Smoke from large wildfires seems to damage the ozone layer

The ozone layer is often seen as a success story for human action to correct a climate emergency – but unfortunately we may be undoing our own hard work. A new study has shown that smoke from wildfires, such as those that recently devastated parts of Australia, can deplete the ozone layer further, delaying its recovery.

The ozone layer sits in the Earth’s lower stratosphere, at altitudes between 15 and 35 km (9 and 22 miles), and reflects a large amount of the Sun’s damaging UV rays. This helps to keep the planet habitable, but in the mid-1980s a hole was discovered in this layer. Within a few years, the world came together to sign the Montreal Protocol into effect, banning the use of ozone-dissolving CFCs. The ozone hole has been shrinking ever since, preventing some of the worst-case climate scenarios.

But unfortunately, symptoms of the current climate crisis threaten to undo that effort. In late 2019 and into early 2020, much of Australia was affected by wildfires. More than 60 million acres (24 million ha) of land was burned, and over a million tons of smoke particles were ejected into the atmosphere, reaching a height of 35 km – the upper region of the ozone layer. Previous work has established that smoke from volcanic eruptions can trigger chemical reactions that dissolve ozone, so the researchers investigated whether wildfire smoke might do the same.

The first clue came in March 2020, just after the fires had been extinguished. Satellite data showed that levels of nitrogen dioxide in the stratosphere had dropped significantly, marking a record low within the prior 20 years of observations. This decline is a smoking gun (pun intended) for a particular chemical reaction by which volcanic smoke degrades ozone.

In normal atmospheric chemistry, dinitrogen pentoxide reacts with sunlight to produce a range of nitrogen species. One of these end products is nitrogen dioxide, which binds with chlorine compounds, preventing them from destroying ozone.

But when emitted particles join the mix, they interrupt that chemistry. Smoke particles convert the dinitrogen pentoxide into nitric acid, which means there’s not as much left to form nitrogen dioxide. That in turn means the chlorine forms more chlorine monoxide, a potent ozone-killer.

To investigate whether the wildfire smoke performs the same process, the team ran complex simulations of atmospheric chemical reactions, and examined models with and without huge clouds of smoke particles being added. And sure enough, all models with the smoke saw drastic drops in nitrogen dioxide.

“The behavior we saw, of more and more aerosols, and less and less nitrogen dioxide, in both the model and the data, is a fantastic fingerprint,” said Susan Solomon, lead author of the study. “It’s the first time that science has established a chemical mechanism linking wildfire smoke to ozone depletion. It may only be one chemical mechanism among several, but it’s clearly there.”

The team estimated that this smoke would have depleted the ozone column by up to one percent. Given the ozone hole is healing at a rate of about one to three percent per decade, this event effectively cancels out much of the healing it's done in the past 10 years.

This ozone depletion isn’t the only climate effect from wildfires of this scale. A study last year found that these fires released some 788 million tons of carbon dioxide into the atmosphere, while another found that parts of the stratosphere warmed by up to 2 °C (3.6 °F) for six months.

Worse still, huge wildfires are expected to increase in frequency as the climate changes, and their environmental effects serve to accelerate that climate change, creating a vicious cycle. Gaining a better understanding of these processes can help us prepare.

The research was published in the journal Proceedings of the National Academy of Sciences.

Source: MIT