Fog composition along the Yangtze River basin: Detecting emission sources of pollutants in fog water.

To investigate the fog chemistry along the Yangtze River basin, a field observation experiment was performed from Shanghai to Wuhan during November 2015. Fifteen fog water samples were collected by using a three-stage Caltech Active Strand Cloud water Collector (CASCC). The three-stage CASCC was mounted on the board of a ship. PH, electrical conductivity (EC), HO, HCHO, S(IV), ten inorganic ions, seven organicacids and sixteen trace metal elements were measured in this study. The pH of fog water samples ranged from weakly acidic (pH4.3) to weakly alkaline (pH7.05) and the EC ranged from 32.4 to 436.3μS/cm. The main cations in fog water were NH and Ca, accounting for 12.35% and 29.07% of those inorganic ions, respectively. In addition, SO and NO contributed to 25.52% and 12.93% to total anion concentrations respectively. Moreover, the dominant kinds of organicacids were formate and oxalate, occupying 45.28% and 28.03% of the total organicacids, respectively. For trace metal elements in fog samples, Al, Fe, Zn, and Ba revealed 34.6%, 16.4%, 19.3%, and 20.9% contributions to these sixteen trace element concentrations, respectively. The results indicated that pollutants were mainly from human activities, including fossil fuel combustion, biomass burning, steel-making, stone quarrying and sand digging. Besides, natural sources including natural background levels and long-range transport of sea salt particles also aggravated the pollution levels in the fog events along the Yangtze River.