Measurements of the chemical composition of stratiform clouds

Abstract

Measurements are reported of the chemical composition of the liquid water and interstitial air in warm (> 0°C), non-precipitating stratus and strato-cumulus clouds at various locations in the eastern United States. Inorganic ionic composition of the cloud water was generally dominated by H +, NH 4 +, NO 3 − and SO 4 2−, similar to the composition of precipitation in this region of the U.S. Concentrations of the corresponding interstitial aerosol species and gaseous HNO 3 were invariably low in comparison to concentrations of the respective ionic species in cloudwater. In contrast, the concentration of NO x (i.e. NO + NO 2 + organic nitrates) was invariably comparable to or in excess of that of cloudwater nitrate. Sulfur dioxide was found at varying concentrations relative to cloudwater sulfate. In many cases, the SO 2 concentration was quite low (< 0.2 ppb) even in the presence of substantial quantities of cloudwater SO 4 2− (> 1 ppb equivalent gas-phase concentration), suggesting large fractional conversion and incorporation into cloudwater. In other cases, in which dilute SO 2 plumes (pso, > 5 ppb) were observed in the cloud interstitial air, the gaseous SO 2 concentration substantially exceeded the cloudwater sulfate concentration. Concentrations of H 2O 2 in cloudwater were found to exhibit strong inverse correlation with interstitial SO 2. Appreciable concentrations of SO 2 in cloud interstitial air and H 2O 2 in cloudwater were only rarely observed to coexist, for the most part only one or the other being present above the limit of detection. These observations are consistent with aqueous-phase oxidation of SO 2 by H 2O 2, as has been inferred previously on the basis of laboratory kinetic studies, and with the hypothesis that depending on relative concentrations, either of these species can be a limiting reagent for in-cloud SO 2 oxidation. The uptake of NO x as cloudwater nitrate is less complete than the uptake of SO 2 as sulfate, and evidence for the occurrence of similar in-cloud processes causing the conversion of NO or NO 2 to cloudwater nitrate has not been found.