Chemical Dynamics of Clouds at Mt. Mitchell, North Carolina

Abstract

The role of clouds as the primary pathway for deposition of air pollutants into ecosystems has recently acquired much attention. Moreover, the acidity of clouds is highly variable over short periods of time. Cloud water collections were made at Mt. Mitchell State Park, North Carolina, using a real-time cloud and rain acidity/ conductivity (CRAC) analyzer during May to September 1987, 1988 and 1989 in an effort to explore extremes of chemical exposure. On the average, the mountain peak was exposed to cloud episodes about 70 percent of experimental days. The lowest pH of cloud water in nearly real-time (∼10 min.) samples was 2.4, while that in hourly integrated samples was 2.6. The cloud pH during short cloud events (mean pH 3.1), whjch results from the orographic lifting mechanism, was lower than that during long cloud events (mean pH 3.5), which are associated with mesoscale or synoptic atmospheric disturbances. On the average, the pH values in nonprecipitating cloud events were about 0.4 pH unit lower than those in precipitating cloud events. Sulfate, nitrate, ammonium and hydrogen ions were found to be the major constituents of cloud water, and these accounted for -90 percent of the ionic concentration. Total ionic concentrations were found to be much higher in non-precipitating clouds (670-3,010 μeq/L) than those in precipitating clouds (220-370 μeq/L). At low acidity, ionic balance is sometimes not obtained. It is suggested that organic acids may provide this balance.

The profile of cloud water ionic concentration versus time was frequently observed to show decrease at the beginning and rising toward the end during short cloud events. Before the dissipation of clouds, a decrease in cloud water pH and an increase in ionic concentration were found. At the same time, temperature and solar radiation increased, and relative humidity and microphysical parameters (liquid water content, average droplet size, and droplet concentration) decreased. These observations suggest that evaporative dissipation of cloud droplets leads to acidification of cloud water. Mean pH of cloud water was 3.4 when the prevailing wind was from the northwest direction, and it was 3.9 when the wind was from the west direction. The effects of variations in cloud liquid water content have been separated from variations in pre-cloud pollutant concentrations to determine the relationship between source intensity and cloud water concentrations.