A Theoretical Method for Computing Vertical Distributions of Acidity and Sulfate Production within Cumulus Clouds

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Abstract
An aqueous chemistry model has been combined with an entraining cumulus cloud model to predict vertical distributions of pH (=−log10[H+]) within a cloud. The cloud model predicts vertical variations of temperature, pressure, entrainment and liquid water content. The aqueous chemistry model incorporates the effects of soluble aerosols, trace gases and aqueous-phase SO2 oxidation on cloudwater pH levels. In the absence of SO2 oxidation, vertical distributions of pH were found to be highly variable, depending on several chemical and meteorological factors. For clouds forming in an environment where the aqueous composition is determined primarily by dissolved sulfate condensation aerosol, pH was found to increase with height above cloud base primarily due to dilution by increasing water content. If cloudwater acidity levels are predominantly determined by dissolved SO2, HNO3 and formic acid gas, pH variations with height were not so clearly defined, with the diluting effects of entrainment, liquid water content and pressure being compensated for by the increased solubility of these gases at lower temperatures. When neutralizing agents such as ammonia and carbonate-laden aerosol are present in cloudy air, acidity can actually increase with height above cloud base. With aqueous-phase oxidation of SO2 allowed, sulfate production rates on the order of several tens of μg m−3 h−1 were predicted to occur in the first kilometer above cloud base, driven predominantly by H2O2 oxidation. In middle or upper cloud levels, sulfate production was considerably slower, with other oxidation pathways (O3 and peroxyacetic acid) becoming more important. Despite this production of sulfuric acid, pH was predicted to increase with height above cloud base due to dilution by higher water contents. These model estimates suggest that several chemical, cloud-microphysical and cloud-dynamical parameters must be well characterized in order to predict cloudwater composition.

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