Chemical composition of rainwater and the acid neutralizing effect at Beijing and Chizhou city,China

【摘要】：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 of4.85.The predominant ions in the precipitation were S_4~(2-),Ca~(2+)and NH_4~+at both sites.The VWM concentrations of ions in rainwater were higher in Beijing,and the S02~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 NK_4~+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 SO_4~(2-)and Ca~(2+)in rainwater samples are non-sea-salt origin at both sites.Coal combustion may be the main source of SO_4~(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 NH_4~+)have played an important role to neutralize the acidity of rainwater in Beijing.