The unexpected effect of fire smoke on the atmosphere

MADRID – Life on Earth evolves in the troposphere, the lowest layer of the atmosphere where oxygen (O2) circulates, which living things breathe. But above the stratosphere is rich ozone (O3), which acts as a UV filter. Without this compound, life would not be possible.. Now, a group of scientists has discovered that fire smoke combines with other human-generated gases to destroy this ozone. After the 2019-2020 summer fires in Australia, The hole in this filter has been enlarged by 10%.. If it were a global phenomenon, huge fires in California, northern Canada, Chile, the Indonesian jungles, and Siberia, or scorching waves in the Mediterranean, would weaken the atmospheric protection that makes this planet such a good place to live.

During its geological evolution, perhaps by simple coincidence, a mechanism appeared on Earth that protects the planet from ultraviolet radiation, especially those of type B and C, which have the greatest potential for cellular and genetic damage. This mechanism, in its simplified version, involves oxygen molecules (O₂), made up of two atoms of atomic oxygen, O). Exposure to ultraviolet light breaks O₂ molecules into O atoms, which then join with other O₂ molecules to form O₃, i.e. ozone. Its molecules absorb solar radiation until they are exhausted, then decompose into oxygen and start over. This has been the case for millions and millions of years. But barely a century ago, engineers from General Motors and DuPont created a gas they called Freon, and within a few years, what was working for millions was out of control.

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

After years of recovery, the Antarctic ozone hole has weakened as never before in 2020. Most scholars believe that it was an exception to this trend. But Susan Solomon, an atmospheric chemist at the Massachusetts Institute of Technology (MIT), and her other colleagues didn’t buy into the anomaly. At the same time that stratospheric ozone depletion was occurring in most of the southern hemisphere, there were very low levels of hydrochloric acid and historically high levels of chlorine monoxide.

Hydrochloric acid (HCl) in the stratosphere comes from the decomposition of chlorofluorocarbons (CFCs) that humans have been emitting for nearly a century. In principle, the chlorine it contains is not reactive, unless it is released, and upon contact with oxygen it turns into chlorine monoxide, the real killer of ozone. Until now it was known that hydrochloric acid (known by many as strong water or salfoman) reacts with cloud and radiation particles, decomposing into chlorine monoxide (ClO₂) or atomic chlorine (Cl). Before those turn into hydrochloric acid, they will have destroyed 1,000 ozone molecules. But such interaction occurred under thermal and radiative conditions that are optimal in the stratosphere in the polar regions. This is why the hole in the ozone layer opens up at the South Pole and sometimes at the North Pole, but not in the rest of the planet. But now it is also cracking in other parts of the world, as published in Nature magazineSuleiman and colleagues. “The fact that hydrochloric acid had fallen to unprecedented levels in mid-latitudes was a warning to me that something serious was going on,” says Solomon.

Chemical levels in the stratosphere have not accumulated. They were missing an ingredient, something to add to the equation. They thought the smoke from the fires might mask the answer. In the months immediately preceding the annual depletion of the ozone layer, Australia experienced a horrific wave of fires that engulfed 30 million hectares, killing more than a billion organisms and spewing nearly a million tons into the atmosphere. of smoke. These particles, thirty kilometers high, were rich in organic carbon. In principle, carbon is not reactive.

After digging through chemistry books and monographs, they verified that HCl from CFCs is soluble in a wide range of carbon-based organic compounds and, even worse, at temperatures that occur in the stratosphere at mid-latitudes. The pieces are starting to fit together. “How it works is very simple,” Solomon explains in an email. “Organic carbon ends up in compounds like alcohols and organic acids. Everyone knows alcohol is a great solvent, you probably used it for cleaning once. The organic acids and alcohols in the molecules cause them to react with hydrochloric acid at much warmer temperatures than they normally would in the stratosphere. Finally, reactions occur at Surface smoke particles that release ozone-destroying chlorine.”

The process is the same as at the poles, Solomon adds, “but only at cooler temperatures, because polar stratospheric clouds don’t have a high solubility until they get very cold.” “This is the new key. It is a monstrous shock for scientists studying the stratosphere. No one expected that fires would have such an effect, ”concludes the American scientist.

Solomon is not new to this. It had already been discovered that particles (mainly sulfates and sulfur) released by the massive eruption of Pinatubo volcano in the Philippines in 1991 not only cooled the planet by at least half a degree over the next two years, but weakened the layer for several months. from ozone. Now, the results of his experiments, compared to data from three satellites (you see what’s happening in the stratosphere from above) show that the thickness of this protective layer has shrunk between 3% and 5% in most of the southern hemisphere. Like every spring, the ozone layer over Antarctica also shrank in 2020. But that year, it did so by 10% more and the hole occupied an additional 2 million square kilometres.

For V. Faye McNeill, an expert in the chemistry and physics of atmospheric molecules at Columbia University (USA), Solomon’s work could have major implications. “As we’ve noted many times, even with the Pinatubo eruption, when particles reach the stratosphere, they can be transported around the world and have a global impact on climate and ozone chemistry.” And now it has been discovered that these aerosols can come from fires. “It’s very relevant, and it’s going to be more important like Huge fires will become more frequent in the coming years due to climate changeMacNeil remembers.

The entry of fires into the equation can help explain many things. It could help explain, for example, that while the ozone hole is closing over Antarctica, the layer is thinning over other parts of the planet without scientists explaining why. It could also shed light on what scientists have observed aboard the German icebreaker. polarstern During the Mosaic expedition. They sailed through the Arctic during the winter and spring of 2019-20 until ice trapped the ship and allowed themselves to be carried away by the mantle while studying the atmosphere with LIDAR, a laser-based detection system that brings so much joy to science. By bouncing the spectrum of light off particles in the stratosphere, they expected to find a narrow layer of particles of volcanic origin. But, as they report in a scientific journal, they found a thick ribbon about 10 kilometers high composed mostly of organic compounds. They only came from the wave of fires that scorched Siberia last summer.

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

Posted by Miguel Angel Criado

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