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Abstract

A background tropospheric chemistry model that is coupled to the general circulation model
ECHAM4 is used to calculate tropospheric ozone with preindustrial, present-day and future
(IS92a) emission scenarios as boundary conditions. The model calculates separate contributions
to tropospheric ozone levels from stratosphere-troposphere exchange (STE) and from photo-
chemical production in the troposphere. In the preindustrial atmosphere, the simulated annual
tropospheric ozone content is 190 Tg 03, of which about 110 Tg 03 originates from the strato-
sphere. In the present-day simulation the ozone content is about 80 Tg 03 larger, mainly due to
O3 precursor emissions from industrial processes in the NH and from biomass burning in trop-
ical regions. In the next few decades, industrial growth is expected to occur mainly at NH (sub)
tropical latitudes, leading to an additional increase of the tropospheric ozone budget by 60 Tg
03. We calculate a global and annual average radiative forcing by tropospheric ozone perturbations of 0.42 w m-2
creases in the next few decades of 0.31 W m-2 for the present-day simulation, and an additional forcing due to ozone in-
The model results indicate that the amount of
tropospheric ozone from stratospheric origin remains relatively unaffected by the changing pho-
tochemistry.