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1. Introduction

A new chemical transport model including both tropospheric and stratospheric chemistry has been developed for studies of chemical and dynamical processes in the tropopause region. Currently the model is used to calculate 3-D tracer distributions for 1996 and has been validated against observations from the same time. The field of possible applications is exemplified by two experiments that have been performed until now. In the first experiment the cross-tropopause flux of ozone was calculated and the effect of heterogeneous chemistry was assessed. The seasonal cycle of the tropospheric ozone budget was computed for cases including gas-phase chemistry only and including heterogeneous reactions on stratospheric sulphate aerosols and Polar Stratospheric Clouds (PSC). In the second experiment the effect of NOx emissions from civil subsonic aircraft on the distributions of ozone and reactive nitrogen was studied.

After a short description of the model and a validation against ozone measurements presented in sections 2 and 3, the two experiments will be described separately in sections 4.1 and 4.2. Section 5 will give a summary of the presented studies and outline plans for future development.