High clouds of smoke sent into the stratosphere by wildfires can eat away at Earth’s ozone because of a powerful mix of smoke, atmospheric chemistry and ultraviolet light, a new study has found.
In late 2019 and early 2020, Australian skies turned dark, darkened by thick columns of wildfire smoke that reached the stratosphere. The recovered satellite data revealed that the smoke somehow interacted with atmospheric molecules to eat up Earth’s ozone (SN: 3/17/22). But it is not clear how exactly this happened.
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Now, scientists are putting together the pieces of that chemical puzzle. Once in the stratosphere, the team says, the smoke particles could interact with stratospheric gases and delay emissions of ozone-depleting chemicals. Add the sun’s rays, and that smoky beer is churned out from chlorine radicals, a type of chemical with an affinity for attacking ozone, researchers report on March 9. nature.
This series of events was responsible for the depletion of about 3 to 5 percent of ozone in parts of the Southern Hemisphere by 2020, researchers estimate. That’s a tiny part of the whole — but it rivals the scale of human emissions of ozone-eating chlorofluorocarbons on its blooms, says MIT chemist Susan Solomon.
Chlorofluorocarbons were once used in air conditioners and refrigerators, but their release into the atmosphere led to a large hole over Antarctica in the Earth’s protective ozone layer, which limits the amount of the sun’s ultraviolet rays reaching the planet’s surface.
In the new study, Solomon and colleagues compared atmospheric observations of chlorine, ozone and other molecules following the Australian wildfires with simulations of atmospheric chemistry. Satellites have measured the amount of certain chemicals in the stratosphere in 2020 — not only ozone, but also the gas chloride chloride and nitrate chloride, among others. Those steps included Solomon’s diligence.
“What we saw in Australia was a tremendous drop of hydrogen chloride” in the satellite data, Solomon says. “I thought, gosh, this looks like Antarctica. How can this happen in Australia?”
Hydrogen chloride gas is a product of the breakdown of chlorofluorocarbons, which can remain in the stratosphere for decades. The cold environment over Antarctica was part of the key to the formation of the ozone hole, because at those temperatures hydrogen chloride gas can dissolve into frozen clouds passing through the stratosphere. The release of that gas is essential to the start of the chain reaction that forms the ozone-depleting chemicals.
The air in Australia is too warm for this process – but satellite data indicated that some still remove hydrogen chloride gas from the atmosphere. Solomon and his team realized that the culprit was organic particles in the smoke. Those particles can absorb hydrogen chloride gas even at warmer temperatures, kicking off the essential first step.
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When hydrogen chloride is absorbed, the smoke particles can act as catalysts, helping to speed up other reactions in the atmosphere. In particular, particles amp the transformation of other chlorine-containing gases floating in the stratosphere, such as chlorine nitrate and hypochlorous acid, into chlorine compounds highly reactive to the sun.
The mixing of the sun’s ultraviolet rays with those new chlorine compounds produces chlorine radicals that release highly reactive chemical molecules and especially enjoy attacking ozone molecules.
The discovery of this fire-related process to destroy ozone is a worrying potential hazard to the ozone layer, Solomon says. The 2010 Montreal Protocol banned the use of chlorofluorocarbons, an act that encouraged success in reducing the hole in the ozone over Antarctica (SN: 2/10/21). The ozone layer is showing signs of recovery since, on the order of 1 percent over the past decade, he says.
However, the smoke from the Australian bushfires more or less “destroyed all the work” for the year Solomon adds.
Climate change is expected to increase the intensity and frequency of wildfires around the world, sending higher fire clouds high into the sky.SN: 12/15/20). If these fires are “one-time,” maybe it’s not so bad for ozone recovery, Solomon says. “But if it happens every fifth year, that’s a different pot of fish.”
The study neatly explains the complicated satellite observations made of the Australian wildfires, says Ross Salawitch, an atmospheric chemist at the University of Maryland in College Park who was not involved in the work. A drop in hydrogen chloride highlights, he says, as well as a surprise in other chlorine compounds, such as chlorine nitrate and chlorine oxide.
But the “icing of the cake,” Salawitch says, is how the discovery of organic components can improve our understanding of what controls the size of the ozone hole. This is important, not only because we want to get each one right, but because “one of the unfortunate consequences of global warming is likely to increase the frequency and ferocity of fires.”
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