A new study from the Massachusetts Institute of Technology (MIT) in the United States has found that the smoke particles that pump fires into the stratosphere drift for more than a year and can trigger chemical reactions that erode the protective layer of ozone that protects Earth from harmful ultraviolet radiation from the sun.
The study, published in the journal Nature, focuses on smoke from the Black Summer megafire in eastern Australia, which burned between December 2019 and January 2020.
These fires - the most devastating recorded in the country - burned tens of millions of hectares and pumped more than a million tons of smoke into the atmosphere.
The MIT team identified a new chemical reaction whereby smoke particles from Australian bushfires intensified ozone depletion.
By triggering this reaction, the fires probably contributed to 3-5% depletion of total ozone in the mid-latitudes of the southern hemisphere, in regions above Australia, New Zealand, and parts of Africa and South America.
The researchers' modeling also indicates that the fires affected the polar regions and eroded the edges of the ozone hole over Antarctica. At the end of 2020, smoke particles from Australian bushfires expanded the Antarctic ozone hole by 2.5 million square kilometers, 10% of its area compared to the previous year.
It is unclear what long-term effect wildfires will have on ozone recovery. The United Nations recently reported that the ozone hole, and global ozone depletion, is on the mend, thanks to a sustained international effort to phase out ozone-depleting chemicals.
But the MIT study suggests that as long as these chemicals persist in the atmosphere, large fires could trigger a reaction that temporarily depletes ozone.
“The 2020 Australian fires were really a wake-up call for the scientific community,” says Susan Solomon, the Lee and Geraldine Martin Professor of Environmental Studies at MIT and a leading climate scientist who first identified the chemicals responsible for the Antarctic ozone hole.
"The effect of forest fires had not been taken into account before in ozone recovery projections," he acknowledges, "and I think that effect may depend on whether fires become more frequent and intense as the planet warms. says the researcher
Solomon and his colleagues found that chlorine-containing compounds, originally emitted from factories in the form of chlorofluorocarbons (CFCs), could react with the surface of fire aerosols.
This interaction triggered a chemical cascade that produced chlorine monoxide, the molecule that most destroys the ozone layer. Their results showed that the Australian bushfires likely depleted ozone through this newly identified chemical reaction.
"But that didn't explain all of the observed changes in the stratosphere," Solomon acknowledges. "There was a lot of chlorine-related chemistry that was totally off the mark."
In the new study, the team took a closer look at the composition of molecules in the stratosphere after the Australian bushfires.
They examined three independent sets of satellite data and found that in the months after the fires, concentrations of hydrochloric acid dropped sharply in the mid-latitudes, while concentrations of chlorine monoxide increased.
Hydrochloric acid (HCl) is present in the stratosphere as CFCs break down naturally over time. As long as chlorine is bound in the form of HCl, it has no chance of destroying ozone. But if the HCl breaks down, the chlorine can react with oxygen to form chlorine monoxide, which destroys the ozone layer.
In polar regions, HCl can break apart by interacting with the surface of cloud particles at frigid temperatures of about 155 Kelvin. However, this reaction was not expected to occur in mid-latitudes, where temperatures are much warmer.
So Solomon wondered if HCl could also interact with smoke particles, at warmer temperatures and in a way that would release chlorine to destroy ozone. If such a reaction were possible, it would explain the imbalance of molecules and much of the ozone destruction observed after the Australian bushfires.
The team searched the chemical literature to see what kinds of organic molecules could react with HCl at warmer temperatures to break it down. "I found that HCl is extremely soluble in a wide range of organic species," he recalls. "It likes to stick to a lot of compounds."
The question then was whether the Australian bushfires released any of those compounds that could have triggered the breakdown of HCl and consequent ozone depletion. When the team analyzed the composition of the smoke particles in the first few days after the fires, the picture was far from clear.
"I looked at these things and I put my hands to my head thinking: there's so much stuff in there, how am I going to find out," Solomon recalls, "but then I realized that it had actually been a few weeks before it was seen. HCl decline, so you really have to look at the data on aged particles from wildfires."
When the team expanded their search, they found that the smoke particles persisted for months, circulating in the mid-latitude stratosphere, in the same regions and at times when HCl concentrations fell.
"It's the aged smoke particles that actually absorb a lot of the HCl," Solomon reveals. "And then you get, surprisingly, the same reactions as in the ozone hole, but over mid-latitudes, at much warmer temperatures." .
When the team incorporated this new chemical reaction into an atmospheric chemistry model and simulated Australian bushfire conditions, they observed a 5% depletion of ozone throughout the mid-latitude stratosphere and a 10% increase in the ozone hole above the antartida.
The reaction with HCl is probably the main route by which forest fires can deplete ozone. But Solomon surmises that there may be other chlorine-containing compounds drifting in the stratosphere, which wildfires could unlock.
"We are now in a kind of race against time," he acknowledges. "Hopefully the chlorine-containing compounds have been destroyed, before the frequency of fires increases with climate change. All the more reason to be vigilant about global warming and these chlorine-containing compounds.
