The Study On Mainly Low Molecular Weight Organic Acids In Rainfall Of Chengdong District In Chengdu City

In the past study of acid rain mainly concentrated in the inorganic acid,while during the research of domestic and foreign scholars in recent years,wefind that organic acids in the rainfall has unignore effects on the rainfall acidity. In view of Chengdu city’s acid rain offen happens, in order to better unde-rstand the causes and effect factors of acid rain, this thesis selected the east r-egion of Chengdu as the main object of study, focused on the low molecularweight organic acids of the monsoon rains during a period of study8months（March2011to2011in October）,57rain water samples were collected,determin-ed,analysised and discussed pH value、three main low molecular organic acidsand major anions and cations ions in the rainfall.The study found that the fr-ee acidity in rainfall in addition to the effects of inorganic acids,organic acidshas reached the extent that can not be ignored, although the organic acid cont-ent accounted for only16%of the inorganic anion content by computing, butthe rainfall acidity contribution rate had been26.454%. After exploring we fo-und that the formic acid and acetic acid derived rainfall have similar sources,which are mainly from the release of plant, vehicle exhaust emissions.etc, thesource of the oxalic acid is much more complex, according to high correlationof the oxalic acid and SO₄^2-,we speculate its source may have certain simila-rity with the SO₄^2-source. More specific conclusions are as follow:1) With the optimization of experimental conditions, select3.2mol/LNaHCO₃-1.0mol/LNa₂CO₃as eluent at last, eluent flow rate:0.5ml/mol, injection volu-me:20μL. Experimental methods under this condition show the precision relati -ve standard deviation is between0.8%and4.4%, and the standard rate of rec-overies is88%to106%.2) After the experimental analysis, the average concentration of the threelow molecular organic acids: formic acid （HCOO^-）, acetate （CH₃COO^-）, oxalate（C₂O₄^-） were1.10mg/L,1.45mg/L,0.32mg/L,formic acid and acetic acid accoun-ted for89.81%of the total measured organic acid content. Acetic acid accou-nted for50.98%,the concentration of total organic acid is2.83mg/L. Organicacids also changes obviously with the season variation, the higher concentrationof summer and autumn,considering the relationship between orga-nic acid concen-tration and rainfall amount frequency the lower organic acid co-ncentration inJuly may due to the rainwater dilution effect. Concentrations of organic acids in the April^September are higher than the other month, this may because ofthe direct release by the plant. Spring, winter have low conce-ntrations of or-ganic acids. Chengdu Chenghua region’s rainfall may take about16%of the t-otal anion, various acids in proportion of the rainfall in the order of: SO₄^2->NO₃^->CH₃COO^->HCOO^->Cl^->C₂O₄^->F^-, we can see that the main four ions in therainfall are SO₄^2-、NO₃^-、CH₃COO^-、HCOO^-. This also shows that the gro-und rainfall acidity mainly effect by the impact of these four acid.By calculate the value of [C_HC00T+C_CH3COOT+C_C2O4]/[C_NO3^-+C_{SO4^2-],we found its value closed to0.25during May to August,this result indicates that during the acid rain studyin addition to considering inorganic acids, organic acids can not be ignored.3} Precipitation pH average value is4.33（range:3.56to6.82）, the frequ-ency of acid rain is as much as80.55%, strongly acidic rainfall frequency rea-ches50%, indicating that acid rain pollution in this place is very serious. Twotypes of acidic substances in the whole period of monitoring the impact ofthe pH value is inconsistent through the analysis of the relationship of pHand organic acids, inorganic acids, organic acids between April and July is themain influencing factors; August^October period, both inorganic and organicacids effects on the pH value; During March to April and July to August,4) By calculating the FA value is0.0203, and the correlation coefficientwith the pH value is0.377, this showed that there are other acidic substancesin addition to the inorganic acid in the rainfall have a obvious effects on pHvalue;Study of the contribution of organic acids to the rainwater free acidityobtained the average contribution of organic acids on the acidity of rainfall fr- eedom is26.454%, the contribution of formic acid and acetic acid is22.257%and oxalic acid contributes15.17%,indicating that the organic acids in rainfallmay be have unignored impact on the pH value, especially formic acid andacetic acid. By calculating the correlation coefficient of organic acids’s contrib-ution rate and pH, the correlation coefficient of formic acid and acetic acid is0.735, and oxalic acid’s correlation coefficient is0.691, this result further evi-denced that the relationship of organic acids on the acidity of rainfall freedomis very obvious. Organic acids’s impact on the free acidity have relationshipwith the pH value, it reaches greatest at pH value:4.0⁵.0, this mainly due tothe control of the organic acid ionization degree.5) By calculated the correlation coefficient of formic acid and acetic acidis0.912, the correlation coefficient of Oxalic acid and formic acid, acetic ac-id is0.792.Formic acid and acetic acid’s extreme similarity shows that bothhave a similar source, oxalic acid with formic acid, acetic acid have lower rel-evance, indicating that oxalic acid may exist other different sources.No obviouscorrelation between organic acids and inorganic cations,in in organic anionsonly SO₄^2-、NO₃^-、Cl^-have obvious relevance with organic acids.We obtainedfour types of main factor in the factor analysis, the first factor: K⁺、Na⁺、Mg²⁺、NH₄⁺、Cl^-、NO₃^-、SO₄^2-; second factor: HCOO^-、CH₃COO^-、C₂O₄^-; third t-he main factor:Ca²⁺; fourth factor: the F-.This indicate that three kinds of organ-ic acids of the rainfall have different and independent sources correspondingto other different inorganic ions. Various ions in the rainfall can be dividedint-o two categories after cluster analysis. One is SO₄^2-, the other is the remaining11ions except for the SO₄^2-.This means the organic acids mainly affectby its own independent source,also may affect by other sources, while the in-organic anions and cations of rainfall mainly affect by the impact of activities,such as vehicle exhaust emissions, factory emissions.etc. Therefore, to someexte-nt organic acids also affect by the impact of human activities, correspond-ing with the front value of the F/A<1, and the monitoring point location wemay know automobile exhaust emissions is another major source of organic ac-ids.