CFCs are not the only culprit – bushfire smoke may also destroy ozone

The Australian bushfires of 2019 and 2020 had the world appalled as they raged and spread, destroying vegetation and homes, and killing or displacing billions of animals in their path. All told, 43 million acres of land were incinerated and over one million tons of smoke particles were injected into the atmosphere. The huge, pyrocumulonimbus smoke clouds reached 35 km into the atmosphere, well into the stratosphere which is home to the ozone layer. The mass and extent of the smoke particles entering into the stratosphere was comparable with the effect of a volcanic eruption. 

In previous research, Susan Solomon, the Lee and Geraldine Martin Professor of Environmental Studies at the Massachusetts Institute of Technology, had discovered that the particles spewed into the atmosphere during volcanic eruptions can destroy ozone through a series of chemical reactions. These tiny particles gather moisture on their surfaces and, once wet, they provide the surface area for many reactions between particles in the stratosphere, including the reactions that culminate in the breakdown of ozone. After the Australian fires subsided in March, 2020, she wondered whether the smoke particles from the fires would have the same effect and hasten the depletion of ozone. 

In a new study, published today in Proceedings of the National Academy of Sciences, Solomon and a team of atmospheric scientists from MIT confirm that the smoke from the devastating fires set off chemical reactions in the stratosphere that contributed to the destruction of ozone. This research is the first to establish a chemical link between wildfire smoke and ozone depletion. 

After the fires subsided in March 2020, the scientists noted a sharp decrease in nitrogen dioxide in the stratosphere, which is the first step in a chemical cascade that is known to end in ozone depletion. The researchers found that this decrease in nitrogen dioxide directly correlated with the amount of smoke that had entered the stratosphere from the fires.

They explained that, during volcanic eruptions, the tiny particles provide a surface for chemical reactions that form nitric acid from dinitrogen pentoxide, a chemical that circulates in the stratosphere. Dinitrogen pentoxide normally reacts in sunlight to produce various compounds of nitrogen, including nitrogen dioxide, which reduces the action of chlorine-containing gases that break down ozone. Thus, when volcanic smoke converts dinitrogen pentoxide into nitric acid, the concentration of nitrogen dioxide decreases and this leaves chlorine-containing gases in excess. These chemicals (such as CFCs and HCFCs) are the main human-made agent that destroys ozone.

As soon as the atmospheric scientists recorded a decrease in the presence of nitrogen dioxide in the stratosphere after the fires, they predicted that this would lead to the destruction of ozone, just as recorded after volcanic eruptions. In fact, in their study they estimate that the chemical chain reaction as a result of smoke particles in the stratosphere depleted the column of ozone by one percent.

“This chemistry, once you get past that point, is well-established,” said Professor Solomon, who is the lead author of the paper. “Once you have less nitrogen dioxide, you have to have more chlorine monoxide, and that will deplete ozone.”

The team used measurements of stratospheric nitrogen dioxide taken by three independent satellites that have surveyed the Southern Hemisphere for varying lengths of time. They compared each satellite’s records in the time following the Australian fires; all three showed a significant decrease in nitrogen dioxide levels in March 2020. For one satellite’s record, the data in that month was a record low among observations spanning the past 20 years.

The researchers then used a global, three-dimensional model to simulate the potential consequences of the wildfire smoke on atmospheric chemistry. This model maps out hundreds of chemical reactions that take place in the atmosphere, from Earth’s surface right up through the stratosphere. As part of the modelling exercise, they injected a “cloud of smoke particles” into the model’s input, to mimic the observations from the fires. They assumed that the particles, like volcanic aerosols, would gather moisture, and they then ran the model multiple times to compare the results, with and without the smoke cloud.

In every simulation incorporating wildfire smoke, the team found that as the mass of smoke particles increased in the stratosphere, concentrations of nitrogen dioxide decreased, matching exactly the real-time observations that were made by the three satellites.

“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 Solomon. “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. It tells us these particles are wet and they had to have caused some ozone depletion.”

Sadly, the one percent ozone loss estimated to have occurred as a consequence of the Australian bushfire smoke equates roughly with the recovery of the ozone layer that has taken place since the worldwide Montreal Protocol agreement to stop producing ozone-depleting gases. The consequences of the fires thus cancel out those hard-won diplomatic gains. Also, it is all too likely that climate change will result in more frequent wildfires in the future, and that ozone depletion via this route will become even more of a problem.  

“The Australian fires look like the biggest event so far, but as the world continues to warm, there is every reason to think these fires will become more frequent and more intense,” said Solomon. “It’s another wakeup call, just as the Antarctic ozone hole was, in the sense of showing how bad things could actually be.”

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By Alison Bosman, Earth.com Staff Writer