Chemical characteristics of acidic precipitation in Tiantong, Zhejiang Province

To explore the chemical characteristics and the causes of acid rain, 90 rain samples were collected from Mar. 2010 to Feb. 2011 in Tiantong region, Zhejiang Province, and measured by ion chromatography. The results showed that mean pH in precipitation was 4.37, and the acid rain frequency was 97% over the year. The ion concentration followed by the order of SO ₄ ^2->NH ₄ ⁺> NO ₃ ^->Cl ^->Ca ²⁺> Na ⁺>Mg ²⁺>K ⁺>F ^-. There was a pronounced seasonal pattern for pH and ion concentration in precipitation, much more serious pollution in winter and spring than in summer and autumn. Equivalence ratio of SO ₄ ^2- to NO ₃ ^- was 1.9, indicating that acid rain was dominated by sulfuric-nitric mixed type. Anthropogenic sources contributed most of SO ₄ ^2-, NH ₄ ⁺, NO ₃ ^- and part of Ca ²⁺, sea salt was responsible for the presence of Na ⁺, Cl ^- and the most amount of Mg ²⁺, and K ⁺ and the most fraction of Ca ²⁺ came from crust. Sea salts posed significant effect on ion concentrations, but little effect on acidity in precipitation. The correlation coefficients of NH ₄ ⁺ to SO ₄ ^2- (r=0.90) and to NO ₃ ^- (r=0.88) were significantly higher than that of Ca ²⁺ to SO ₄ ^2- (r=0.67) and to NO ₃ ^- (r=0.73), suggesting that NH ₄ ⁺ can neutralize more significantly the rain acidity than Ca ²⁺. This could be further confirmed by 1.47 of equivalence ratio of NH ₄ ⁺ to Ca ²⁺. Different from other cities, alkaline ions, especially Ca ²⁺, was lower in the precipitation of Tiantong region, resulting in relatively high rain acidity.