Title: Model study of the influence of cross-tropopause O<sub>3</sub> transports on tropospheric O<sub>3</sub> levels
Abstract: Cross-tropopause transport of O3 is a significant factor in the tropospheric budget and distribution of O3. Nevertheless, the distribution in the troposphere of O3 that originates from the stratosphere is uncertain. We study this with a chemistry — general circulation model with relatively high spatial and temporal resolution. The model simulates background tropospheric CH4-CO-NOx-HOx photochemistry, and includes a tracer for stratospheric O3. Since this tracer is not photochemically produced in the troposphere but only destroyed, comparing its budget and distribution with that of total tropospheric O3 yields an estimate of the contribution of the stratospheric O3 source in the troposphere. Model results suggest that transport from the stratosphere and net photochemical formation in the troposphere, considering present-day emissions, are of comparable magnitude. The model predicts efficient transport of upper tropospheric O3-rich air to the surface by large-scale subsidence in the subtropics and by synoptic disturbances in the NH middle and high latitudes. O3 from the stratosphere contributes significantly to surface O3 in winter and spring when the photochemical lifetime of O3 is relatively long. In summer and in the tropics, little O3 from the stratosphere reaches the surface due to strong photochemical destruction, so that surface O3 is largely determined by photochemical production. Photochemically produced O3 maximizes in the free troposphere where the O3 surface warming efficiency is higher compared to the boundary layer.