Microphysical Effects on Cloud Water Acidity: A Case Study in a Nonprecipitating Cloud Event Observed at mt. Mitchell, North Carolina

Abstract

There is increasing recognition that deposition of cloudwa- ter may significantly contribute to the decline of forest, particularly at elevated sites where immersion in clouds occurs frequently.1-5 Concentrations of major ions in nonprecipitating clouds and fogs have been reported to be significantly higher than those commonly observed in acidic precipitation.6-9 Lower dilutions and higher scavenging efficiencies due to reduced mass transfer limitations of gas phase absorption and longer residence times may explain, in part, the higher concentrations found in fog and cloud (droplet characteristic diameter: 1-100 mm) than in rain (droplet characteristic diameter: 0.1-3.0 mm).10 Fog and cloud water deposited on surfaces exposed to the atmosphere by wind-driven interception or settlement of the largest drops can enhance the damaging effects of substances previously deposited by dry deposition.3 This is especially true at the high elevation locations, while acidic exposure due to clouds may be about five fold due to rain over the same time period.5 Temporal variation of cloud water acidity during a cloud event is affected by cloud microphysical processes such as condensational growth and evaporative dissipation of cloud droplets.1-5 In warm clouds, wet deposition operates via the removal of aerosol particles by condensation (nucleation scavenging). This is done by transforming some of the aerosol particles into cloud droplets as a result of heterogeneous nucleation from the vapor, and via the removal of aerosol particles by impaction (impaction scavenging). Another method is by attaching aerosol particles to cloud and rain drops through the mechanism of Brownian diffusion, inertia and hydrodynamic forces, and via the transport of the scavenged materials to the ground.11. In this paper, chemical and microphysical data obtained at Mt. Mitchell, North Carolina during one orographic cloud event (nonprecipitating) are examined to investigate the relationship between temporal variation of cloud acidity and cloud microphysics. © 1992 Air & Waste Management Association.