity has not evident relativity to SO42". In every spotting precipitation the ions which has buffer action to precipitation acidity are different, but calcium ion' buffer action is more evident, and the buffer action of NHL*"1" is more prominent. As a whole the chemical ions influencing the precipitation acidity of Chengdu during the period from 1996 to 2000 can be divided into two species, one concludes NO3\ NHL^ SO42\ Ca2+> Mg2"\ Na+ and the other concludes Cl\ K"\ F. Analysis considered that the first species probably is the influencing result of the anthropogenic discharge sulfur dioxide and nitrogen oxide with atmospheric particulates in Chengdu and Sichuan basin. We concluded the ions of NO3" and NH4+ were offered by Chengdu itself. Ions in the second species probably come from soil mostly. In the precipitation of Chengdu circumjacent area the source of Na+and Mg2+ has biggish relativity. Ions of SO42" in precipitation and part suffer ions have strong relativity, but the buffer ions are inadequacy sameness on other spotting. On every spotting the relativity between precipitation's pH and positive ion and negative charged ion is not strong. In the whole cantonal area the coefficient of correlation between pH and Ca2+ is comparatively bigger.Concentrations of SC?42\ Ca2+ in precipitation and SC>2> TSP in atmosphere have correlation in March, and have no prominent correlation in the other months. Concentrations of NO3" in precipitation and NOx in atmosphere have no evident correlation. Mg2+> K"\ Na+ in precipitation of Chengdu urban area mostly come fromsoil.Weather status has signified effects on the forming of acid rain in Chengdu. Pollutions come from elevated source in Sichuan basin has big effect on the forming of acid rain in Chengdu urban area. In Chengdu the main emission sources of SO2 such as steam power plant have no evident effect on the content of SO42" in precipitation.Based on the instance that the correlated layout and the total developmental status in Chengdu keep steady, the efficient way to controlling the acid rain in Chengdu is carrying through the total amount control of SO2 and NOx within the range of whole province, and especially controlling the quantity of pollutant discharged from NOx control source in Chengdu urban area.
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