Analysis of Ionic Composition and Sources in Cloudwater at Mount Lu

Since acid rain was first observed, it has been studied worldwide. Because clouds are the precursor of rain, the acidification of clouds is thought to be the significant reason causing acid rain. To investigate the cloud pollution process and mechanism, atmospheric chemistry about clouds and precipitation was conducted at Mount Lu in order to get an overview of the characteristics of ion composition and the transformation of acidic pollutants. Research on cloud chemistry is highly significant for the investigation of acid rain formation in China and acidic pollutant transformation in the entire East Asian region.The monitoring campaign was divided into2periods:spring and summer. A total of141cloud samples were collected during Aug.2011-Sep.2011and Mar.2012-May.2012. All samples were analyzed for18indicators, including electric conductivity (EC), pH. F-, Cl-,NO3-,SO42-,NH4+, K+, Na+,Ca2+, Mg2+and organic acids. This paper mainly discussed the cloud chemical composition, and the influence of air mass direction and motion path was analyzed by the HYSPLT.The cloud water was acidic with the volume weighted mean (VWM) pH3.69in summer and3.90in spring. Because of the impact of spring dust storms and rainfall, the acidity of cloud water and frequency of strongly acidified clouds in spring were lower than in summer. The cloud water pH of Mount Lu ranged from2.75to5.84and the volume weighted mean (VWM) pH was3.79. The cloud water pH in spring was reduced1.49in the past25years. Compared to domestic sites, the acidity of cloud water of Mount Lu was higher than Mount Tai located in northern China, and was very similar to Mount Heng located in southern China, Compared to other sites in Asia, cloud water of Mount Lu was relatively acidic, while sites in the United States, Canada and Italy were relatively acidic compared to Mount Lu.The predominant ions in cloud water collected were SO42-(359.53μeqL-1), NO3-(288.73μeqL-1), NH4+(193.00μeqL-1) and Ca2+(63.08μeqL-1), accounting for about76%of the total ion concentration. Cloud water in summer had higher ion concentrations than in spring, but the range of ion concentrations in spring was wider, and the maximum was more than70times of minimum. The proportion of SO42-and NO3-in spring clouds were significantly higher than in summer, and other ions such as NO3-, Ca2+, Na+, Cl-, F-, Mg2+were slightly lower than in summer. SO42-was the domination acidification factor and contributed63%of the acidity of cloud water. About99.24%of sulfates in cloud water were nss-sulfate indicating a regional source. The sulfate to nitrate ratio of cloud water was1.86, lower than25years ago. NH4+was the domination neutralization factor and neutralized55%of the acidity of cloud water, while Ca2+neutralized11%of the acidity of cloud water. There were high correlations between the main acidification factor and neutralization factor. The reason why most of cloud events in this study were strongly acidic was that there were a large number of acids not being neutralized, and the ratios of non-neutralized acid in clouds was32%. Formic acid, acetic acid and oxalic acid, the most abundant organic acids in cloud water, contributed3%of the acidity of cloud water. Formic acid and acetic acid were mainly from plants emissions. As oxalic acid had a high correlation with SO42-, it may be mainly from anthropogenic emissions. Mg2+had a complex source and was significantly influenced by marine sources. Compared with the other sites worldwide, the concentration of SO42-on Mount Lu was relatively low. Sites in Japan had the highest Na+and Cl" Ca2+in clouds, more than15times of Mount Lu and American sites. In addition to Mount Lu, the NH4+/Ca2+in cloud water at sites were greater than1, and the NH4+/Ca2+of Mount Lu showed an increasing trend from1987to2012, which were0.23,1.14and4.53respectively in1987,1993,2011-2012.Ion concentrations had obvious differences at the early, middle and late stages of cloud events. Analysis showed that SO42-and NH4+tended to be more easily scavenged by small particles, while NO3-tended to be more easily scavenged by large particles. The insignificant change of Ca2+concentration in clouds indicated that the influence of dust on clouds was relatively stable. Factor analysis found that ion concentrations of cloud water of Mount Lu were mainly influenced by anthropogenic sources and soil dust. The marine source was relatively clean and spring clouds could parse out the influence of the marine source as shown by factor analysis.Cloud air masses were classified as four sorts based on back-trajectory. Cloud air masses from north and northwest of the continent, went through the north and transferred to Mount Lu bringing clouds with characteristics of highest ions concentration and highest proportion of Ca2+in clouds. Cloud air masses from nNortheast Asia, went through the Korean peninsula, South Korea or coast, north and east of China and transferred to Mount Lu bringing clouds with characteristics of higher ions concentration and highest concentrations of Na+and Cl-. Cloud air masses from the east coast, influenced by pollutants from Japan, went through cities such as Shanghai and Hangzhou and transferred to Mount Lu bringing clouds with characteristics of the highest acidity. Cloud air masses from the south, southern coast of China and southeast Asia, went through the south and transferred to Mount Lu bringing clouds with characteristics of lower ions concentration and the lowest acidity; Cloud air masses from southeast Asia, moved in a northeast direction and transferred to Mount Lu bringing clouds with characteristics of higher acidity. Local air masses were relatively clean and brought clouds with characteristics of the lowest ion concentration.All in all. Clouds of Mount Lu were severely affected by human activities for the last25years. Cloud acidity of Mount Lu substantially increased, as did ion concentrations, which had put on speed of rain acidification.