Model study of the influence of cross-tropopause O3 transports on tropospheric O3 levels

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-NO_x-HO_x 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.