A three-dimensional chemistry general circulation model simulation of anthropogenically derived ozone in the troposphere and its radiative climate forcing

We present results from the tropospheric chemistry/climate European Center Hamburg Model by comparing two simulations that consider a preindustrial and a contemporary emission scenario. Photochemical O-3 production from anthropogenically emitted precursors contributes about 30% to the present-day tropospheric O-3 content, which is roughly equal to the natural photochemical production. Transports of stratospheric O-3 into the troposphere contribute about 40%. As a result of anthropogenic emissions, the O-3 maximum over remote northern hemisphere (NH) areas has shifted from winter to spring, when photochemical production of O-3 is relatively efficient. Over NH continents the preindustrial seasonal variability is relatively weak whereas a distinct surface O-3 summer maximum appears in the contemporary simulation. In the (sub)tropical southern hemisphere (SH), anthropogenic biomass burning emissions cause an increase of O-3 mixing ratios in the dry season (September-November). We calculate a relative increase in O-3 mixing ratios due to anthropogenic emissions of about 30% in the pristine SH middle and high latitudes to about 100% in the polluted NH boundary layer. The model simulations suggest that the absolute increase of tropospheric O-3 maximizes in the middle troposphere. Through convection, upper tropospheric O-3 mixing ratios are significantly affected in the tropical regions and, during summer, in the middle and high NH latitudes. Under these conditions the radiative forcing of climate by increasing O-3 is relatively large. We calculate a global and annual average radiative forcing by tropospheric O-3 perturbations of 0.42 W m(-2), i.e., 0.51 W m(-2) in the NH and 0.33 W m(-2) in the SH.