MIT study links Australian bushfires to ozone depletion via smoke chemistry

A wildfire can loft smoke into the stratosphere, where particles may linger for more than a year. A study from the Massachusetts Institute of Technology in the United States shows that as long as these particles stay suspended, they can participate in chemical reactions that erode the ozone layer. The ozone shield protects the earth from harmful ultraviolet radiation from the sun.

The study, published in Nature, examines smoke from the Black Summer megafire that burned across eastern Australia from December 2019 to January 2020. The blaze covered vast areas and released more than a million tons of smoke into the atmosphere.

The MIT researchers identify a novel chemical reaction in which smoke particles from Australian bushfires intensify ozone depletion. By driving this reaction, the fires likely contributed to a 3–5 percent drop in total ozone in the mid-latitudes of the southern hemisphere, affecting regions including Australia, New Zealand, parts of Africa, and portions of South America.

Smoke particles from wildfires seem to erode the ozone layer PEXELLER

The researchers’ models also indicate that the fires altered conditions in polar regions and wore away the edges of the ozone hole over Antarctica. By the end of 2020, smoke from Australian bushfires had broadened the Antarctic ozone hole by about 2.5 million square kilometers, roughly 10 percent of its surface compared to the previous year.

Threat to the recovery of the ozone layer

The long-term impact of wildfires on ozone recovery remains uncertain. A United Nations assessment has noted improvements in the ozone hole and global depletion due to sustained international efforts to phase out ozone-depleting substances.

Yet MIT researchers warn that if the lingering chemicals persist in the atmosphere, large fires could trigger reactions that temporarily reduce ozone. Susan Solomon, the Lee and Geraldine Martin Professor of Environmental Studies at MIT and a pioneer in identifying the chemicals responsible for the Antarctic ozone hole, calls the 2020 Australian fires a wake-up call for the scientific community.

The impact of wildfires had not been fully considered in ozone recovery projections. Solomon notes that the effect could be tied to how often fires occur and how intense they become as the planet warms.

The ozone layer is under threat again Pixabay

The study headed by Solomon and MIT graduate student Peidong Wang, with collaborators from the Guangzhou Institute for Environmental and Climate Research, the National Oceanic and Atmospheric Administration, the National Center for Atmospheric Research, and Colorado State University, builds on a discovery made in 2022. That earlier work linked forest fires to ozone depletion through a chemical process tied to smoke and chlorine chemistry.

The team found that chlorine-containing compounds originally emitted by human activities, in the form of chlorofluorocarbons, can interact with the surface of fire aerosols. This interaction can initiate a cascade that produces chlorine monoxide, a potent ozone-depleting molecule. The results suggest that Australian bushfires likely depleted ozone through this newly identified chemical pathway.

Solomon explains that the earlier explanation did not account for all observed stratospheric changes. The current study examines the composition of molecules in the stratosphere after the Australian fires and analyzes three independent satellite data sets. It shows that hydrochloric acid concentrations fell in mid-latitudes in the months following the fires, while chlorine monoxide levels rose.

Hydrochloric acid interacts with smoke

Hydrochloric acid, present as chlorine-containing compounds in the stratosphere, naturally degrades over time. When chlorine remains bound in the form of hydrochloric acid, it cannot destroy ozone. If HCl breaks down, chlorine can react with oxygen to form chlorine monoxide, which damages the ozone layer.

Smoke particles from wildfires seem to erode the ozone layer PEXELLER

In polar regions, HCl can break apart by interacting with cloud surfaces at extremely low temperatures around 155 Kelvin, but this reaction was not expected to occur in mid-latitudes where temperatures are higher.

Solomon wondered if HCl could interact with smoke particles at elevated temperatures and release chlorine to destroy ozone. If such a reaction is possible, it could explain the instability of molecules and much of the ozone destruction observed after the Australian bushfires.

The team scanned chemical literature to identify organic molecules that could react with HCl at higher temperatures. Solomon recalls that HCl is remarkably soluble in many organic species and tends to cling to numerous compounds.

Smoke particles from wildfires seem to erode the ozone layer PEXELLER

As the researchers expanded their search, they found that smoke particles persisted for months, circling the mid-latitude stratosphere in the same regions and coinciding with periods when HCl concentrations dropped. Solomon notes that the older smoke particles absorb much of the HCl, and the same reactions observed in the ozone hole can occur at mid-latitudes but at higher temperatures.

When the team simulated this new chemical reaction using atmospheric chemistry models that mirror Australian bushfire conditions, they estimated about 5 percent ozone depletion across the entire stratosphere and about a 10 percent increase in the ozone hole observed at mid-latitudes near Antarctica.

The interaction with HCl is likely a key pathway for wildfire-driven ozone loss, though Solomon suggests there could be other chlorine-containing compounds drifting in the stratosphere released by fires.

The researchers emphasize that time is of the essence as these processes unfold and may become more pronounced with changing fire regimes.

This work reframes how scientists view wildfire impacts on atmospheric chemistry and ozone recovery, highlighting the need for continued observation and modeling of fire-derived aerosols and halogen chemistry.

A comprehensive view of the findings is available in the Nature article cited in this summary.