Many people are aware that ozone protects us from harmful ultraviolet (UV) solar radiation, and that ozone amounts are decreasing. Perhaps not so many are aware of its great variability in our atmosphere during the seasons and even during days. Episodes of low stratospheric ozone have been at observed at many locations, transported by upper air winds. Among most notable have been Australian episodes, close to the earth's South Pole where ozone amounts have recently been very low during the Antarctic Spring. This feature provides a brief examination of stratospheric ozone and its depletion, emphasizing the seasonal 'ozone hole' over Antarctica.
If not for ozone, the stratosphere would not exist. Ozone's great absorption of harmful near UV radiation at approximately 12 - 40 km altitudes causes heating, which defines the stratosphere as a region at which temperature increases with increasing altitude. Stratospheric ozone measurements are obtained many ways. Ground observations have been taken since the 1920's, later balloon and aircraft observations, and then satellite observations. A Total Ozone Mapping Spectrometer obtained measurements on the Nimbus 7 satellite from 1978-1991 and Russia's Meteor 3 satellite during 1991-1994, and presently operates on NASA's Earth Probe satellite and Japan's Advanced Earth Observing satellite. Many ground-based observations are taken, which is how the 'ozone hole' was first discovered at the British Antarctic Survey's Halley station during 1985. (Nimbus 7 measurements indicated such activity prior to that, but not so convincingly.)
Recent atmospheric ozone time-series profiles from Alfred Wegener Institute's Neumayer arctic research station also clearly indicate remarkable Antarctic ozone decreases between late August to early December of each year. The plots indicate ozone Dobson Unit amounts. You may notice some maximum amounts during Antarctic Spring (e.g., day 275) of about 65 DU ! For reference, 400 Dobson Units refer to a column of ozone which would be 4 millimeters thick at standard pressure and temperature at earth's surface (.4 atm-cm). Not a large amount to protect us, but sunlight contributes to ozone formation (and its dissociation), a sort of natural protection. Thus, a person might assume that the long Antarctic night is responsible for the 'ozone hole'. It contributes, but stratospheric chlorine chemistry and the circumpolar vortex (flow around the cold South Pole during winter) are more responsible.
Perhaps most responsible is sulfates from volcanic eruptions, such as Mount Pinnatubo during June 1991. Nearly all of the seasonal ozone reduction is observed in the lower stratosphere, where such volcanic aerosols tend to reside. Such a decrease is much less during the Arctic Spring; and in fact, ozone amounts at most northern hemisphere locations are maximum during Spring.
Why is this relevant ? Stratospheric chlorine (catalyst mainly responsible for ozone depletion reactions) , particularly from chloroflourocarbons is estimated to have quadrupled since 1950, which is causing ozone depletions of as much as 20% per decade at the South Pole to amounts typically about 2-9% at mid-latitudes, with very little change at the equator, where most ozone forms and is transported elsewhere by stratospheric air flow. Only relatively recently have significant actions such as the Montreal Protocol been taken to decrease use of chloroflourocarbons, which is estimated by some researchers to allow ozone to develop to normal amounts during the next century.
Next week, I plan to include a brief discussion of UV radiation forecasts and their use for daily activities.
Text is copyright of Joseph Bartlo, though may be used with proper crediting.