其他摘要 | In recent years, Due to the change of climate and the worsen of the atmosphere quality, there has been an increasing focus on the effect of organic aerosol on regional climate and enviromental response. Low molecular weight(LMW for short) carboxylic acids have benn identified as major constituents of the organic aerosol. These acids are water-soluble and volatile compounds, it has been suggested they may alter the hygroscopic properties of atmospheric particles and act as more acitive cloud condensation nuclei in atmosphere, As a result, they become more and more importan in atmospheric chemistry.The high solubility of LMW carboxylic acids makes rain an ideal scavenging medium. Entering the aqeuous phase, they made major contribution to rainwater acidity. In remote area, they constituents 65% of the free acidity.Southwest of China, such as Guiyang and Chongqing, is one of the most serious areas of acid rain in our country. The ecosystem of this region is more fragility and irreverseble after disturbed because they belong to Karst region in the world. It is more important to research the acid rain and possible reasons in such region.
Guiyang city,typical of acid rain area,and Shangzhong town,less human activities’s region were selected as the research rrgion. From 2006-4 to 2007-4, 221 rainwater sample were collected and seven LMW carboxylic acids and formaldehyde were determined,in addition, the major inorganic anions and cations were analyzed to elucidate their occurrence, variation, possible sources and deposition.By the way,the origin of free acidity in rainwater especially LMW carboxylic acids contributions were evaluated for enviromental policy. The main conclusions are as follows:
1、Seven LMW carboxylic anions and inorganic anions were simultaneously determined by ICS-90 chromatography combined with RFC-30(eluent generator) about 30min. The Dionex anoin separator column(250×4mm) IonPac AS11-HC and it’s corresponding guard column(50×4mm)AG11-HC were used. The conductivity of eluent was chemically suppressed by a self-regenerating suppressor(ASRS ULTRA II 4mm) that used the column effluent to supply the proton generated by water electrolysis.The eluent KOH was generated by RFC-30(Reagent free controllor) on line with 1mmol/L(0~6min) and 1mmol/L~30mmol/L (6~31min). The flow of eluent was 1.5ml/min and column temperature was kept at 39℃ by CTS-10. Under this chromatography condition,the linear correlation coefficients in the range of 0.9995~0.9999. The relative standard deviations(RSD%) were below 5%. Quantitative recoveries of spiked samples ranged from 80~120% .This method has been effectively applied to the determination of these anions in rain samples in GuiZhou province.
2、The pH of rainwater in Guiyang city ranged from 2.49 to 6.92 with an annual volume weighted average(VWA for short) 3.28 which is typical acidic value.About 73.8% of the rainwater had a pH of less than 5.6 and 42% of the rainwater showed a pH of less than 4.0 which indicate acidic rain severely polluted the atmosphere in Guiyang city. By contrast, the VWA concentration of H+ is 5.83 in Shangzhong Town. The pH of rianwater ranged from 5.01 to 6.50 with 18.4% of the sample’s pH below 5.6.
The conductivity of rainwater in Guiyang city ranged 10~1028μS/cm with a mean 248μS/cm which showed the serious air pollution in this region. In Shangzhong Town, the conductivity of rainwater ranged 2.2~52.8μS/cm with a mean 18.1μS/cm which showed the slight air pollution in this region.
3、SO42-、NO3-、H+、Ca2+ and NH4+ were the dominate ions in rainwater in Guiyang city, their VWM concentration were 154.1μmol/L、33.9μmol/L、520.7μmol/L、226.4μmol/L and 158.3μmol/L respectively. The annual wet deposition of H+ was 496.9mmol/m2/yr and 78% of H+ deposited in sumertime. The annual wet deposition of SO42-、Ca2+、NH4+、Mg2+、Na+、K+、NO3-、Cl- were 151.5mmol/m2/yr、195.3mmol/m2/yr、138.7mmol/m2/yr、50.9mmol/m2/yr、26.8mmol/m2/yr、11.4mmol/m2/yr、32.3mmol/m2/yr和12.6mmol/m2/yr respectively.
In Shangzhong Town,SO42-、NO3-、Ca2+ and NH4+ were the dominate ions in rainwater , their VWM concentration were 43.1μmol/L、19.3μmol/L、33.0μmol/L and 49.5μmol/L respectively. The annual wet deposition of H+ was 0.5mmol/m2/yr much lower than that in Guiyang city. The annual wet deposition of SO42-、Ca2+、NH4+、Mg2+、Na+、K+、NO3-、Cl- were 13.9mmol/m2/yr、10.6mmol/m2/yr、15.9mmol/m2/yr、2.1mmol/m2/yr、5.9mmol/m2/yr、3.0mmol/m2/yr、6.2mmol/m2/yr and 3.2mmol/m2/yr respectively.
4、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 14.24μmol/L、9.35μmol/L and 2.79μmol/L in rainwater in Guiyang city and 4.95μmol/L、1.35μmol/L and 2.31μmol/L in the rainwater in Shangzhong town. Carboxylic acids were estimated to account for 18.7% of the free acidity in rainwater in Guiyang city. In Shangzhong town, this contribution is much bigger, 58.1% of the free acidity was originated from LMW carboxylic acids.
During research period, the wet deposition of formic acetic and oxalic acids were 13.5mmol/m2/yr 、8.9mmol/m2/yr and 2.6mmol/m2/yr repectively. It was deposited 26.7mmol/m2/yr and 14.5mmol/m2/yr for formic and acetic acids through dry deposition in Guiyang city. There were 1.59mmol/m2/yr 、0.43mmol/m2/yr and 0.04mmol/m2/yr wet deposition for formic acetic and oxalic acids, 0.1mmol/m2/yr and 0.17mmol/m2/yr dry deposition for fromic and acetic acids in Shangzhong town.
5、The town Shangzhong also witnessed higher concentratin of the acids in the summer season than in the winter. Direct emissions from growing vegetations or from soils account for the main provenance of the acids in the rural area. This is, however, not the case in the city Guiyang, where the organic acids were found higher in the non-growing season than in the growing season. This seasonal variation of the acids indicates their anthropogenic sources and the relatively weak scavenging effect of precipitation during the wintertime. Mteorological parameters have an important impact on the concentration levels of organic acids in precipitation. During the wet precipitation events, the rain fluxes of organic acids generally decreased as a function of time and the scavenging of atmospheric organic acids by wet precipitation mostly take place during the early stage of the precipitation. The air-mass back-trajectory anlysis indicated that rainwater in Guiyang city with inland back-trajectory coming from the north had the highest concentrations of organic acids whlie the marine back-trajectory coming from the southeast had the lowest concentrations.
6、From statistical analysis (such as correlation analysis,principal component analysis and classify analysis)and indicator of origin of LMW carboxylic acids in atmosphere, it was suggeted that the LMW carboxylic acids in Shangzhong town were mainly originated from the direct emissions from vegetations or soils. However, the origin of carboxylic acids in Guiyang were more complicated, and human being activity provided a large amount unsaturated hydrocarbons, which could be the precursor of carboxylic acids.
7、Guiyang city ,the acid rain belt, is the most severely polluted area in China from the pH and conductivity of the rainwater. By contrast, the town of Shangzhong received little air pollition and acid rain during research period. The Guiyang city data exhibits concentrations of water-soluable ions which are roughly 1~4 times higher than at Shangzhong town resulting in the significantly lower acidic deposition (about 99.9%) in Shangzhong town compared to Guiyang city. LMW carboxylic acids contribute 19% to the free acidity in rainwater in Guiyang city,on contrary, they were estimated to contribute 58% to the rainwater acidity in Shangzhong town which were the major acid contributor. From origin anlysis, it was suggested the biogenic emissions were main source in Shangzhong town and anthropogenic sources were equal important to LMW carboxylic acids in Guiyang city.
8、Extensive experiments have been conducted to study the inorganic anions in atmosphere precipitation especially sulfuric and nitric acids. However, little is known about the characteristaics and the role of organic compounds in rainwater. This is the first complete and systematically study on the chemical composition of rainwater and the possible sources of acid rain in Southwest of China, a severely acid rain polluted region. LMW carboxylic acids were found an important constituents to the rainwater and contribute a large fraction to the free acidity in rainwater especially in remote area. |
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