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Particles in wildfire smoke may activate molecules that destroy the ozone layer, according to new research suggesting that future ozone recovery may be slowed by increasingly intense and frequent fires.
A study published in the journal Nature found that smoke from Australia’s 2019-2020 bushfires temporarily depleted the ozone layer by 3% to 5% by 2020.
Smoke from the 2019–20 wildfires, which circulated around the world, was ejected from a pyrocumulonimbus cloud into the stratosphere, the second layer in the Earth’s atmosphere.
Related: Smoke from Black Summer wildfires depleted the ozone layer, study finds
In the ozone layer – part of the stratosphere – molecules of ozone gas absorb high-energy ultraviolet rays from the sun. This reduces the amount of radiation reaching the Earth’s surface.
Lead researcher Prof. Susan Solomon, an atmospheric scientist at the Massachusetts Institute of Technology in the US, said the destruction of the ozone layer by smoke particles was similar to the process of the Antarctic ozone hole that forms each spring, “but at much higher temperatures “.
Smoke aerosols, the researchers found, can activate chlorine to form compounds that then destroy ozone molecules.
Solomon said chlorine levels in the stratosphere had declined since the 1987 Montreal Protocol phased out the use of ozone-depleting hydrochlorofluorocarbons. “There’s a huge science policy success story there,” she said. “The slow recovery of the ozone layer is on the order of 1% per decade in mid-latitudes.”
But she warned that more frequent fires could slow ozone recovery. “Suddenly, in a year , we had a loss of 3% to 5%. It will recover if that’s the only year it happens, but not if it keeps happening.
“The question in my mind is, will the man-made chlorine be diluted and destroyed from the atmosphere faster than global climate change will increase the frequency and intensity of these types of fires? I think it will be a race.”
Dr. Martin Jucker, a lecturer at the University of New South Wales who was not involved in the study, agreed that the ozone hole may be recovering more slowly than expected due to more bushfires in the future.
Related: Australia is facing unprecedented grass fires next summer, ‘fuelled’ by global warming
“Of particular concern to Australia is the expansion of the ozone hole further towards the equator, which means that the ozone layer could thin much closer to where millions of Australians live,” he said.
Dr. Laura Revell, an atmospheric scientist at the University of Canterbury in New Zealand, said: “The first signs of ozone recovery in Antarctica have been visible since about the mid-2010s. If there are no major changes, we expect chlorine levels to in the stratosphere will gradually decrease this century and that the hole in the ozone layer will become smaller year after year.
“It is concerning that while the ozone hole usually forms over Antarctica because of the cold temperatures there, wildfire aerosols appear to be able to promote ozone losses at the relatively warmer temperatures present in densely populated mid-latitudes.”
Solomon and her colleagues determined that the ozone destruction process is caused by hydrochloric acid dissolving in the smoke aerosols in the stratosphere.
Hydrochloric acid dissolved about a thousand times more easily in the smoke aerosols than in “the normal sulfuric acid and water stratospheric particles,” Solomon said.
“From a scientific point of view, it’s very exciting to see this brand new effect,” she said. “From a planetary point of view…it would just be tragic if humanity screwed up the ozone hole solution by deciding that we [allow] many more fires like this if we don’t limit climate change.”
Solomon added that it was important to determine whether smoke from bushfires in Australia — where native forests are dominated by eucalyptus trees — differed in composition from fires in other areas. “I don’t really see a chemical reason why that would be, but it needs to be looked at.