Chemical characteristics of rainwater in Sichuan basin, a case study of Ya'an.

Rainwater chemistry was investigated at a semi-rural site in Ya'an, Sichuan basin with rain samples collected from May 2013 to July 2014. The rainwater pH values ranged from 3.25 to 6.86, with an annual volume-weighted mean (VWM) of 4.38, and the acid rain frequency was 74 %. Such severe acidification, 15 % of the total events showed a pH below 4.0, attributed to the deficiency of Ca(2+), significant anthropogenic pollution contribution, and rainy pattern to this area. The annual VWM of total ions concentration was 477.19 μeq/L. NH4 (+) was the most abundant ionic species, followed by SO4 (2-), NO3 (-), Ca(2+), Cl(-), Na(+), K(+), Mg(2+), and F(-) in a descending order. The total ionic concentrations presented a seasonal trend of lower values in autumn and summer but higher ones in winter and spring. Based on enrichment factor, correlation analysis and principle component analysis, three factors were identified: factor 1 (NH4 (+), SO4 (2-), NO3 (-), K(+), and Cl(-), 47.45 % of the total variance) related to anthropogenic sources (coal/fuel combustion, biomass burning and agriculture), factor 2 (Ca(2+), Mg(2+), Na(+), and Cl(-), 34.01 % of the total variance) associated with natural sources, and factor 3 (H(+), 11.78 % of the total variance) related to free acidity. Back trajectory analysis indicates that the rainwater chemistry in Ya'an was mainly affected by regional air masses from Sichuan basin. Long-range transported air masses from southwest with heavy anthropogenic pollution increased the total ion concentration and acidity of rainwater. Considering its special topography, anthropogenic emissions from regional and long-range transport (especially from southwest) must be controlled effectively to improve the acid rain condition of non-urban areas in Sichuan basin.