From AMS Glossary
A chemical species that possesses an unpaired electron.
These compounds, such as chlorine monoxide (ClO), hydroxyl radical (OH), and nitric oxide (NO), are characterized by a high chemical reactivity. Free radicals are often generated in the atmosphere from the photodissociation of more stable species. Their subsequent chemistry is central to such environmental issues as stratospheric ozone depletion and tropospheric oxidant production.