Chemical ozone loss in the tropopause region on subvisible ice clouds, calculated with a chemistry-transport model

A global chemistry-transport model has been used to investigate the role of subvisible ice clouds in chemical ozone loss in the tropopause region. The three-dimensional subvisible cloud representation is based on 6-hourly European Centre for Medium-Range Weather Forecasts high cloud fields, assuming observed cloud particle radii and densities. The resulting seasonal average subvisible cloud occurrence agrees well with that observed by the Stratospheric Aerosol and Gas Experiment (SAGE) II instrument. The clouds are located along cold and warm fronts, associated with cyclonic activity. On the ice clouds, heterogeneous reactions occur similarly as on polar stratospheric clouds. The calculated chemical perturbations show high spatial variability, owing to the relatively detailed cloud representation. Halogen activation occurs only when the clouds are located at or above potential vorticity levels of 2–3 PVU. The cloud-induced chlorine activation is stronger in June than in December with maximum ClO levels close to 100 pptv. Consequently, the calculated chemical ozone loss is stronger in June than in December with local maxima of ∼4%. The calculated chlorine activation is significantly lower compared to previous model results and shows an opposite seasonal behavior. Changing the cloud composition to a mix of liquid and ice reduces the chlorine activation even further. This illustrates the strong sensitivity to cloud properties. In addition, the reactive gas uptake on the ice cloud surfaces is poorly described for midlatitude tropopause conditions. Although these uncertainties prevent an accurate calculation of chemical ozone loss, it seems unlikely that heterogeneous chemistry on subvisible clouds can explain the observed negative ozone trends in the midlatitude lowermost stratosphere. Nevertheless, the ozone loss cannot be neglected, being of comparable magnitude as ozone formation by NOx from commercial air traffic