Home Sciences New enemy for the ozone layer: forest fires

New enemy for the ozone layer: forest fires

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The ozone layer seemed to be recovering after the international agreements that identified CFC gases as the cause of the hole that was destroying this layer of the atmosphere. However, it has now been discovered that another factor may undo the gains made so far: large forest fires. For the first time it has been proven that they are destroying this protective shield against solar radiation.

The Australian bushfires in 2019 and 2020 were historic in how far and how fast they spread, as well as how long they lasted. In total, the devastating ‘Black Summer’ fires razed more than 18 million hectares of land and killed or displaced nearly 3 billion animals. fires too they injected more than a million tons of smoke particles into the atmosphere, reaching up to 35 kilometers in height, a mass and range comparable to that of an erupting volcano.

Now, atmospheric chemists at the Massachusetts Institute of Technology (MIT) in the US have discovered that the smoke from these fires triggered chemical reactions in the stratosphere that contributed to the destruction of ozone, a layer that protects the Earth from ultraviolet radiation from of the sun.

The study is 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, which is the first step in a chemical cascade known to end in ozone depletion.

The researchers found that this drop in nitrogen dioxide is directly related to the amount of smoke the fires released into the stratosphere. They estimate that this smoke-induced chemistry depleted the column of ozone by 1 percent. That is the same percentage in which the ozone layer has recovered in the last ten years thanks to the international agreements signed to stop its destruction.

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That means those wildfires undid those hard-won international achievements in a short span of time. If future wildfires become stronger and more frequent, as climate change is projected to do, the projected recovery of ozone could be delayed by years.

“The Australian fires appear to be the largest such event to have occurred so far, but as the world continues to warm, there is every reason to think that these fires will be more frequent and intense“It’s another wake-up call, just like the ozone hole in Antarctica was.

Smoke up to the stratosphere

Massive wildfires are known to generate pyrocumulonimbus, towering clouds 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 Australia’s bushfires reached the stratosphere, up to 35 kilometers.

In 2021, study co-author Pengfei Yu of Jinan University conducted a separate study of the impacts of the fires and found that accumulated smoke warmed parts of the stratosphere by as much as 2 degrees Celsius, a warming that persisted. for six months. The study also found evidence of ozone destruction in the southern hemisphere after the fires.

Solomon wondered if smoke from the fires might have depleted ozone through chemistry similar to volcanic aerosols. Large volcanic eruptions can also reach the stratosphere, and in 1989 Solomon discovered that particles from these eruptions can destroy ozone through a series of chemical reactions.

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Just like volcanic eruptions

In the new study, Solomon and colleagues looked at how nitrogen dioxide concentrations in the stratosphere changed after the Australian fires. If these concentrations fell significantly, it would be a sign that smoke from wildfires depletes ozone through the same chemical reactions as some volcanic eruptions.

The team analyzed nitrogen dioxide observations taken by three independent satellites that have surveyed the southern hemisphere for varying lengths of time. They compared the record from each satellite in the months and years before and after the Australian fires. All three records showed a significant drop in nitrogen dioxide in March 2020.

To verify that the decrease in nitrogen dioxide was a direct chemical effect of smoke from the fires, the researchers ran atmospheric simulations using a global three-dimensional model that simulates hundreds of chemical reactions in the atmosphere, from the surface to the stratosphere.

Now scientists are investigating other reactions triggered by wildfire smoke that could further contribute to ozone depletion. At the moment, the main driver of ozone depletion remains chlorofluorocarbons, or CFCs, chemicals like old refrigerants that have been banned under the Montreal Protocol, yet remain in the stratosphere. But as global warming leads to stronger and more frequent wildfires, their smoke could have a serious and lasting impact on ozone.

“Wildfire smoke is a toxic mix of organic compounds that are complex beasts”Solomon says. “And I’m afraid the ozone is being hit by a whole series of reactions that we’re now working frantically to unravel,” he adds.

Reference article: https://news.mit.edu/2022/wildfire-smoke-ozone-depletion-0228

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