Factors governing cloud water composition in the Appalachian mountains

Abstract

Cloud events collected during the summers of 1986—1988 at 5 sites in the Appalachian Mountains of the eastern USA were classified on the basis of meteorology. The data consisted of cloud water chemical concentrations, liquid water content (LWC), mixed-layer air trajectories, synoptic type from weather maps, and whether or not the cloud was precipitating. An analysis of variance (anova) supported the hypothesis that the meteorological variables describe the cloud water chemical composition (P < 0.001); there were differences between precipitating and non-precipitating events (> few hours duration) and among synoptic-trajectory classes. The highest H⁺, SO₄^2-, NH₄⁺ and NO₃^- concentrations occur when a site is in the warm sector of the cyclone or in a cap cloud; lower concentrations occur during easterly (marine) flow or post-cold-frontal cloud events. The difference among classes was similar when cloud LWC was accounted for; this supports the hypothesis that the synoptic-trajectory classes describe differences in the transport and mixing of aerosol and gases rather than LWC differences. The chemical concentrations for non-monitored cloudy hours were predicted from the meteorological classes and the resulting mean concentrations for the entire growing season were compared to the sampled cloud events. Insufficient data exists for examining annual variability but, when all 3 years of data were used together, differences between sampling sites could be observed. Two southern sites received cloud water with higher solute concentrations than two northern sites. Differences in solute concentrations between 2 sites at different elevations on the same mountain were observed, with higher solute concentrations observed near cloud base.