Cloud/fog samples were collected during spring of 2007 in the highly polluted North China Plain in order to examine the impact of pollution and dust particles on cloud water chemistry. The volume weighted mean pH of cloud water was 3.68. The cloud acidity was shown to be associated with air mass origins. Cloud water with its air mass trajectories originating from the southern part of China was more acidic than those from northern China. Anthropogenic source and dust had obvious impact on cloud water composition as indicated by the very high mean concentrations of SO4 2- (1331.65μeqL-1), NO3 - (772.44μeqL-1), NH4 + (1375.92μeqL-1) and Ca2+ (625.81μeqL-1) in the observation periods. During sandstorm days, cloud pH values were relatively high, and the concentrations of all the ions in cloud water reached unusual high levels. Significant decreases in the mass concentrations of PM2.5 and PM10 were observed during cloud events. The average scavenging ratio for PM2.5 and PM10 was 52.0% and 55.7%, respectively. Among the soluble ions in fine particles, NO3 -, K+ and NH4 + tend to be more easily scavenged than Ca2+ and Na+. ; Department of Civil and Environmental Engineering