Meteorological and Chemical Factors Influencing Cloudwater Composition in a Non-Precipitating, Liquid-Water Updraft

Abstract

An aqueous chemical model has been combined with an entraining updraft model to predict cloudwater composition above cloud base. Meteorological factors such as liquid water content, entrainment, pressure and temperature were allowed to vary as they might in a cloudy environment. Chemical reactions producing sulphuric acid, formic acid and hydrogen peroxide (H2O2) are extremely sensitive to the microphysical, dynamical, and radiative properties within a cloud. Several sulphate production pathways are sensitive to cloudwater pH, which is largely determined by sulphate aerosol concentration and cloud liquid water content. Hydrogen peroxide production may occur in both the gas and aqueous phase, and is sensitive to the amount of light available to initiate photochemical reactions. Near cloud base, H2O2 may be rapidly destroyed by SO2 in polluted areas. At higher cloud levels, where light intensities are higher and SO2 concentrations are usually lower, H2O2 may be produced at rates of up to 0.5 ppb h~l. Vertical or horizontal variations in meteorological factors alone will produce significant changes in cloudwater composition. Entrainment of cleaner mid-tropospheric air, pressure reductions, and increases in liquid water content lead to a decrease in solute concentrations in cloudwater. In contrast, reductions in temperature and aqueous-phase acid generation cause solute concentrations to increase in cloudwater. At high altitude sites, the composition of cloudwater will vary significantly with altitude. Near cloud base, concentrations of sulphate and nitrate in cloudwater will probably be highest, although cloud liquid water contents and droplet sizes will be lower. At higher elevations above cloud base, liquid water contents and drop sizes are larger, while solute concentrations may be lower. These model estimates suggest that several chemical, cloud-dynamical and cloud-microphysical parameters must be well characterised in order to predict cloudwater composition.