Chemical composition of clouds at Mt. Mitchell, North Carolina, USA

Abstract

Using a recently developed cloud and rain acidity/conductivity (CRAC) real time analyzer, clouds atop Mt. Mitchell State Park, North Carolina — the highest peak (˜ 2038 m MSL) in the eastern US — were investigated from May to October during 1987 to 1989. Measurements for chemical and physical climatology were performed to estimate exposure of pollutant at the forest canopy resulting from direct capture of cloud droplets. The mountain peak is exposed on the average to cloud episodes about 70% days per year. Results are reported for integrating times of cloud collection, approximately 5 min, during several precipitating and non-precipitating cloud events. Cloud water was found to have higher acidity and higher concentrations of sulfate, nitrate and ammonium than previously observed in rain droplets. The pH of the cloud water was in the range of 2.4 to 4.9 during 1987 through 1989 field seasons, while the pH of the rain water was in the range of 3.5 to 5.5. The cloud water collected in summer season was more acidic than those collected in any other seasons (spring and fall). The maximum of average cloud pH value (i.e., low acidity) was found when the predominant wind direction was from south-east. On the other hand, the low average cloud pH values (i.e., high acidity) were found when the predominant wind direction was west through northeast. Total concentrations of major chemical species in cloud water at Mt. Mitchell State Park are significantly higher (˜ 1250 to ˜ 3020 μeq/1) than those reported for other rural locations. According to the time-sequential chemical concentration, a concave trend with total concentration decreasing at the beginning and rising toward the end of event was observed. It was also observed that changes in the individual ionic concentration are proportional to the changes in total concentration. It suggests that condensation and evaporation of cloud droplets are the important processes, during various stages of a cloud event, in determining the total pollutant concentration of cloud water.