Smoke from forest fires erodes the ozone layer

Smoke particles from forest fires can erode the ozone layer, according to a study by the Massachusetts Institute of Technology (MIT), which shows that the Australian fires widened the ozone hole by 10% in 2020.

A forest fire can pump smoke into the stratosphere, where the particles drift for more than a year; The new work confirms that, while they remain suspended, they can trigger chemical reactions that erode the ozone layer, in charge of protecting the Earth from the sun's harmful ultraviolet radiation.

The research is published in the journal 'Nature' and focuses on smoke from the mega-fire in eastern Australia, between December 2019 and January 2020.

The team identified a new chemical reaction whereby smoke particles from Australian bushfires worsened ozone depletion.

By triggering this reaction, the fires probably contributed to a 3-5% reduction in total ozone in the mid-latitudes of the southern hemisphere, in the regions covering Australia, New Zealand and parts of Africa and South America.

How much of the ozone layer was destroyed by bushfires?

According to a study published in the scientific journal Nature on Thursday, the Australian bushfires that occurred from 2019 to 2020 caused a significant depletion of the ozone layer, resulting in 3 to 5 percent of its destruction.

The lead author of the study, Susan Solomon, an atmospheric chemist at the Massachusetts Institute of Technology (MIT), explained that the depletion was so severe that it resulted in the lowest recorded levels of ozone in some areas.

This damage temporarily negated the progress made by the Montreal Protocol, a 1987 agreement that aimed to phase out ozone-depleting substances.

The study also noted that the ozone hole expanded for a prolonged period, which is a cause for concern.

The fires affected the polar regions

The researchers' computer simulations also indicate that the fires affected the polar regions, eroding 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 kilometres, 10% of its area compared to the previous year.

Despite signs of recovery, this MIT study suggests that as long as these chemicals persist in the atmosphere, large fires could trigger a reaction that temporarily depletes ozone.

"The effect of wildfires has not been accounted for in ozone recovery projections before, and I think that effect may depend on whether fires become more frequent and intense as the planet warms," ​​says Susan Solomon.

The new study builds on a discovery made in 2022 by Solomon and colleagues, in which they first identified a chemical link between wildfires and ozone depletion.

They then discovered that chlorine-containing compounds, originally emitted by factories in the form of chlorofluorocarbons (CFCs), could react with the surface of fire aerosols.

Wildfires can deplete ozone

This interaction triggers a chemical cascade that produces 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.

For the new work, the team examined three independent sets of satellite data and looked at the role and behaviour of hydrochloric acid (HCl), present in the stratosphere as CFCs break down over time.

In principle, as long as the chlorine is bound as 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.

As indicated in the publication, "these changes in atmospheric chemical composition suggest that aerosols from wildfires affect the depleting chemistry of stratospheric chlorine and ozone."

In polar regions, HCl can break up when it interacts with the surface of cloud particles at frigid temperatures, however, this reaction was not expected to occur in mid-latitudes, where temperatures are much warmer.

The reaction with HCl is probably the main pathway by which wildfires can deplete ozone, but Solomon surmises that there may be other chlorine-containing compounds drifting in the stratosphere, which wildfires could unlock.

"Now we are in a kind of race against time," she says. "Hopefully the chlorine-containing compounds have been destroyed, before the frequency of fires increases with climate change".

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