Particles in bushfire smoke can activate molecules that destroy the ozone layer, according to a new research that suggests future ozone recovery may be delayed by increasingly intense and frequent fires.
A study published in the journal Nature has found that smoke from the 2019-20 Australian bushfires temporarily depleted the ozone layer by 3% to 5% in 2020.
Smoke from the fires, which circulated around the globe, was ejected into the stratosphere, the second layer in Earth’s atmosphere, by a pyrocumulonimbus cloud.
In the ozone layer – part of the stratosphere – molecules of ozone gas absorb high-energy ultraviolet rays from the sun. This lessens the amount of radiation that reaches the Earth’s surface.
The lead researcher, Prof Susan Solomon, an atmospheric scientist at the Massachusetts Institute of Technology in the US, said the ozone destruction by smoke particles was similar to the process of the Antarctic ozone hole forming each spring, “but at much warmer temperatures”.
Smoke aerosols, the researchers found, can activate chlorine to form compounds that then destroy ozone molecules.
Solomon said chlorine in the stratosphere had been decreasing since the 1987 Montreal protocol phased out the use of ozone-depleting chlorofluorocarbons. “There’s a tremendous science policy success story there,” she said. “The slow recovery of the ozone layer is on the order of 1% per decade in the mid-latitudes.”
But she warned that more frequent fires could delay ozone recovery. “All of a sudden, in one year [2020], we had a 3% to 5% loss. It’ll recover if that’s the only year that it happens, but not if it keeps happening.
“The question in my mind is: is the man-made chlorine going to get … diluted and destroyed out of the atmosphere faster than global climate change is going to increase the frequency and intensity of this kind of fire? I think it’s going to be a race.”
Dr Martin Jucker, a lecturer at the University of New South Wales who was not involved in the research, agreed that the ozone hole might recover more slowly than expected as the result of more bushfires in future.
“Of particular interest for Australia is the extension of the ozone hole further equatorward, which means that the ozone layer can become thinner 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: “Early signs of Antarctic ozone recovery have been visible since approximately the mid-2010s. In the absence of any major changes, we expect that stratospheric chlorine concentrations will gradually decrease this century and that the ozone hole will get smaller year by year.
“Of concern is that while the ozone hole usually forms over Antarctica because of the cold temperatures there, wildfire aerosols appear to be capable of promoting ozone losses at the relatively warmer temperatures present at mid-latitudes which are heavily populated.”
Solomon and her colleagues identified that the ozone-destruction process was triggered by hydrochloric acid in the stratosphere dissolving in the smoke aerosols.
Hydrochloric acid dissolved about a thousand times more readily in the smoke aerosols than in the “the normal sulphuric 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 be just tragic to have mankind screw up solving the ozone hole by deciding that we’re going to [allow] a lot more of these fires if we don’t mitigate climate change.”
Solomon added that it was important to determine whether the smoke from fires in Australia – where native forests are dominated by eucalypts – differed in composition from fires in other areas. “I don’t really see a chemical reason why that would be so, but it needs to be looked at.”