Characteristics And Sources Of Organic Acids In Atmospheric Deposition At Mount Lu

After Europe and North America, Southern of China has become the world’s third largest acid rain area and organic acids contribute much to acid rain. Along with the government control of SO2 and NOX two main precusor substances of acid rain, contribution of organic acid to the wet precipitation acidity should not be ignored. Organic acids, one important composition in atmosphere, featured by high polarity,hygroscopicity and low vapour pressure, could effect the size distribution of TSP, liveness of cloud condensation nuclei and the cloud process, and local climates and ecosystems were influenced finally. Mount Lu, located in the heavy acid rain area of southern China, stands in the boundary of free troposphere, hence, research of organic acids’s concentration, composition and source at Mount Lu and discussing relationship of organic acids among different various environments were of significance to atmospheric chemistry and acid rain.Cloud water, rainwater and TSP samples were collected at Mount Lu from August to September of 2011 and March to May of 2012. Ionic chromatography system (ICS-2500,Dionex, USA) was adopted to analyze formic acid, acetic acid, oxalic acid, methane sulfonic acid, propionic acid,lactic acid and butyrate in cloud water and rainwater and corresponding acid ions in TSP.The volume-weighted mean (VWM) concentration of organic acids in cloud water was 38.42μeq/L, ranging from 7.45 to 111.46 μeq/L. In rainwater samples, organic acid concentrations varied from 12.39 to 68.97 μeq/L (VWM of 33.39 ueq/L). Formic acid, acetic acid, and oxalic acid were the most common organic acids in both cloud water and rainwater. The VWM concentration of organic acids in TSP was 1523.92 ng/m3, and the range of concentration of organic acids was 40.48 ng/m3 ～5695.83 ng/m3.Acetic acid and oxalic acid were the preponderant species in TSP, occupying 70.58% and 17.67% of total organic acid respectively. The level of concentration of organic acids in atmospheric deposition at Mount Lu was similar to that of forest ecosystem and other mountain sites but higher than urban and marine site.Different seasonal difference of organic acids appeared in cloud water, rainwater and TSP at Mount Lu. The VWM concentration of organic acids in cloud water in summer was 48.81μeq/L, higher than that in spring (34.38μeq/L). The adverse comparative result occurred in TSP,1698.69 ng/m3 in spring and 929.68ng/m3 in summer. However,there was much less discrimination in rainwater between the two seasons,34.77μeq/L in spring and 33.04μeq/L in summer. High concentration of TSP and high ratio of mass between organic acids and TSP lead to higher concentration of organic acids in spring. Different seasonal difference of organic acids in cloud water and rainwater were owed to different source of organic acids and the combined action of cloud water and TSP on rain generated indistinctive calendar variation of organic acids in rainwater.Cloud water and rainwater Mount Lu were greatly acidated and contribution of organic acids to acidity was noticeable. Weighted mean of pH in cloud water was 3.80 and the value in rainwater was 4.44. The contribution of organic acids to free acidity in cloud water varied from 0.35%-23.55%, with a weighted mean at 2.50% and 2.20% in summer,2.83% in spring. Higher contribution of organic acids to free acidity occurred in rainwater, whose range was 2.08%--52.55%. The weighted mean value was 17.66% with the value at 12.24% in summer and 19.89% in spring. The contribution of organic acids to acidity was lower during summer than during spring in both cloud water (2.20% in summer vs 2.83% in spring) and rainwater (12.24% in summer vs 19.89% in spring).The formic-to-acetic acid ratio (F/A) was applied to decide the main source (primary emissions and secondary sources) in cloud water and rainwater. The result showed that organic acids were dominated by primary emissions in 71.31% of the cloud water samples and whole rainwater samples.Positive matrix factorization (PMF) was adopted to analyze the sources of organic acids in atmospheric wet deposition. And four factors were decided in cloud water, including biogenic emissions (61.8%), anthropogenic emissions (15.28%), marine emissions (15.07%) and soil emissions (7.85%).Five factors were analyzed in rainwater with biogenic emissions (62.66%) as the dominant source followed by biomass burning(21.82%) and the rest sources were anthropogenic emissions, marine emissions and soil emissions accounted for 2.59%、7.29%、5.64%, respectively.The findings from this study imply a high concentration level of organic acids at Mount Lu influenced less by human activity. High contribution of organic acids to rainwater acidity increase ecological risks in the local environment.