Studies on non-precipitating cumulus cloud acidification

A one-dimensional cumulus cloud model is used to study the chemical transformations occurring during the formation, growth and dissipation of a cloud. A parabolic vertical velocity profile is introduced to create an updraft which entrains and detrains air laterally. The cloud forms in the upper half of the updraft structure where lateral detrainment occurs, all entrainment being from below the cloud base. As the updraft velocity increases in magnitude, the cloud grows and deepens. Downdraft is introduced into the fully developed cloud so that lateral entrainment into the parcel is from regions above cloud top. The cloud dissipates and eventually disappears as the downdraft penetrates and reaches the cloud base. Predicted liquid water contents and cloud structure are consistent with field observations.

Detailed gas- and aqueous-phase chemistries are included in the cloud model. First-order rate constants for the oxidation of S(IV) to S(VI) increase with increasing concentrations of H₂O₂ in the background and with increasing cloudwater content. The cloudwater pH depends primarily on the degree of oxidation of S(IV) to S(VI) by H₂O₂ which is the most important oxidant of S(IV) in the aqueous phase. HO₂, OH, Cl₂⁻ and the intermediate SO₄⁻ also make significant contributions to the oxidation, while for these runs, O₃contributes less than 2% to the overall oxidation. Cumulus clouds are shown to behave like large variable volume reactors with entrainment of background air from below-cloud regions and release of processed air from in-cloud regions. The first-order rate constant for the oxidation of S(IV) to S(VI) ranges from 1 % min⁻¹ to 33% min⁻¹, depending on cloud properties and trace species concentrations.