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Using a sophisticated radiative transfer model, the biological UV radiation absorbed by
tropospheric ozone is compared to that absorbed by the same amount of stratospheric ozone for
various solar zenith angles (SZAs) and different skies. For solar zenith angles less than 60., the model results suggest that the addition of each 5 Dobson Unit (DU) ozone in the troposphere,
can offset the increase in erythemally-weighted UV (EER) radiation which is caused by 10 DU
ozone depletion in the stratosphere, if a typical cloud is present in the sky. Clouds and
non-absorbing aerosols were found to elevate the effectiveness of tropospheric ozone by
increasing the pathlength of the photons. In relation to ozone depletion in the stratosphere, a new index: which is called offset index: (OFI) is introduced. This index: can quantify the changes in
tropospheric constituents (e.g., ozone, cloud, aerosol, etc.) which is required to be added into the
troposphere in order to offset the increased biologically UV (e.g.,UVB, EER, cataract, keratitis)
are computed by the model for a typical latitude.
resulted from a real ozone depletion in the stratosphere. The offset indices of cloud and aerosols