TROPOSPHERIC OZONE CHEMISTRY - THE IMPACT OF CLOUD CHEMISTRY

The effect of clouds and cloud chemistry on tropospheric ozone chemistry is tested out in a two-dimensional channel model covering a latitudinal band from 30 to 60-degrees N. Three different methods describing how clouds affect gaseous species are applied, and the results are compared. The three methods are: A first order parameterization scheme for the removal of sulphur and other soluble gases by liquid droplets. A parameterization scheme for SO2, O3, and H2O2 removal is constructed. The scheme is based on the solubility of gases in liquid droplets, cycling times of air masses between clouds and cloud free areas and on the chemical interaction of SO2 with H2O2 and 03 in the liquid phase. Gas-aqueous-phase interactions and aqueous-phase chemical reactions are included in the reaction scheme for a number of components in areas where clouds are present. In all three methods, a full gas-phase chemistry scheme is used. Particular emphasis is given to the study of how the ozone and hydrogen peroxide levels are affected. Significant changes in the distributions are found when aqueous-phase chemical reactions are included. The result is loss of ozone in the aqueous phase, with pronounced reductions in ozone levels in the middle and lower troposphere. Ozone levels are reduced by 10 to 30% with the largest reductions in the remote middle troposphere, bringing the values in better agreement with observations. Changes in H2O2 are harder to predict. Although, in one case study, hydrogen peroxide is produced within the aqueous phase. concentrations are mostly comparable or even lower than in the other cases. Hydrogen peroxide levels are, however, shown to be very pH sensitive. pH values around 5 seem to favour high H2O2 levels. High H2O2 concentrations may be found particularly in the upper part of the clouds under favourable conditions.