A two-dimensional atmospheric chemistry modeling investigation of Earth's Phanerozoic O3 and near-surface ultraviolet radiation history

We use the Cambridge two-dimensional (2-D) chemistry-radiation transport model to investigate the implications for column O-3 and near-surface ultraviolet radiation (UV), of variations in atmospheric O-2 content over the Phanerozoic (last 540 Myr). Model results confirm some earlier 1-D model investigations showing that global annual mean O-3 column increases monotonically with atmospheric O-2. Sensitivity studies indicate that changes in temperature and N2O exert a minor influence on O-3 relative to O-2. We reconstructed Earth's O-3 history by interpolating the modeled relationship between O-3 and O-2 onto two Phanerozoic O-2 histories. Our results indicate that the largest variation in Phanerozoic column O-3 occurred between 400 and 200 Myr ago, corresponding to a rise in atmospheric O-2 to similar to 1.5 times the present atmospheric level (PAL) and subsequent fall to similar to 0.5 PAL. The O-3 response to this O-2 decline shows latitudinal differences, thinning most at high latitudes (30-40 Dobson units (1 DU = 0.001 atm cm) at 66 degrees N) and least at low latitudes (5-10 DU at 9 degrees N) where a "self-healing" effect is evident. This O-3 depletion coincides with significant increases in the near-surface biologically active UV radiation at high latitudes, +28% as weighted by the Thimijan spectral weighting function. O-3 and UV changes were exacerbated when we incorporated a direct feedback of the terrestrial biosphere on atmospheric chemistry, through enhanced N2O production as the climate switched from an icehouse to a greenhouse mode. On the basis of a summary of field and laboratory experimental evidence, we suggest that these UV radiation