Life on Earth develops in the troposphere, the lower layer of the atmosphere where oxygen (O2) abounds, which living beings breathe. But above is the stratosphere, rich in ozone (O3), which acts as a filter for ultraviolet radiation. Without this compound, life would not be possible. Now, a group of scientists has discovered that smoke from wildfires combines with other man-made gases to destroy that ozone. After the 2019-2020 summer fires in Australia, the orifice in this filter was enlarged by 10%. If it were a global phenomenon, the mega-fires in California, northern Canada, Chile, the jungles of Indonesia, Siberia or the incendiary waves of the Mediterranean would be weakening the atmospheric protection that makes this planet a good place to live .

Throughout its geological evolution, perhaps by simple chance, a mechanism appeared on Earth that protects the planet from ultraviolet (UV) rays, especially those of type B and C, which have greater potential for cellular and genetic damage. This mechanism, in its simplified version, involves oxygen molecules (O₂), formed by two atomic atoms of oxygen, O). Your exposure to UV radiation breaks down O₂ molecules into O atoms. These join with other O₂ molecules to form O3, ie ozone. Its molecules absorb solar radiation until they are depleted, then they break down into oxygen and start over. It has been like this for millions and millions of years. But just a century ago, engineers at General Motors and Du Pont created a gas they called freon, and within a few years, what worked for millions was out of whack.

Freon was the first of many chlorofluorocarbons (CFCs) on which the democratization of first refrigerators, then air conditioners and sprays was based. Several decades later, Mexican Mario Molina demonstrated in 1974 that CFCs were rising into the stratosphere and opening a hole in the ozone layer. Under the action of radiation, the chlorine in these gases becomes highly reactive and, in geometric progression, decomposes ozone into oxygen. The alarm was such that, in 1987, the Montreal Protocol prohibited the use of CFCs. The bad thing is that these remain for decades in the stratosphere. The good thing is that five years ago the ozone layer was recovering. But fires can burn that recovery.

The fires in the austral summer of 2019 and 2020 released about 900 thousand tons of smoke into the atmosphere, with particles associated with chlorine that destroy ozone.EVAN COLLIS/DFES WORKOUT (EFE)

After years of recovery, the Antarctic ozone hole weakened like never before in 2020. Most scientists thought it was the exception to the trend. But Susan Solomon, an atmospheric chemist at the Massachusetts Institute of Technology (MIT) and other colleagues, did not believe in the anomaly. At the same time that stratospheric ozone depletion was occurring over most of the southern hemisphere, there were unusually low levels of hydrochloric acid and historically high levels of chlorine monoxide.

The hydrochloric acid (HCl) present in the stratosphere comes from the breakdown of CFCs that humans have been emitting for nearly a century. In principle, the chlorine it contains is non-reactive, unless it is released and, in contact with oxygen, turns into chlorine monoxide, the real ozone killer. Until now, it was known that HCl (known by many as strong water or salfuman) interacted with cloud particles and radiation, breaking down into chlorine monoxide (ClO₂) or atomic chlorine (Cl). Before reverting to hydrochloric acid, they will have destroyed 1,000 ozone molecules. But such a reaction occurred under thermal and radiation conditions whose optimum occurs in the stratosphere of the polar regions. This is why the hole in the ozone layer opens up in Antarctica and occasionally in the Arctic, but not in the rest of the planet. But it is now cracking in other parts of the globe as well, Solomon and his colleagues report in the journal Nature. “The fact that HCl dropped to unprecedented levels in the mid-latitudes was a warning to me that something serious was going on,” says Solomon.

The chemical levels in the stratosphere didn’t match. They were missing an element, something to add to the equation. And they thought that the smoke from the fires might hide the answer. In the months just before the annual depletion of the ozone layer, Australia experienced a terrible wave of fires that covered 30 million hectares, killing more than a billion living beings and spewing almost a million tons of smoke into the atmosphere. These particles, thirty kilometers high, were rich in organic carbon. In principle, carbon is not reactive.

“It’s a brutal shock for scientists who study the stratosphere. No one expected the fires to have such an effect.”

Susan Solomon, atmospheric chemist at the Massachusetts Institute of Technology (MIT)

After combing through chemistry books and studies, they found that the hydrochloric acid in CFCs is soluble in a wide range of carbon-based organic compounds and, what’s worse, at temperatures that occur in the mid-latitude stratosphere. The pieces began to fall into place. “How it works is very simple,” explains Solomon via email. “Organic carbon ends up in compounds like alcohols and organic acids. Everyone knows alcohol is a great solvent, maybe you’ve used it for cleaning once. The organic acids and alcohols in the particles cause them to react with hydrochloric acid at much higher temperatures than would normally occur in the stratosphere. Finally, reactions occur on the surface of the smoke particles that release the ozone-depleting chlorine.” The process is the same as at the poles, “but only at much colder temperatures, because the polar stratospheric clouds do not have such high solubility until that they become very cold”, adds Solomon. “This is the new key. It is a brutal shock for scientists who study the stratosphere. Nobody expected that fires would produce such an effect”, concludes the American scientist.

Solomon is not new to this. He has already found that particles (mainly sulfates, sulfur) released by the massive eruption of the Pinatubo volcano in the Philippines in 1991 not only cooled the planet by at least half a degree over the next two years, but also weakened the ozone layer for months. . Now, the results of his experiments, compared with the data from three satellites (which come from the top of what passes in the stratosphere) show that the thickness of this protective mantle was covered between a 3% and a 5% in most of it South hemisphere. As every spring, the ozone layer over Antarctica also thinned in 2020. But that year, it happened 10% more and the hole occupied another 2 million square kilometers.

“It is very relevant and will be even more so as megafires become more frequent in the coming years due to climate change”

V. Faye McNeill, specialist in atmospheric chemistry and particle physics at Columbia University, United States

For V. Faye McNeill, a specialist in chemistry and atmospheric particle physics at Columbia University (United States), Solomon’s work could have major implications. “As we’ve observed many times, even with the Pinatubo volcanic eruption, when particles reach the stratosphere, they can be transported around the globe and have a global effect on climate and ozone chemistry.” And now it turns out that these aerosols can come from fires. “It’s very relevant and will be even more so as megafires become more frequent in the coming years due to climate change,” recalls McNeill.

The entry of fires into the equation can help explain many things. This could help explain, for example, that while the hole in the ozone layer is closing over Antarctica, in other parts of the planet the layer is weakening without scientists being clear about the causes. It could also shed light on what scientists aboard the German icebreaker observed. polar stern during the MOSAIC expedition. They sailed through the Arctic during the winter and spring of 2019-20 until ice trapped the ship and they let themselves be carried away by the ice cap while studying the atmosphere with LIDAR, a laser detection system that is bringing a lot of joy to science. By reflecting the light spectrum of particles in the stratosphere, they expected to find a thin layer of particles of volcanic origin. But, as they published in a scientific journal, what they found was a thick band about 10 kilometers high composed mainly of organic compounds. They could only come from the wave of wildfires that burned Siberia the previous summer.

If Solomon’s discovery were global, it would all fit into the largest hole ever recorded in the ozone layer over the Arctic. In 2020 there were also mega-fires in Siberia, and in 2021 in Canada and, almost every summer, in Western America, the Mediterranean, Chile and again in Australia.

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