Chemical composition of precipitation in Shenzhen, a coastal mega-city in South China: Influence of urbanization and anthropogenic activities on acidity and ionic composition.

Rainwater samples from Shenzhen in south China were collected over the period of a year, and the chemical compositions were measured with the main purpose of understanding the acidification of rainwater and the controlling factors. The pH value of precipitation ranged from 3.72 to 6.77, with a volume-weighted mean (VWM) value of 4.29, and the acid rain frequency was 97%. The VWM concentrations of anions and cations followed the order of SO > Cl > NO and Na > Ca > NH > Mg > K, respectively. Air mass back-trajectory and positive matrix factorization analyses indicated that sources of ions in rainwater were mainly from sea salt, soil dust and anthropogenic activities. Compared with other areas in China, the rainwater of Shenzhen has the lowest values of the NP/AP, ∆pH and NF values of Ca and NH, indicating that the lack of the capacity for neutralization could be the main reason for the severe acid rain problem. It is noteworthy that the rain acidification tendency is obvious, and the pH value has reduced by 1.0 units since the 1980s. Based on a comparison of the chemical compositions of the rainwater from different historical periods, the NO concentration was found to have increased consistently, whereas the NH concentration maintained a decreasing trend since 1980. On the other hand, the nss-SO and nss-Ca concentrations increased after 1980 and then decreased after 1994. Meanwhile, the decreasing pH was accompanied by a decreasing NP/AP ratio. These results suggest that the changes in human activities at different stages of urban development can lead to a synergistic change in the chemical characteristics of precipitation. Both an increase in the acidic species emissions (especially NO) due to rapid economic development and a decrease in the alkaline ions concentration due to urbanization have resulted in the rain acidification tendency in Shenzhen.