Effect of aircraft on ultraviolet radiation reaching the ground

Changes in ozone levels for a range of scenarios, including those for present and projected future aircraft emissions and for present and future halogen loadings, are calculated using the Commonwealth Scientific and Industrial Research Organization two-dimensional chemical transport model, These changes are applied to measured ozone columns and vertical profiles based on measurements to produce vertical profiles of ozone for each scenario considered, which are traceable to measurements. A radiative transfer model is then used to investigate changes in biologically active radiation reaching the surface of the Earth resulting from current and future fleets of aircraft and those resulting from changing levels of halogen compounds in the atmosphere. It is shown that equal changes in ozone column for these scenarios do not produce equal changes in biologically weighted fluxes reaching the ground. This is because aircraft affect ozone mainly in the upper troposphere, whereas the effects of halogens are greatest in the middle and lower stratosphere. The magnitude of the ratio of the biologically weighted flux change to the ozone column change is greater for the case of the aircraft, due to the larger contribution to multiple scattering in the troposphere. For the same reason, projected fleets of supersonic aircraft are shown to have a smaller effect on UV radiation for a given change in ozone column than subsonic aircraft. While aerosols reduce the UV radiation reaching the ground for all scenarios investigated, they have minimal impact on the ratios of UV changes to ozone column changes because the bulk of the aerosol loading is below the altitudes where ozone changes due to aircraft or halogens occur.