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Study Reveals Chemical Link Between Wildfire Smoke and Ozone Depletion – Is It Up With That?


If wildfires become larger and more frequent, they could stall ozone recovery for years.

Peer-reviewed publications

MASSACHUSETTS INSTITUTE OF TECHNOLOGY

The Australian bushfires in 2019 and 2020 are a history of how widespread and fast they were and how long they burned intensely and for a long time. All told, the terrifying “Black Summer” fires burned more than 43 million acres, and killed or displaced nearly 3 billion animals. The fires also spewed more than 1 million tons of smoke particles into the atmosphere, up to 35 kilometers above Earth’s surface – a mass equivalent to an erupting volcano.

Now, atmospheric chemists at MIT have discovered that smoke from those fires causes chemical reactions in the stratosphere that contribute to ozone destruction, which shields Earth from incoming ultraviolet radiation. Research by the group, appearing in Proceedings of the National Academy of Scienceswas the first to establish a chemical link between wildfire smoke and ozone depletion.

In March 2020, shortly after the fires subsided, the team observed a sharp drop in nitrogen dioxide in the stratosphere, the first step in a chemical cascade thought to end in depletion. ozone layer reduction. The researchers found that this drop in nitrogen dioxide directly correlates with the amount of smoke the fire releases into the stratosphere. They estimate that this smoke-generating chemistry depletes the ozone column by 1%.

To put this in context, they note that the removal of ozone-depleting gases under a worldwide agreement to cease their production resulted in an ozone recovery of about 1% from the Ozone declines earlier in the last 10 years – meaning the wildfires have canceled out – short-lived diplomatic gains. If future wildfires grow stronger and more frequent, as they are predicted by climate change, the projected recovery of ozone could be delayed for years.

Lead author Susan Solomon, Professor Lee and Geraldine Martin of Environmental Research at MIT. “It’s another wake-up call, like the ozone hole over Antarctica, meant to show how bad things can really 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 numerous institutions including the University of Saskatchewan, the University of California, and University of Science and Technology. South, the National Center for Atmospheric Research and the University of Colorado at Boulder.

Chemical traces

Large wildfires are thought to produce pyrocumulonimbus – towering plumes of smoke that can reach the stratosphere, the layer of the atmosphere that lies between 15 and 50 kilometers above the Earth’s surface. Smoke from the Australian bushfires has reached the stratosphere, reaching up to 35 km high.

In 2021, Solomon’s co-author, Pengfei Yu at Jinan University, did a separate study on the effects of the fires and found that the accumulated smoke had warmed parts of the stratosphere. up to 2 degrees Celsius – a warming that lasts for six months. . The study also showed signs of ozone layer destruction in the Southern Hemisphere following the fires.

Solomon wondered if smoke from the fires could deplete the ozone layer through a chemical similar to volcanic aerosols. Large volcanic eruptions can also reach the stratosphere, and in 1989, Solomon discovered that particles in these eruptions could destroy the ozone layer through a series of chemical reactions. As particles form in the atmosphere, they collect moisture on their surface. When wet, the particles can react with chemicals circulating in the stratosphere, including dinitrogen pentoxide, which reacts with the particles to form nitric acid.

Usually, dinitrogen pentoxide reacts with the sun to form many different types of nitrogen, including nitrogen dioxide, a compound that binds with chlorine-containing chemicals in the stratosphere. As volcanic smoke converts dinitrogen pentoxide to nitric acid, the nitrogen dioxide drops, and the chlorine compounds go the other way, turning into chlorine monoxide, the main man-made agent that destroys ozone.

“This chemical, once you get past that point, is well established,” says Solomon. “Once you have less nitrogen dioxide, you have to have more chlorine monoxide, and that depletes ozone.”

Put in the cloud

In the new study, Solomon and her colleagues looked at how nitrogen dioxide levels in the stratosphere change after the fires in Australia. If these concentrations dropped significantly, it would signal that wildfire smoke depletes ozone through chemical reactions like some volcanic eruptions.

The team looked at observations of nitrogen dioxide taken by three independent satellites that surveyed the Southern Hemisphere over different time periods. They compared each satellite’s record for the months and years leading up to and following the fires in Australia. All three records show a significant drop in nitrogen dioxide in March 2020. For one satellite’s record, the drop represents a record low among observations spanning the past 20 years. .

To test whether the reduction in nitrogen dioxide was a direct chemical effect of the smoke, the researchers performed atmospheric simulations using a three-dimensional global model that simulated hundreds of reactions. chemical reactions in the atmosphere, from the surface upward through the stratosphere. .

The team injected a cloud of smoke particles into the model, simulating what was observed from the Australian bushfires. They assumed that the particles, like volcanic aerosols, gather moisture. They then ran the model several times and compared the results with the simulation without the smoke cloud.

In every simulation that incorporated wildfire smoke, the team found that as the amount of smoke particles increased in the stratosphere, the concentration of nitrogen dioxide decreased, consistent with observations by the three satellites.

“The behavior that we see, more and less aerosols and less and less nitrogen dioxide, in both the model and the data, is a great fingerprint,” says Solomon. “This is the first time that science has established a chemical mechanism linking wildfire smoke to ozone depletion. It could be just one chemical mechanism among many, but it clearly is. It tells us these particles are wet and that they must be causing some ozone depletion. “

She and her colleagues are looking at other responses to wildfire smoke that could contribute to further ozone depletion. Currently, the main cause of ozone depletion remains chlorofluorocarbons, or CFCs – chemicals such as old refrigerants that were banned under the Montreal Protocol, although they continue to exist in the stratosphere. But as global warming leads to stronger, more frequent wildfires, their fumes can have serious, long-lasting effects on the ozone layer.

“Fire smoke is a toxic mixture of organic compounds that are complex beasts,” says Solomon. “And I’m afraid ozone is being hampered by a series of reactions that we’re working frantically to unravel.”

This research was supported in part by the National Science Foundation and NASA.

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Written by Jennifer Chu, MIT News Office


JOURNEYS

Proceedings of the National Academy of Sciences

ARTICLE TITLE

On the stratospheric chemistry of mid-latitude wildfire smoke

ARTICLE PUBLICATION DATE

February 28, 2022

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