Clouds play an important role in the transport and transformation of atmospheric pollutants from the gas and particle phases. Although many laboratory and some observational studies addressed acid rain formation in the 1980s and early 1990s, only a few studies have monitored cloud water composition in recent years. Our current knowledge of cloud chemistry and composition beyond acidity and major ions is rather poor. Little observational data exist on trace metals and organic species in clouds in the U.S., and observations of cloud composition in the Southwest are particularly rare.

The chemical composition of clouds in northern Arizona was investigated during the North American Monsoon seasons in 2005 and 2006. Intercepted clouds were sampled on the top of Mt. Elden (9299 ft) near Flagstaff (AZ) with an automated Caltech Active Strand Cloudwater Collector (CASCC). Cloud water collection was initiated by an optical fog detector providing information on cloud frequency and liquid water content during the monsoon season at this location. Collected cloud samples were analyzed for acidity, major ions, trace metals, and organic carbon concentrations. Preliminary findings will be presented and discussed in relationship to prior studies on the East Coast and California.