Characterization of cloud water chemistry at Mount Tai, China: Seasonal variation, anthropogenic impact, and cloud processing

To investigate cloud chemistry in the fast developing region of north China, five intensive observation campaigns were performed at the summit of Mount Tai (Mt. Tai) during 2007–2008. A total of 482 cloud water samples were collected using single-stage and two-stage cloud collectors. Inorganic ions, organic acids and species associated with S(IV) oxidation in cloud water were analyzed. The results showed that 80% of the cloud samples were acidic (pH < 5.6), and 55% of the samples were strongly acidified (pH < 4.5). The clouds at Mt. Tai contained much higher anthropogenic and crustal ion concentrations than observed at many other rural sites; the most abundant ions in the cloud water were NH4+ (volume weighted mean concentration of 1215 μeq L−1), SO42− (1064 μeq L−1), NO3− (407 μeq L−1) and Ca2+ (193 μeq L−1), suggesting large influences of anthropogenic emissions on cloud water. Seasonal variations of cloud composition were observed, showing high fractions of NH4+ and SO42− in summer and more soil/crustal ions in spring and winter. Backward trajectory analysis showed that the cloud water pH in air masses arriving from the south was typically much lower than when air was transported from the north. Higher nitrate fraction and low pH were found in air masses from the industrialized coastal regions of China, indicating an increased contribution of HNO3 to cloud acidification due to higher NOx emissions. The drop size dependence of cloud chemical composition was examined. Smaller droplets were more acidified than larger ones. The drop size dependence tendency of ion concentrations varied with cloud evolution. SO42−, NH4+ and NO3− were more enriched in larger droplets in the initial stages of the clouds, and tended to be higher in smaller drops with cloud development. In-cloud aqueous sulfate formation was estimated using a selenium tracer method. In-cloud aqueous production on average accounted for 27% of the measured cloud water sulfate, with a range from 5% to 62% for individual cloud events.