Ozone in the troposhere is determined by photochemical production and destruction, dry deposition and cross-tropopause transport of ozone from the stratosphere. In the Arctic troposphere, transport and photochemical reactions involving nitrogen oxides and volatile organic compounds (VOCs) as a result of human emissions also produce ozone resulting in a background mixing ratio of 30 to 50 nmol mol−1 (ppb). Nitrogen oxides play a key role in recycling active free radicals (such as reactive halogens) in the atmosphere and indirectly affect ozone depletion. Ozone depletion events (ODEs) are phenomena associated with the sea ice zone. They are routinely observed at coastal locations when incoming winds have traversed sea ice covered areas.