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 cloudwater was generally dominated by H+, NH2−4, NO3 and SO2−4, similar to the composition of precipitation in this region of the U.S. Concentrations of the corresponding interstitial aerosol species and gaseous HNO3 were invariably low in comparison to concentrations of the respective ionic species in cloudwater. In contrast, the concentration of NOx (i.e. NO + NO2 + 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 SO2 concentration was quite low (<0.2 ppb) even in the presence of substantial quantities of cloudwater SO2−4 (> 1 ppb equivalent gas-phase concentration), suggesting large fractional conversion and incorporation into cloudwater. In other cases, in which dilute SO2 plumes (pso2 > 5 ppb) were observed in the cloud interstitial air, the gaseous SO2 concentration substantially exceeded the cloudwater sulfate concentration.

Concentrations of H2O2 in cloudwater were found to exhibit strong inverse correlation with interstitial SO2. Appreciable concentrations of SO2 in cloud interstitial air and H2O2 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 SO2 by H2O2, 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 SO2 oxidation. The uptake of NOx as cloudwater nitrate is less complete than the uptake of SO2 as sulfate, and evidence for the occurrence of similar in-cloud processes causing the conversion of NO or NO2 to cloudwater nitrate has not been found.

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