Scientists link wildfire smoke to ozone depletion

Smoke generated by Australia’s 2019-2020 wildfires — which emitted a mass of particles comparable to that of an erupting volcano — contributed to the breakdown of the Earth’s ozone layer, a new study has found.  

These devastating “Black Summer” fires scorched more than 43 million acres of land while displacing nearly 3 billion animals and injecting more than 1 million tons of smoke particles into the atmosphere, according to the scientists, who published their findings in the Proceedings of the National Academy of Sciences on Monday.

That smoke, they explained, reached up to 35 kilometers above the Earth’s surface and set off chemical reactions in the stratosphere that were destructive to ozone. 

“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,” lead author Susan Solomon, a professor of environmental studies at the Massachusetts Institute of Technology, said in a statement.

“It’s another wakeup call, just as the Antarctic ozone hole was, in the sense of showing how bad things could actually be,” she added.

Shortly after the fires subsided in March 2020, the authors said they observed a plunge in nitrogen dioxide levels in the atmosphere, which is the first step in what they described as “a chemical cascade” resulting in ozone depletion. That drop, the researchers found, directly correlated to the amount of smoke the fires bellowed into the atmosphere.

In total, the scientists estimated that this smoke-induced reaction depleted the column of ozone by 1 percent. As a basis of comparison, they explained, the global phaseout of ozone-depleting gases has led to about a 1 percent ozone recovery for decreases over the past decade.

The new study establishes the first direct link between wildfire smoke and ozone depletion, which stemmed from an initial inkling Solomon had that smoke from fires might deplete ozone through a chemical reaction similar to that of volcanic aerosols, a news release accompanying the study said. Solomon had previously discovered that particles from these eruptions — which can reach the stratosphere — can destroy ozone in this manner. 

“This chemistry, once you get past that point, is well-established,” Solomon said. “Once you have less nitrogen dioxide, you have to have more chlorine monoxide, and that will deplete ozone.”

The scientists used nitrogen dioxide data from three independent satellites that have been surveying the Southern Hemisphere. They then compared each satellite’s record of the months and years that preceded the fires to the data from March 2020. 

To confirm the direct link between the smoke and ozone depletion, they also injected a cloud of smoke particles into a three-dimensional model to simulate the suspected chemical reactions.

“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 said.

Acknowledging that this may be just “one chemical mechanism among several” responsible for ozone depletion, Solomon stressed that the link is “clearly there.”

“Wildfire smoke is a toxic brew of organic compounds that are complex beasts,” she added. “And I’m afraid ozone is getting pummeled by a whole series of reactions that we are now furiously working to unravel.”

–Updated on March 1 at 8:48 a.m.