环境地球化学国家重点实验室知识库

The chemical compositions were measured in rainwater samples collected during 2011–2012 from two representative cities, Beijing in north China and Chizhou city in south China. The rainwater was highly acidic with a volume-weighted mean (VWM) pH of 4.56, ranging from 3.77 to 5.67, and about 94% of the samples had pH below 5.0 in Chizhou. The pH values of rainwater in Beijing ranged from 3.78 to 6.62, with a VWM value of 4.85. The predominant ions in the precipitation were SO₄^2 −, Ca^2 + and NH₄⁺ at both sites. The VWM concentrations of ions in rainwater were higher in Beijing, and the SO₄^2 − concentration of rainwater in Beijing was about twice that in Chizhou. However, due to the weaker neutralization of acidity, the rainwater from Chizhou had relatively low pH values. According to the results of linear regression analysis, the percentage of the potential acidity counteracted by Ca^2 + and NH₄⁺ was higher in rainwater in Beijing (90.7%) than that in Chizhou (70.8%). Using Na as an indicator of marine origin, and Al for the terrestrial inputs, the proportions of sea salt and terrestrial elements were estimated from elemental ratios. More than 98% of SO₄^2 − and Ca^2 + in rainwater samples are non-sea-salt origin at both sites. Coal combustion may be the main source of SO₄^2 −, and local and remote soil dust may be an important source of Ca^2 + in Beijing rainwater. The high concentrations of alkaline ions (Ca^2 + and NH₄⁺) have played an important role to neutralize the acidity of rainwater in Beijing.