| 其他摘要 | Low Molecular Weight(LMW) Carboxylc Acids is the important and ubiquitous chemical constituents in the atmosphere. A comprehensive study on the chemical compositions of wet precipitation was carried out from June 2007 to June 2008 in Anshun, west of Guizhou Province,China. During this periord,118 rainwater samples were collected and seven LMW carboxylic acids were determined;in addition, the major inorganic anions and cations were also analyzed to elucidate their occurrence, variation, possible sources and deposition. To gain an insight into the state of air pollution in southwest China and regional variations in rain acidity, we compared the chemistry of precipitation among major cities and remote area in Guizhou. The main conclusions are as follows:
(1) The pH of rainwater in Anshun varied from 3.57 to 7.09 with an annual volume weighted average(VWA) 4.89 which is typical acidic value. About 53.0% of the rainwater had a pH of less than 5.6. The conductivity of rainwater varied from 6.01～298.00μS/cm with a mean 46.52μS/cm, which showed the serious air pollution in this region.
(2) The concentrations in the precipitation of Anshun showed in this order: SO42->Ca2+>NH4+>NO3->Mg2+>K+>Na+>Cl->H+>HCOOHt>CH3COOHt>(COOH)22-. SO42-, Ca2+, Mg2+, NH4+ and NO3- were the dominate ions in rainwater. Their VWM concentrations were 140.9, 124.2, 36.2, 45.4 and 31.50μmol/l. It demonstrated SO42- and NO3- were important contributors of acidity. Ca2+ , NH4+ and Mg2+ buffered the precipitation acidity. Significant correlations were found soil-derived species among Ca2+ and Mg2+ and sea-salt species between Na+ and C1-. Other relative good correlations were observed between SO42- and NO3- due to their similar sources. Enrichment factor showed the sources of inorganic ions. Burning coal used by human activity is responsible for the sources of SO42-, while the soil and lime dust claimed the sources of Ca2+. NO3- mainly originated from the NOx emitted by factories. Mg2+ was mainly from terrestrial input with limited amount from marine. NH4+ had sever sources such as nitrogen fertilizer production and consumption. Cl- in precipitation of Anshun was primarily from sea-source,but anthropogenic sources including the emission of HCl and Cl2 clamed the other main origin.
(3) Seven LMW carboxylic acids were determined in rainwater, they are formic, acetic, oxalic, pyruvic, propanic, methanesulfonic, and lactic acid. Formic,acetic and oxalic acids were found to be the three dominate carboxylic acids with the VWM concentrations of 8.77, 6.90, 2.84 μmol/l respectively. The total concentration of organic acids was 19.0 μmol/l, accounting for 12.6 % of the total anions. And carboxylic acids were estimated to account for 32.9 of the free acidity in rainwater(when pH<5). During the research period, the wet deposition flux of formic,acetic and oxalic acids were 10.81, 10.46, and 3.94 mmol/m2/yr repectively. Based on Henry’s Law, the dry deposition flux was 4.78 and 1.63mmol/m2/yr for formic and acetic acids.
(4) The concentration of organic acids in seasons followed in the order: Winter>Spring>Summer>Autumn,showing remarkable seasonal variations. Furthermore, the acids often decreased with time in a precipitation event, indicating that organic acids are primarily scavenged from blow cloud. And the organic acids were found higher in the non-growing season than in the growing season. This seasonal variation indicated these LWM acids was not primarily from emission by vegetation’s growth.
(5) With the high coefficient of 0.80, formic is significantly correlated with acetic acid, suggesting that two kinds of acids shared similar sources. From other statistical analysis,such as correlation analysis and principal component analysis in precipitation, it was suggeted that the LMW carboxylic acids in Anshun should be highly complicated; that is to say, both biogenic and anthropogenic sources might contribute to organic acids in atmosphere.Based on the Henry’s Law, we proposed the ratio of formic and acetic acids in the precipitation((F/A)aq) as the indicator of the sources, and thus found that the pollution were direct emissions from natural or anthropogenic sources .
(6) Comparison with acid precipitation in other urban and rural cities in Guizhou was disscussed. There was a urban–rural-remote decreasing trend in the contributions of LMW carboxylc Acids acids to free acidity in rainwater within Guizhou. The rainwater in Guizhou showed remarkable spatial variations in concentration,deposition flux and possible sources of organic acids.Direct emission from vegetation accounted for main sources in Shangzhong Town, representative of remote area,while biomass burning and vehicular emissions in Anshun, representative of rural area. And organic acids in Guiyang might come primarily from products of the photochemical reactions of unsaturated hydrocarbon.A more complicate situation was found in Zunyi,that sourses of organic acids existed seasonal variation; the secondary sources were responsible for the organic acids in the Spring and Winter, while primary emissions claimed the sources in the Summer and Autumn. |
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