  Astrophysical ionizing radiation events such as supernovae, gamma-ray bursts,
and solar proton events have been recognized as a potential threat to life on
Earth, primarily through depletion of stratospheric ozone and subsequent
increase in solar UV radiation at Earth's surface and in the upper levels of
the ocean. Other work has also considered the potential impact of nitric acid
rainout, concluding that no significant threat is likely. Not yet studied
to-date is the potential impact of ozone produced in the lower atmosphere
following an ionizing radiation event. Ozone is a known irritant to organisms
on land and in water and therefore may be a significant additional hazard.
Using previously completed atmospheric chemistry modeling we have examined the
amount of ozone produced in the lower atmosphere for the case of a gamma-ray
burst and find that the values are too small to pose a significant additional
threat to the biosphere. These results may be extended to other ionizing
radiation events, including supernovae and extreme solar proton events.
