Chemical effects of UV fluctuations inferred from total ozone and tropospheric aerosol variations

A regional-scale photochemistry model with 50 km horizontal resolution and 20 layers in the vertical is described. The model's domain includes Europe and most of the North Atlantic and the Arctic region. Model results for the summer months April-September 1996 are presented and compared to measureme nts. Emphasis is on the Aegean Sea where extensive measurements were made a s part of the European Union project Photochemical Activity and Solar Ultra violet Radiation (PAUR). The model sensitivity with respect to changes in U V radiation, caused by moderate variations in the total ozone column (25 Do bson units), are explored and shown to be small. Aerosol optical depth was calculated from Mie scattering theory using the modeled total sulphate colu mn and used to calculate perturbations of the photolysis rates due to sulph ate aerosols. Only small effects on monthly averaged ozone are found with t he inclusion of aerosols. For individual days, occasional large variations in calculated ozone levels are seen when the aerosol optical depth is fixed at a high value. The inclusion of aerosols result in a slight reduction of high ozone events over central parts of Europe.