EFFECTS OF ANTHROPOGENIC EMISSIONS ON TROPOSPHERIC OZONE AND ITS RADIATIVE FORCING

Tropospheric ozone changes since preindustrial times due to changes in emissions have been calculated by the University of Oslo global three -dimensional photochemical model. The radiative forcing caused by the increase in ozone has been calculated by two independent radiative tra nsfer models; the University of Reading model (Reading), and the Unive rsity of Oslo/Norwegian Institute for Air Research model (OsloRad). Si gnificant increases in upper tropospheric ozone concentrations are fou nd at northern midlatitudes (15-40 ppbv, depending on season) at about 10 km altitude. In the tropical regions the largest increase (about 2 0 ppbv for all seasons) is found at about 15 km altitude. The increase is found to be caused mainly by enhanced in situ production due to tr ansport of precursors from the boundary layer, with a smaller contribu tion from increased transport of ozone produced in the boundary layer. The lifetime of ozone in the troposphere decreased by about 35% as a result of enhanced concentrations of HO2. The calculated increase in s urface ozone in Europe is found to be in good agreement With observati ons. The calculations of radiative forcing include the effect of cloud s and allow for thermal adjustment in the stratosphere. The global and annual averaged radiative forcing at the tropopause from both models (0.28 W m(-2) and 0.31 W m(-2), for the Reading and OsloRad models, re spectively) are in the lower part of the Intergovernmental Panel on Cl imate Change [1995] estimated range. The calculated radiative forcing is similar in magnitude to the negative radiative forcing by sulfate a erosols, but displaced southward in source regions at northern midlati tudes. The increase in tropospheric ozone is calculated to have cooled the lower stratosphere by up to 0.9 K, with possibly half of this coo ling occurring in the past 2 to 3 decades.