Modification of aerosol mass and size distribution due to aqueous-phase SO₂ oxidation in clouds: Comparisons of several models

Abstract

Models of aerosol scavenging and aqueous-phase oxidation of SO₂ by H₂O₂ and O₃ in a cloud updraft are compared. Bulk models considering only a single droplet size are compared with size-resolved models that explicitly simulate multiple aerosol and drop sizes. All models simulate growth of cloud drops on a lognormal ammonium bisulfate aerosol distribution, and subsequent aqueous-phase chemistry during adiabatic ascent. In agreement with earlier published studies, it is found that relative to bulk models, the size-resolved cloud chemical models consistently calculate 2–3 times more oxidation via the SO₂ + O₃ pathway, due to calculated variability of cloud water pH among cloud drops. All models calculate high scavenging of the input dry aerosol mass, but the calculated number of cloud drops formed varies from 275–358 drops cm⁻³. Differences in the calculated number of cloud drops formed result from the treatment of gaseous species uptake, solution thermodynamics, applied water condensation mass accommodation coefficient, and bin size range definitions over which the input aerosol distribution is numerically approximated. The difference in calculated cloud drop number can under many conditions propagate to appreciable variations in cloud albedo. It is found that the modifications to the aerosol size and mass spectrum are sensitive to the number of cloud drops formed, and differences in the processed aerosol spectra were found to induce up to 13% differences in calculated light extinction properties of the modified particle distributions. These significant discrepancies among cloud aerosol chemistry interaction models, even when used to simulate relatively simple conditions, suggest that parameterizations of these processes used in larger-scale cloud, regional and longer-term climate models can contain high levels of uncertainty.