Study: Increased wildfire risk could delay ozone layer's recovery

The smoke released by major wildfires likely contributes to the destruction of ozone, the vital atmospheric shield that protects the planet by absorbing most of the sun's harmful ultraviolet radiation, according to a new study published in Nature on Wednesday.

Why it matters: Wildfires are projected to become more frequent and intense over the coming decades due to global warming and land-use change.

• A UN-backed panel of scientists estimated in January that the recovery of Earth's ozone layer was back on track. But increased levels of wildfire smoke in the atmosphere could delay that recovery, the researchers behind the new study found.

Details: The researchers studied the aerosol from major fires in Australia during the austral summer of 2019–2020 to try to better understand the chemical mechanism behind wildfire smoke contributing to ozone degradation.

• During that time, which was later dubbed the "black summer," several devastating fires scorched about 59 million acres, killed at least 33 people and killed, wounded or displaced millions of animals.
• Blazes during that fire season also produced massive pyrocumulonimbus cloud towers that injected around 0.9 teragrams (nearly 1 trillion grams) of wildfire smoke into the lower stratosphere.

How it works: The researchers hypothesized that oxidized organics, particularly organic acids, within smoke released by wildfires increase the solubility of hydrochloric acid (HCl) in the lower stratosphere.

• As HCl dissolves into smoke particles under stratospheric conditions, reactive chlorine species are activated and contribute to the destruction of ozone molecules, the researchers proposed.
• Just one reactive chlorine atom in the stratosphere can destroy tens of thousands of ozone molecules before it reacts with another gas and finally stabilizes. After reacting with chlorine atoms, ozone breaks down into normal oxygen, which does not absorb harmful ultraviolet radiation from the Sun.
• The researchers tested their hypothesis by comparing simulations of the proposed chemical process with atmospheric observations of the Southern Hemisphere mid-latitudes following the 2019–2020 Australian wildfires.
• The simulations produced inconsistencies in the levels of chlorine species and ozone that were "in remarkable agreement" with the observations, which supported their hypothesis.

By the numbers: The researchers' model also suggested that smoke released by the Australian wildfires chemically depleted between 3% to 5% of the total ozone column in the Southern Hemisphere mid-latitudes in June and July of 2020.

• The researchers noted that the change is relatively small compared to other forms of ozone loss in the mid-latitudes, but they stressed the effect could become more pronounced if wildfires become more frequent or intense in the future.

Between the lines: The researchers note the chemistry described in the study should also occur from wildfire smoke released in Northern Hemisphere, as well as from biomass burning, pollution and possibly aircraft emissions.

What they're saying: Susan Solomon, a climate scientist at MIT and the lead author of the study, told Axios she believes the ozone layer's full recovery may be a race between the decay of harmful chemicals called chlorofluorocarbons in the atmosphere and increasingly intense wildfires sending more smoke into the atmosphere.

• Solomon said previous studies have shown links between wildfire smoke and ozone decay, but the new study was the first to evaluate the dissolution of stratospheric chlorine compounds into smoke particles.
• "That's exactly the same thing as what happens in polar stratospheric clouds, except that it's happening at temperatures that are much, much warmer, because the oxidized organics have such a high solubility of hydrochloric acid," she said.

Alan Robock, a climate scientist at Rutgers University who is not associated with the study, said the findings will help climatologists make more accurate predictions about how smoke in the stratosphere may affect weather and atmospheric circulation.

• "We need to understand how the whole system works in order to predict what will happen and also to understand how global warming can affect climate change," Robock told Axios.

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