Forest fires could increase Arctic ozone depletion – aerosol layer over the Arctic promotes ozone-depleting reactions in the stratosphere –

Double climate impact: The increasing fires in the Arctic taiga and tundra are also damaging the ozone layer and contributing to Arctic ozone depletion, as measurements suggest. Accordingly, the smoke of such fires can rise to the limit of the stratosphere. In the winter of 2019/2020, the aerosol density at this altitude increased tenfold due to Siberian forest fires. The suspended particles in turn can promote ozone-depleting reactions in the ozone layer.

For a long time there was only a real ozone hole over the South Pole. In recent years, however, stratospheric ozone has been depleted more and more frequently over the Arctic. In 2011, 2016 and in spring 2020, the ozone densities dropped so much that an ozone hole opened up over the northern polar region. The primary cause is the changed air currents in the Arctic caused by climate change, which strengthen the ring-shaped polar vortex and cause the stratosphere to cool down extremely. This in turn promotes ozone depletion.

Atmosphere measurement using LIDAR during the polar night. ©

Laser look into the arctic atmosphere

But there is apparently a second factor driving Arctic ozone depletion: the smoke from increasing taiga and tundra fires in the Arctic. Due to warming and falling precipitation, there are more and more widespread fires in the northern polar region, some of which even last through the winter. In Siberia in particular, these fires have repeatedly reached record levels in recent years.

Kevin Ohneiser from the Leibniz Institute for Tropospheric Research (TROPOS) and his colleagues have now discovered the consequences of this for the Arctic ozone layer. As part of the international MOSAiC expedition, from September 2019 to May 2020, they regularly examined the composition of the atmosphere over the central Arctic using LIDAR. This laser measurement reveals, among other things, how many aerosols are in the air column. This was supplemented by satellite data and LIDAR measurements on Spitsbergen.

Aerosol values ​​increased tenfold

The measurements revealed something surprising: “From the first day of the MOSAiC measurements at the end of September 2019, we observed a conspicuous aerosol layer with a broad maximum at an altitude of about ten kilometers, directly above the local tropopause,” reports Ronny Engelmann from TROPOS. The aerosol concentration in the lower stratosphere increased tenfold over the entire winter half-year.

The data showed that these suspended particles did not originate from a volcanic eruption, as initially assumed. Instead, most of this aerosol layer showed clear wildfire smoke signatures, the team reports. Traceability using analysis of air currents revealed the source: The smoke came from the exceptionally strong and long-lasting forest fires that raged in Siberia in the summer of 2019. The fire season was one of the strongest in the last 20 years.

Novel “elevator effect”

What is unusual, however, is that the smoke from the fire was able to rise to such great heights at all. So far, this is only known from warmer regions, as Ohneiser and his colleagues explain. The fires can heat the air so much that the smoke rises into the stratosphere in so-called fire clouds. Such “elevator clouds” were also observed in the catastrophic fires in Australia in 2019 and in the western United States in 2020.

The strange thing, however: Typically, the towering cloud towers of these fire clouds are clearly visible in the sky. In addition, they usually trigger thunderstorms. But in Siberia both were missing. Instead, a new, cloud-free form of self-buoyancy could have come into play there: “We suspect that the dark, carbonaceous smoke particles were heated up so much by the sunlight that their surrounding air slowly rose,” explains Ohneiser. “This is the only plausible explanation for efficient vertical transport over several kilometers.”

associated with ozone depletion

The rise of the smoke to the lower limit of the stratosphere had consequences for the ozone layer: where the aerosol values ​​were higher, the researchers also detected an abnormally strong loss of stratospheric ozone. A layer with extremely low ozone densities formed at altitudes of between 15 and 20 kilometers, but the ozone was also measurably depleted at the level of the aerosol layer at an altitude of ten to 15 kilometers.

“We find a clear connection between the occurrence of forest fire smoke in the lowest part of the stratosphere and the abnormally strong ozone depletion,” says Albert Ansmann from TROPOS. He and his colleagues suspect that the surface of the smoke particles favors the ozone-depleting reactions, similar to what is already known from volcanic sulphate aerosols.

double effect

If this is confirmed, then climate change could affect the Arctic ozone layer in two ways: On the one hand, it promotes ozone depletion due to the changed air currents in the northern polar region. On the other hand, climate change has an indirect effect by allowing the Arctic forests and tundra to dry out, thus promoting forest fires. Their smoke then also promotes the depletion of arctic ozone.

“This expands the debate about the consequences of climate change to include a new and previously unconsidered aspect,” concludes Ohneiser and his colleagues. (Atmospheric Chemistry and Physics, 2022; doi: 10.5194 / acp-21-15783-2021)

Source: Leibniz Institute for Tropospheric Research e. V

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