A zonally averaged transport and chemistry model for atmospheric trace species is developed, based on an existing statistical-dynamical climate model. The modeled region is a single hemisphere from 1000 mbar to 240 mbar. The model is applied to a study of the budget of tropospheric ozone, in particular to an estimation of the relative importance of the stratospheric and in situ photochemical sources. A parameterization of cross-tropopause exchange is developed based on the concept of tropopause folding. It is used to provide transfers of air and ozone across the upper boundary. A basic tropospheric chemistry scheme is also incorporated, featuring methane oxidation and subsequent reactions, carbon monoxide oxidation and oxygen-hydrogen reactions. Nitrogen oxide reactions are reduced to a minimum by specification of NOx distribution in space and time. Nonmethane hydrocarbon reactions are omitted. Transport and surface deposition of relevant species are performed.

The model produces ozone distributions which are in general agreement with observations and other modeling studies. These are analyzed by considering, &khgr;&khgr;, the mixing ratio of ozone of stratospheric origin with respect to the ''complete'' ozone mixing ratio. The sources and sinks of &khgr;&khgr; are parameterized in terms of the relevant processes, and it can be treated as a quasi-conservative variable with respect to transports. Zonally averaged fields of this mixing ratio are produced and show the influence of ozone of stratospheric origin to be very small (5% at most) close to the surface, in this zonally averaged analysis. At 300 mbar, in middle to high latitudes, the influence is more significant (up to about 50% in high-latitude winter). These results are in accord with the very limited observational evidence. ¿ American Geophysical Union 1992