Atmospheric chemists discovered that smoke from Australia’s ‘Black Summer’ wildfires triggered chemical reactions in the stratosphere that contributed to ozone depletion. The research is the first to show a link between wildfire smoke and ozone depletion.
The Australian wildfires of 2019 and 2020 were unprecedented in terms of how far and fast they spread, as well as how long and powerfully they burned. The devastating “Black Summer” fires scorched more than 43 million acres of land, killing or displacing nearly 3 billion animals. The fires also released over a million tons of smoke particles into the atmosphere, reaching up to 35 kilometers above the Earth’s surface – a mass and reach comparable to an erupting volcano.
MIT atmospheric chemists have discovered that the smoke from those fires triggered chemical reactions in the stratosphere, which contributed to the destruction of ozone, which protects the Earth from incoming ultraviolet radiation. The study, published in the Proceedings of the National Academy of Sciences, is the first to demonstrate a chemical link between wildfire smoke and ozone depletion.
The team noticed a sharp drop in nitrogen dioxide in the stratosphere in March 2020, shortly after the fires had subsided. This is the first step in a chemical cascade that is known to end in ozone depletion. The researchers discovered that the decrease in nitrogen dioxide is directly proportional to the amount of smoke released into the stratosphere by the fires. They estimate that this smoke-induced chemistry depleted the column of ozone by 1 percent.
The Australian fires look like the biggest event so far, but as the world continues to warm, there is every reason to think these fires will become more frequent and more intense. It’s another wakeup call, just as the Antarctic ozone hole was, in the sense of showing how bad things could actually be.
Susan Solomon
To put this in context, they note that the phaseout of ozone-depleting gases under a worldwide agreement to stop their production has led to about a 1 percent ozone recovery from earlier ozone decreases over the past 10 years — meaning that the wildfires canceled those hard-won diplomatic gains for a short period. If future wildfires grow stronger and more frequent, as they are predicted to do with climate change, ozone’s projected recovery could be delayed by years.
“The Australian fires look like the biggest event so far, but as the world continues to warm, there is every reason to think these fires will become more frequent and more intense,” says lead author Susan Solomon, the Lee and Geraldine Martin Professor of Environmental Studies at MIT. “It’s another wakeup call, just as the Antarctic ozone hole was, in the sense of showing how bad things could actually be.”
The study’s co-authors include Kane Stone, a research scientist in MIT’s Department of Earth, Atmospheric, and Planetary Sciences, along with collaborators at multiple institutions including the University of Saskatchewan, Jinan University, the National Center for Atmospheric Research, and the University of Colorado at Boulder.
Chemical trace
Massive wildfires are known to produce pyrocumulonimbus clouds, which can reach the stratosphere, the layer of the atmosphere between 15 and 50 kilometers above the Earth’s surface. The smoke from Australia’s wildfires reached as high as 35 kilometers into the stratosphere.
In 2021, Solomon’s co-author, Pengfei Yu of Jinan University, conducted a separate study of the fires’ effects and discovered that the accumulated smoke warmed parts of the stratosphere by up to 2 degrees Celsius, with the warming lasting six months. The study also discovered evidence of ozone depletion in the Southern Hemisphere as a result of the fires.
Solomon wondered whether smoke from the fires could have depleted ozone through a chemistry similar to volcanic aerosols. Major volcanic eruptions can also reach into the stratosphere, and in 1989, Solomon discovered that the particles in these eruptions can destroy ozone through a series of chemical reactions. As the particles form in the atmosphere, they gather moisture on their surfaces. Once wet, the particles can react with circulating chemicals in the stratosphere, including dinitrogen pentoxide, which reacts with the particles to form nitric acid.
Fig: Chemical link between wildfire smoke and ozone depletion
Dinitrogen pentoxide normally reacts with the sun to produce a variety of nitrogen species, including nitrogen dioxide, a compound that binds to chlorine-containing chemicals in the stratosphere. When volcanic smoke converts dinitrogen pentoxide to nitric acid, nitrogen dioxide decreases and chlorine compounds morph into chlorine monoxide, the primary man-made agent that depletes ozone.
“Once you get past that point, this chemistry is well-established,” Solomon says. “With less nitrogen dioxide, you have to have more chlorine monoxide, which depletes ozone.”
Cloud injection
In the new study, Solomon and her colleagues looked at how concentrations of nitrogen dioxide in the stratosphere changed following the Australian fires. If these concentrations dropped significantly, it would signal that wildfire smoke depletes ozone through the same chemical reactions as some volcanic eruptions.
The researchers examined nitrogen dioxide observations from three independent satellites that have surveyed the Southern Hemisphere for varying lengths of time. They compared the records of each satellite in the months and years preceding and following the Australian fires. In March 2020, all three records showed a significant decrease in nitrogen dioxide. The drop represented a record low among observations spanning the last 20 years for one satellite’s record.
The researchers conducted atmospheric simulations using a global, three-dimensional model that simulates hundreds of chemical reactions in the atmosphere, from the surface up through the stratosphere, to ensure that the nitrogen dioxide decrease was a direct chemical effect of the fires’ smoke.
The team injected a cloud of smoke particles into the model to simulate the Australian wildfires. They assumed that the particles gathered moisture, similar to volcanic aerosols. They then ran the model several times and compared the results to simulations that did not include the smoke cloud. The team discovered that as the amount of smoke particles increased in the stratosphere, nitrogen dioxide concentrations decreased, correlating with the observations of the three satellites.
“The behavior we saw, of more and more aerosols, and less and less nitrogen dioxide, in both the model and the data, is a fantastic fingerprint,” Solomon says. “It’s the first time that science has established a chemical mechanism linking wildfire smoke to ozone depletion. It may only be one chemical mechanism among several, but it’s clearly there. It tells us these particles are wet and they had to have caused some ozone depletion.”
She and her colleagues are investigating other reactions triggered by wildfire smoke that could contribute to ozone depletion. For the time being, the main cause of ozone depletion is chlorofluorocarbons, or CFCs – chemicals such as old refrigerants that have been banned under the Montreal Protocol but still linger in the stratosphere. However, as global warming causes stronger, more frequent wildfires, their smoke could have a serious, long-term impact on ozone.
“Wildfire smoke is a toxic brew of organic compounds that are complex beasts,” Solomon says. “And I’m afraid ozone is being battered by a cascade of reactions that we’re now working feverishly to unravel.”
