Record low ozone at Scott Base
NIWA measurements of atmospheric ozone above Arrival Heights, near Scott Base in Antarctica, have reached the lowest values recorded. On September 30 the ozone level recorded there was 124 Dobson Units (DU) which is a new record low measurement for the site.
Dr Stephen Wood of NIWA, currently at Scott Base, said that these measurements are consistent with the picture seen by satellite measurements of this year’s Antarctic ozone hole.
"The ozone holes that form each year over Antarctica are triggered by three factors: Chlorine pollutants in the atmosphere combine with extremely cold temperatures above Antarctica and the return of sunlight in spring to chemically destroy ozone.
"They remove as much as 60–70% of the ozone over Antarctica and the ozone holes usually reach their largest area sometime around the middle of September. The lowest ozone values usually occur in late September or early October."
Dr Wood said this year's Antarctic ozone hole is already the largest on record, and it formed surprisingly early.
"Figures from NASA satellite measurements show the hole covered an area of approximately 29 million square kilometres in early September, exceeding the previous record from 1998, and these record sizes persisted for several days. The lowest ozone levels recorded in Antarctica this spring by satellite instruments are already below 100 DU this year and are close to the minimum ever recorded previously.
"According to WMO reports, the area with temperatures cold enough to trigger the ozone depletion was 10–20% larger in early September than in any of the previous 10 years. The large area is one reason for the early development of the hole because the further north the cold air reaches the earlier it becomes sunlit. Wave motions in the atmosphere also appear to have contributed to the early depletion by distorting the shape of the hole and bringing parts of the cold mass of air even further north."
He said the area covered by the hole is so large that its effects are being seen farther north than usual.
"Normally, ozone over New Zealand during springtime averages around 350 DU, but this year we have seen days with ozone as low as 260 DU when atmospheric circulation patterns pushed the hole closer to New Zealand. The ozone hole is usually defined as the area where ozone is less than 220 DU, so we can’t say that the ozone hole actually came over New Zealand.
"At this time of the year, lower ozone amounts over New Zealand are not as much of a concern as in the summer because the sun is still relatively low in the sky and levels of erythemal (sun-burning) UV are still approximately half of midsummer values.
"However, the larger and more intense Antarctic ozone hole does bring an increased risk of high UV levels in New Zealand when the circulation pattern that contains the ozone hole breaks up, usually in early December."
Dr Wood said that despite the amount of chlorine pollutants, especially from chlorofluorocarbons, having already started to decrease in the lower atmosphere due to measures introduced under the Montreal Protocol, there has not yet been a measurable response in ozone levels.
"Scientists expect that the Antarctic ozone hole will continue to form for at least the next 20 years. There is also the possibility that climate change will mean a trend toward colder temperatures in the stratosphere and this will delay the expected recovery in ozone."
Click here to see NASA satellite images of the hole on 23 August and 27 September.
