| Water soluble organic compounds (WSOC) are one kind of the major compounds in atmospheric pollutants. They exist in gas, liquid and particle phases. WSOC in particles can affect the atmospheric visibility, the climate and human health. WSOC in precipitation may also contribute to rain acidity. Research of WSOC in atmosphere is drawing more and more attention.In the past 20 years, with rapid urbanization and industrialization, more and more motor vehicles and industrial activities emitted a lot of pollutants to atmosphere, resulting in the visibility reduction of Shenzhen, and acid rain occurs frequently in Shenzhen now. The quality of atmospheric environment appears a trend of deterioration. In nowadays some research of precipitation has already been taken. But they usually paid attention to the inorganic chemical compositions of rain water. The results showed that the main inorganic anions of precipitation in Shenzhen are SO42-, NO3- and Cl- and the main organic anions are formic, acidic and oxalic. So far the analysis of WSOC in precipitation of Shenzhen was not integrated, such as the knowledge of the contribution to rain acidity of WSOC in rain water is still lacked. The purpose of this study is to better understand the components of WSOC in Shenzhen precipitation. We mainly concern the chemical composition and concentrations of WSOC in rain water, analyze the possible reasons of the concentration variation, calculate the contribution of WSOC to rain acidity in precipitation, and discuss the possible sources of WSOC component. As there are still limited research about the WSOC components in rain water in China, our study can add experimental datas to this field studys. Additionally, this study may also provide analysis methods for other similar study. We collected a series of rain water samples in 100 precipitations from the April 2004–the December 2006 in Shenzhen. The collected samples were analyzed with Ion Chromatography and Total Organic Carbon Analyzer. The results showed mono- and di-carboxylic acids are the main chemical compositions of WSOC in rain water of Shenzhen. The most WSOC components in rain were formic, acetic and oxalic acid, which can contribute over 70% of the total organic acids and 4% of the total anions.The analysis of the organic acids concentration variation in different time shows that the variation of organic acids may correlate with temperature, precipitation and the rate of rainfall.In our study, the average pH of the rain water in Shenzhen is 4.30. The frequency of acid rain is 94%. This result indicates that the acid rain is one of the most important environmental problems in Shenzhen. The calculation results show that the organic acids in rain water make little contribution to the precipitation acidity in Shenzhen, which is mainly due to the inorganic acid. However, in some rain events, the contribution of organic acid to the TFA and pH of rain water is significant, so it can not be ignored. Organic acids may act as potential substances which can make siginificant conctibution to the rain acidity sometimes in Shenzhen. The average concentration of WSOC in rain water is 643.8μmol/L, the reason of the WSOC variations in precipitation may relate to rainfall and temperature. The contribution of organic acids to the WSOC concentration is 0.5%, which is much smaller than other research. There is a good correlation between formic and acetic in rain water of Shenzhen which indiacated that these two species may have similar sources or similar emission intensity.
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