The massive forest fires that ravaged south-east Australia in 2019-2020 have triggered only now understood chemical reactions. These reactions consumed part of the ozone layer, expanding and prolonging the hole in this part of the stratosphere. A study, published in Nature, describes how smoke combined with molecules containing chlorine in the stratosphere – remnants of chemicals that are now banned – to cause destruction.

Os incêndios australianos produziram a maior nuvem de fumaça já registrada, liberando cerca de um milhão de toneladas de fumaça a alturas de até 30 quilômetros. Isso está na estratosfera, a faixa da atmosfera que contém a camada de ozônio, responsável por proteger a Terra dos raios ultravioleta nocivos, diz o coautor do estudo Kane Stone, químico atmosférico do Instituto de Tecnologia de Massachusetts (MIT), em Cambridge, em nota sobre o estudo.

In the months following the forest fires, the hole in the ozone layer, which appears annually over Antarctica, was larger and lasted longer than in previous years. But Stone says the researchers didn't know why.

Study co-author Susan Solomon, atmospheric chemist at MIT, suggests that smoke may have caused, even in hot air, a chemical reaction that usually needs cold conditions to occur. Satellite data analyzed after the fires revealed abnormal changes. The stratosphere "looked like another planet after those fires," she says.

About 80% of chlorine in the atmosphere is a legacy of chlorofluorocarbons, chemicals used in aerosols and refrigerator equipment from the 1930s on. Its use has been virtually eliminated since an international treaty was implemented in 1987. The remaining chlorine is in harmless forms for the ozone layer -- hydrochloric acid and chlorine nitrate. However, when hydrochloric acid dissolves in water droplets, it forms molecules capable of reacting with the ozone layer and destroying it. This is usually restricted to poles, where the air is cold enough to trigger the process, according to Stone.

The team used a computer model to predict how various organic acids contained in the smoke particles would cause chlorine to react with the ozone layer. The changes produced in the simulations mirrored the changes in stratospheric chemistry observed after the fires. "Forest fire smoke in hot temperatures does things in Australia that couldn't happen otherwise," Solomon says.

Chlorine-containing molecules that remained before being banned are slowly decomposing and the annual hole in the ozone layer is shrinking. But Solomon says more frequent forest fires resulting from climate change could jeopardize the recovery of the ozone layer. "It's like a race. Will chlorine fall from the stratosphere fast enough in the next 40 to 50 years so that the likely increase in intense and frequent forest fires does not prolong the hole in the ozone layer?"

Not all forest fire smoke hits the stratosphere, says David Peterson, a meteorologist at the United States Naval Research Laboratory in Monterey, California. But when an intense fire combines with the humid air above, firestorms form chimney-like clouds that pump smoke into the air. Understanding what causes some high storm clouds to inject smoke into the stratosphere will be crucial in figuring out how much impact the fires will have on ozone recovery, he says.

Haywood would like to see the new chemistry integrated into a climate model to predict how ozone destruction can be affected if intense forest fires become more common.

Source: Um só Planeta