DISTRIBUTION AND BUDGET OF O-3 IN THE TROPOSPHERE CALCULATED WITH A CHEMISTRY GENERAL-CIRCULATION MODEL

We present results of global tropospheric chemistry simulations with t he coupled chemistry/atmospheric general circulation model ECHAM. Ulti mately, the model will be used to study climate changes induced by ant hropogenic influences on the chemistry of the atmosphere; meteorologic al parameters that are important for the chemistry, such as temperatur e, humidity, air motions, cloud and lain characteristics, and mixing p rocesses are calculated on-line. The chemical part of the model descri bes background tropospheric CH4-CO-NOx-HOx photochemistry. Emissions o f NO and CO, surface concentrations of CH4, and stratospheric concentr ations of O-3 and NOy are prescribed as boundary conditions. Calculati ons of the tropospheric O-3 budget indicate that seasonal variabilitie s of the photochemical production and of injection from the stratosphe re are represented realistically, although some aspects of the model s till need improvement. Comparisons of calculated O-3 surface concentra tions and O-3 profiles with available measurements show that the model reproduces O-3 distributions in remote tropical and midlatitudinal si tes. Also, the model matches typical profiles connected with deep conv ection in the Intertropical Convergence Zone (ITCZ). However, the mode l tends to underestimate O-3 concentrations at the poles and in relati vely polluted regions. These underestimates are caused by the poor rep resentation of tropopause foldings in midlatitudes, which form a signi ficant source of tropospheric O-3 from the stratosphere, too weak tran sport to the poles, and the neglect of higher hydrocarbon chemistry. A lso, mixing of polluted continental boundary layer air into the free t roposphere may be underestimated. We discuss how these model deficienc ies will be improved in the future.