Smoke particles from forest fires can trigger chemical reactions in the atmosphere that contribute to the depletion of the protective ozone layer. This is the finding of a study that examined changes in atmospheric chemistry after the Australian wildfires of December 2019 and January 2020. Accordingly, the smoke particles activate chlorine compounds that were once released into the atmosphere through the release of chlorofluorocarbons (CFCs), which have since been banned. If wildfires become more frequent as a result of global warming, this could endanger the recovery of the ozone layer.
Chlorofluorocarbons (CFCs) have been used since the 1930s, for example as a propellant for spray cans and in the production of foams, as a coolant for refrigerators and freezers, or as a cleaning agent. But in the 1980s, CFCs were found to be depleting the ozone layer, which protects the earth from the sun’s UV rays. An ozone hole had formed over the South Pole, which still occurs today in the Antarctic spring. CFCs have therefore been banned in industrialized countries since 1995 and worldwide since 2010. But the harmful chemicals remain in the atmosphere for decades, slowing down the recovery of the ozone layer.
In December 2019 and January 2020, devastating bushfires raged in eastern Australia. Several million hectares of land went up in flames and around 900,000 tons of smoke rose into the stratosphere. Satellite data showed that this was accompanied by major changes in atmospheric chemistry, including a significant drop in ozone levels. However, it was initially unclear how the smoke from the forest fires damaged the ozone layer.
Forest fire aerosols start chemical cascade
A team led by Susan Solomon from the Massachusetts Institute of Technology (MIT) in Cambridge has now discovered which chemical reactions are triggered by the smoke particles and lead to ozone depletion. “We find that the aerosol from wildfires, which contains a mixture of oxidized organic matter and sulfur compounds, activates reactive chlorine species in the stratosphere, thereby increasing ozone depletion rates,” explains the team.
As early as 2022, Solomon and her team published a study that found that chlorine-containing compounds that originally entered the stratosphere in the form of CFCs can react with the surface of forest fire aerosols. This interaction sets in motion a chemical cascade that produces chlorine monoxide, a molecule that reacts strongly to ozone, breaking it down. “But that doesn’t explain all the changes observed in the stratosphere,” says Solomon. “There was a whole bunch of chlorine-related chemical reactions that got completely out of hand.”
Reactions with hydrochloric acid
For the current study, the researchers analyzed the composition of the molecules in the stratosphere after the forest fires in more detail. Using three independent sets of satellite data, they found that after the fires, the concentration of hydrochloric acid (HCl) in the stratosphere decreased significantly in mid-latitudes, regions overlying Australia, New Zealand and parts of Africa and South America. At the same time, chlorine monoxide (ClO) levels skyrocketed. “The fact that HCl levels were declining at an unprecedented rate in the mid-latitudes was kind of a danger signal to me,” says Solomon.
Like other chlorine compounds, the HCl in the stratosphere is a breakdown product of CFCs. However, as long as the chlorine is bound in the hydrochloric acid, it cannot harm the ozone layer. In the Antarctic winter, the compound can break apart when the molecule comes into contact with the surface of clouds at very low temperatures. Then, when the sun returns to this area in spring, photochemical reactions of these chlorine radicals with the ozone lead to ozone depletion and the formation of the ozone hole over the South Pole. According to current knowledge, however, these reactions should not occur in mid-latitudes at much warmer temperatures.
Race against time
However, Solomon and her team showed that smoke particles that remain in the stratosphere for months after the forest fires contain compounds that can break down HCl even without extreme cold. “It’s the aged smoke particles that pick up most of the HCl,” reports Solomon. “Amazingly, the same reactions as the ozone hole occur, but at mid-latitudes and at much warmer temperatures.” When the team incorporated this new chemical reaction into an atmospheric chemistry model and simulated the conditions of the Australian wildfires, they observed a three- five percent ozone depletion in the stratosphere at mid-latitudes and a ten percent increase in the ozone hole over Antarctica – a finding that agrees well with the data measured in reality.
“The Australian fires of 2020 were a real wake-up call for science,” says Solomon. “The impact of forest fires has so far not been taken into account in the forecasts for the recovery of the ozone layer.” However, the current study shows that smoke aerosols can also convert the breakdown products of CFCs into reactive chlorine compounds. “Now it’s kind of a race against time,” says Solomon. “We can only hope that the chlorine-containing compounds are depleted before the frequency of fires increases with climate change. All the more reason to be vigilant about global warming and these chlorine-containing compounds.”
Source: Susan Solomon (Massachusetts Institute of Technology (MIT), Cambridge, USA) et al., Nature, doi: 10.1038/s41586-022-05683-0