Modelling cloud effects on ozone on a regional scale: A case study

We have investigated the influence of clouds on ozone on a regional scale (Europe) with a regional scale photochemical dispersion model (LOTOS). The LOTOS-model calculates ozone and other photo-oxidant concentrations in the lowest three km of the troposphere, using actual meteorologic data and emissions. We accounted for the altered radiative transfer in clouds and aqueous-phase chemistry (with and without iron and copper reactions in the aqueous phase). These cloud effects have been tested separately to distinguish their individual contribution to the ozone formation/degradation. Furthermore we estimated the effect of wet deposition parameterizing a upper-limit scavenging rate. Model results are shown for a cloudy period in August 1990. We find for this period that the radiative effect of clouds lead, locally, to ozone reductions in the planetary boundary layer by as much as 22%; overall to a reduction of ozone of 4%. This coincides with. a reduction of 14% of the net ozone formation. When aqueous-phase chemistry was added the net ozone formation and concentration were further reduced, up to 20% and 5%, respectively. Although aqueous-phase chemistry can lead to a strong reduction of ozone formation in the cloud (up to 110%), the overall effect is limited by the cloud volume fraction, cloud liquid water content and pH. The iron and copper reactions change the aqueous-phase reaction pathways substantially and render the droplet roughly a four times smaller sink for ozone.