Organic acids contribute to rainwater acidity at a rural site in eastern China

We collected 43 valid rainwater samples at Lin'an, a rural site in eastern China, between March 2014 and February 2015. We measured the concentrations of seven low molecular weight water-soluble organic acids using ion chromatography with the elution gradient method. We detected formic, acetic, oxalic, succinic, glutaric, malonic, and methanesulfonic acid in at least 70% of all samples, reflecting the ubiquity of these acids in the precipitation of Lin'an. The total concentration of these organic acids in individual samples was between 2.63 and 114.77 mu eq L-1 (mean 16.64 mu eq L-1). Formic, acetic, and oxalic acid were the most abundant organic acids in the rainwater samples, with volume-weighted mean concentrations of 9.58, 3.89, and 2.01 mu eq L-1, respectively; these three acids accounted for 93% of the total mean organic acid concentration. The average contribution of organic acids to precipitation total free acidity was 13.71% in Lin'an, which was lower than has been recorded in other rural and mountainous areas of southwestern China. but much higher than has been recorded in some urban and semi-urban areas. The mean ratio of formic to acetic acid in rainwater was 2.40, and the mean ratio of malonic and succinic acid in rainwater was 0.62. These ratios indicated that, in Lin'an, formic and acetic acid mainly originated from primary biogenic sources, but malonic and glutaric acid mainly originated from motor vehicle emissions. With the exception of oxalic acid, the volume-weighted mean concentrations of other organic acids and total organic acids in the growing season (summer and spring) were higher than those in non-growing season (autumn and winter), suggesting that vegetation around the Lin'an sampling site was an important source of atmospheric organic acids. As the volume-weighted mean concentration of oxalic acid was higher during the non-growing season, this may indicate that aerosolized oxalic acid was less well diluted due to less rainfall during the drier non-growing season.