Cloud chemistry in the eastern United States, as sampled from three high-elevation sites along the Appalachian Mountains

Atmospheric deposition of acidic cloud water is thought to be one of the ca uses for the recent forest decline in industrialized areas of the world. Th e present paper presents results from the Mountain Acid Deposition Program (MADPro), a part of EPA's Clean Air Status and Trends Network, (CASTnet). W e used automated cloud water collectors at three selected mountain sires (W hiteface Mt., NY; Whitetop Mt., VA; and Clingman's Dome, TN) to take hourly samples from non-precipitating clouds during temperate (non- freezing) sea sons of each year from 1994 to 1997. Samples were promptly analyzed for pH, conductivity, and concentration of dissolved ions. Cloud liquid water cont ent (LWC) and meteorological parameters were measured at each site. Mean-lo ud frequencies and LWC of clouds were higher at Whiteface Mt., NY, than in the Southern Appalachians. The four most prevalent ions found in cloud wate r samples were usually, in order of decreasing concentration: sulfate (SO42 -) hydrogen (H+), ammonium (NH4+), and nitrate (NO3-). Within cloud events the concentration of these major ions tended to co-vary. Typically there wa s an inverse relationship between LWC of the cloud and ionic concentration of the cloud water. During the sampling season, the highest ionic concentra tions were seen during mid-summer. Ionic concentrations of samples from the southern sites were significantly higher than samples from Whiteface Mt., but further analysis indicates that this is at least partially due to the n orth-south difference in the LWC of clouds. MADPro results are shown to be comparable with previous studies of cloud chemistry in North America. (C) 1 999 Elsevier Science Ltd, All rights reserved.