Cloud and the corresponding precipitation chemistry in south China: Water-soluble components and pollution transport

To investigate the transport and deposition of pollutants in south China, the chemical composition of cloud water and rainwater was studied at Mount Heng, which is located in the heart of China's acid rain affected region. A total of 194 cloud samples and 38 event-based precipitation samples from March to May 2009 were analyzed for a wide variety of water-soluble compounds. The volume-weighted mean (VWM) pH of cloud water was 3.80, indicating an acidic nature. Compared with cloud water, precipitation was much less acidic with a VWM pH value of 4.35. The concentration results show that in cloud water, SO ₄^2- was the dominant anion, followed by NO ₃^-, controlling the acidity, while NH₄ ⁺ accounted for 69.8% of the total cations, dominating the cloud water neutralization. In rainwater, a similar situation appeared, but with a slightly higher Ca²⁺concentration. In both cloud water and precipitation the acidities were found to be mainly due to sulfuric acid, followed by nitric acid. The calculation of neutralization factors and the application of multiple linear regression analysis showed the higher NH ₄⁺ contribution to the neutralization process in both types of samples. The very weak correlations of ions in rain with the ones in cloud water on the event basis suggested that cloud and precipitation events may be associated with different air masses. With relatively high ionic concentrations and higher frequency, air masses from the east and south directions were associated with the high cloud deposition of the major species at our site.