Effects of a large asteroid impact on ultra-violet radiation in the atmosphere

The effects of sulfate aerosols produced as a result of an asteroid impact on the ultra-violet (UV) radiation are investigated by radiative transfer calculations. After an impact, a reduction in the solar incident radiation and ozone depletion are expected to occur, each of which, in turn, are counteract their on effects on the UV radiation. We estimate reasonable ranges for the amounts of sulfate aerosols and ozone depletion after an impact, and calculate the UV radiation at the Earth's surface, besides absorption in the stratosphere, by changing the aerosol and ozone concentrations within the ranges. The calculation results reveal that the UV-B (0.28-0.315 μm) radiation depends on both aerosol and ozone concentrations. The reflection of UV-B radiation by sulfate aerosols cancels out the increase in surface UV-B radiation due to ozone depletion. This study suggests that, immediately after the Chicxulub impact event, the UV-B radiation at the Earth's surface would not increase as compared to the pre-impact levels, since large amount of sulfate aerosols would exist in the stratosphere. Several years after the impact, the UV-B radiation would increase, because most of the sulfate aerosols would be removed from the atmosphere but their amount would still be enough to destroy ozone and keep it below the harmful level for terrestrial life. In case of the Chicxulub impact, an increase in the UV-B radiation would have occurred several years after the impact and might have contributed to the mass extinction at the end of the Cretaceous period