Chemical characteristics and possible causes of acid rain at a regional atmospheric background site in eastern China

Abstract

A total of 43 rainwater samples were collected from March 2014 to February 2015 at a regional atmospheric background station, in Lin’an, Zhejiang province, eastern China. We analyzed all rainwater samples for pH, electrical conductivity, and concentration of SO₄ ²⁻, NO₃ ⁻, Cl⁻, F, NH₄ ⁺, Ca²⁺, Na⁺, Mg²⁺, and K⁺. The pH of the 43 analyzed rainfall events ranged from 3.39–6.37, with a volume-weighted mean (VWM) of 4.74; 72% of the precipitation events qualified as acid rainfall (those where the rain pH was <5.6), indicating that acid rain pollution was severe in Lin’an area. Although the ionic concentrations in the Lin’an rainwater were lower than those found in most urban areas in China and in rural areas in India, they were still higher than those in Waliguan (a global atmospheric background station in northwestern China) and in Newark, NJ, USA. We found that SO₄ ²⁻ and NO₃ ⁻ were the major anions in the precipitation, accounting for an average of 88% of the total inorganic anions. The SO₄ ²⁻ concentration was between 3.8–303.9 μeq L⁻¹ with a VWM value of 49.8 μeq L⁻¹, while the NO₃ ⁻ concentration was between 3.0–206.5 μeq L⁻¹ with a VWM value of 32.6 μeq L⁻¹. The ratios of SO₄ ²⁻ to NO₃ ⁻ ranged from 0.71 to 2.56 with an average of 1.53, indicating that SO₄ ²⁻ was the most significant anion causing rainfall acidification in Lin’an area. This is similar to observations made in 2008. Compared to observations in Lin’an area in 2008, the contribution of SO₄ ²⁻ to rainwater acidity had decreased due to sharp reduction in SO₂ emissions, while the contribution of NO₃ ⁻ had increased due to a rapid growth in the use of motor vehicles in eastern China. We observed that the alkaline component NH₄ ⁺ made the largest contribution toward neutralizing rainwater acidity. The neutralization factor (NF) for NH₄ ⁺ was between 0.26 and 1.03 with an average of 0.63, and the NF for Ca²⁺ was between 0.09 and 1.21 with an average of 0.30. The ratio of Ca²⁺ to NH₄ ⁺ was between 0.15 and 4.70 with an average of 0.48, lower than that observed in Chinese urban areas and close to that observed in non-urban Chinese sites. These results indicated that Ca²⁺ was still a major factor in buffering rainwater acidity in most urban areas in China, while NH₄ ⁺ seemed to be the most important basic ion for neutralizing rainwater acidity in non-urban areas. In the Lin’an area, the Cl⁻ and K⁺ in rainwater were mostly of marine origin; however, SO₄ ²⁻, Ca²⁺ and Mg²⁺ were generated mainly from the earth’s crust and anthropogenic emissions.