Controversy over tropical ozone hole

Have experts missed a huge tropical ozone hole that has existed since the 1980s? — asks Geographical. Or could it be more a question of definitions?
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In July, an extraordinary research paper, documenting a huge, previously undetected ozone hole over the tropics, prompted a flurry of news stories.

Said to be seven times the size of the well-known ozone hole over Antarctica, the discovery is cause for ‘great global concern’, according to Qing-Bin Lu, a professor at the University of Waterloo, Canada, and author of the report.

His research suggests that, unlike the Antarctic hole, which only opens in spring, the tropical hole remains open year-round, putting roughly half the world’s population at higher risk from ultraviolet radiation.

Most surprisingly of all, Lu claims that the hole has existed since the 1980s.

However, the paper and the results it reports aren’t supported by all ozone watchers.

Some experts, including Paul Young, an atmospheric scientist at Lancaster University and lead author of the forthcoming Scientific Assessment of Ozone Depletion (2022), sponsored by the World Meteorological Organization and the United Nations Environment Programme, categorically refute Lu’s conclusions. ‘There is no tropical ozone hole,’ Young says. ‘If it was there, I will stick my neck out and say that it would have been found.’

So how can scientists have such different takes?

Some of the controversy comes down to the use of different definitions. On average, the concentration of ozone (a highly reactive gas composed of three oxygen atoms) in the ozone layer is 300 Dobson Units, which, when compressed to the atmospheric pressure at the Earth’s surface, would form a layer three millimetres thick.

The conventional definition of an ozone hole is when this concentration drops below 220 Dobson Units. However, Lu defines a hole as ‘an area of ozone depletion greater than 25 per cent relative to the undisturbed atmosphere (when there were no significant chlorofluorocarbons in the stratosphere)’.

Chlorofluorocarbons are man-made, ozone-depleting chemicals used widely since the 1930s, which contributed significantly to the Antarctic ozone hole but which are now largely banned.

‘No ozone hole over the tropics would be observed by the conventional definition of an ozone hole,’ Lu notes in his report, ‘as the total ozone over the tropics was above 220 Dobson Units even with a loss by 56 per cent.’

Full article here.
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Paper: Observation of large and all-season ozone losses over the tropics (2022)

Abstract

This paper reveals a large and all-season ozone hole in the lower stratosphere over the tropics (30°N–30°S) existing since the 1980s, where an O3 hole is defined as an area of O3 loss larger than 25% compared with the undisturbed atmosphere. The depth of this tropical O3 hole is comparable to that of the well-known springtime Antarctic O3 hole, whereas its area is about seven times that of the latter. Similar to the Antarctic O3 hole, approximately 80% of the normal O3 value is depleted at the center of the tropical O3 hole. The results strongly indicate that both Antarctic and tropical O3 holes must arise from an identical physical mechanism, for which the cosmic-ray-driven electron reaction model shows good agreement with observations. The whole-year large tropical O3 hole could cause a great global concern as it can lead to increases in ground-level ultraviolet radiation and affect 50% of the Earth’s surface area, which is home to approximately 50% of the world’s population. Moreover, the presence of the tropical and polar O3 holes is equivalent to the formation of three “temperature holes” observed in the stratosphere. These findings will have significances in understanding planetary physics, ozone depletion, climate change, and human health.