Large forest fires, phenomena that are becoming more common due to Climate Change Impacthuge amounts of smoke and particulate matter are emitted into the atmosphere, which reduces the presence of nitrogen dioxide (NO2) and causes damage to the ozone layer in the mid-latitudes.
Recurring this type of catastrophic fire could jeopardize the success in protecting the ozone layer achieved through the implementation of the Montreal Protocol, according to a study led by atmospheric chemists from the Massachusetts Institute of Technology (MIT, United States), whose results have been published ( 1 March) in the magazine PNAS.
Research focuses on forest fires Australia in 2019 and 2020where more than 175,000 square kilometers (an area equivalent to half the size of Italy) have been burned, but in the summary of the conclusions a very clear extrapolation appears: “The results indicate that an increase in the activity of wildfires in the world of warming could delay the restoration of the ozone layer.”
so-called fires black summer from Australia pumped more than one million tons of smoke particles into the atmosphere, reaching a height of 35 kilometers above the surface of the earth; Mass and extent similar to an erupting volcano.
Atmospheric chemists at the Massachusetts Institute of Technology found that smoke from those fires triggered chemical reactions in the stratosphere that contributed to the destruction of the ozone layer, which protects Earth from ultraviolet radiation from the sun.
The research authors note that this is the first time that a chemical link has been established between smoke from wildfires and stratospheric ozone depletion.
In March 2020, shortly after the fires subsided, the team noticed a sharp drop in nitrogen dioxide in the stratosphere, the first step in a chemical chain known to end with the depletion of the ozone layer. The researchers found that this decrease in nitrogen dioxide is directly related to the amount of smoke that fires release into the stratosphere. Calculations described in this study indicate that this smoke-induced reaction depleted column ozone by 1 percent.
“Australian fires look like the biggest event yet, but as global warming continues, there is every reason to believe that these fires will become more frequent and intense,” said study lead author Susan Solomon, professor of environmental studies at MIT. “It’s another wake-up call, just like the Antarctic ozone hole, in terms of showing how bad things can be.”
Study co-authors include Ken Stone, a research scientist in the Department of Earth, Atmospheric and Planetary Sciences at MIT, along with collaborators from several institutions, including the University of Saskatchewan, University of Jinan, the National Center for Atmospheric Research and the University of Colorado at Boulder.
After chemical marks
An earlier study of major fires in Australia, led in 2021 by Bingfei Yu of Jinan University, showed that fire and smoke warmed parts of the stratosphere by up to 2 degrees Celsius over the course of six months. The study also found evidence of the destruction of the ozone layer in the southern hemisphere after the fires.
Continuing this line of research, the study now being published focused on the potential effects of smoke and particulate matter on the ozone layer. They discovered that when molecules form in the atmosphere, they accumulate moisture and can react with chemicals circulating in the stratosphere, including nitrogen oxide (nitrogen pentoxide), which reacts with molecules to form nitric acid.
Normally, dinitrogen pentoxide reacts with the sun to form different types of nitrogen, including nitrogen dioxide, a compound that binds to chlorine-containing chemicals in the stratosphere. When volcanic smoke converts dinitrogen pentoxide into nitric acid, the nitrogen dioxide falls and the chlorine compounds take another route, turning into chlorine monoxide, the main human agent that destroys ozone.
In the new study, Solomon and colleagues looked at how nitrogen dioxide concentrations changed in the stratosphere after the Australian fires. If these concentrations drop significantly, it would be a sign that smoke from wildfires is depleting 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 decrease in nitrogen dioxide in March 2020. For the satellite record, the decrease represents the lowest level ever among observations spanning the past 20 years.
To verify that the drop in nitrogen dioxide was a direct chemical effect of smoke from the fires, the researchers conducted atmospheric simulations using a global 3D model that simulates hundreds of chemical reactions in the atmosphere, from the surface to the stratosphere. .
The team injected a cloud of smoke particles into the model to simulate what was observed in Australian bushfires. They thought the particles, like volcanic aerosols, accumulated moisture. Then they ran the model several times and compared the results to a simulation without the smoke cloud.
In each simulation that included wildfire smoke, the team found that as the amount of smoke particles increased in the stratosphere, nitrogen dioxide concentrations decreased, consistent with observations from all three satellites.
“The behavior that we saw, from more and more aerosols and less and less nitrogen dioxide, both in the model and in the data, is a pretty cool fingerprint,” Solomon says. “This is the first time that science has created a chemical mechanism linking wildfire smoke with ozone depletion. It might just be one chemical mechanism among several, but it’s clearly there. It tells us these particles are wet and had to cause some ozone depletion.”
Studies are underway
The authors of the new study are looking at other reactions from wildfire smoke that could further contribute to the depletion of the ozone layer. For now, the main driver of ozone depletion remains CFCs, which are chemicals like old refrigerants that were banned under the Montreal Protocol, but remain in the stratosphere. But as global warming leads to stronger and more frequent wildfires, their smoke could have a dangerous and lasting effect on the ozone layer.
“Burst fire smoke is a very complex, toxic mixture of organic compounds,” says Professor Solomon. “And I fear that ozone is going through a whole chain reaction that we are now frantically working to dismantle.”
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