In the Australian summer of 2019/2020 there were devastating fires that destroyed around 12.5 million hectares of forest and bush. Using satellite data, researchers have now examined the effects of this fire summer on the composition of the atmosphere. Accordingly, the smoke particles have significantly changed the composition of the gases in the stratosphere. Some of these changes have the potential to damage the ozone layer. How serious this influence is, however, is disputed among scientists.
The ozone layer in the atmosphere protects us from the sun’s UV-B and UV-C radiation. However, the ozone molecules, which consist of three oxygen atoms, can be destroyed by various chemical reactions. In particular, the halogenated hydrocarbons CFC and CFC, which were widely used refrigerants and solvents up until the 1980s, react easily with O3 and caused the ozone hole, especially over Antarctica. Long-lived CFCs have been banned since 1987 and emissions of ozone-depleting substances have been severely limited worldwide, so that it is estimated that the ozone layer will have regenerated by the middle of the 21st century – provided that further serious disruptions do not occur.
Forest fires as ozone killers?
A team led by Peter Bernath from the Old Dominion University in Norfolk, Virginia, has now come across such a possible disruption. “Large wildfires introduce smoke and products of biomass combustion into the mid-latitude stratosphere, where they destroy ozone,” the researchers write. Using data from the so-called ACE satellite (Atmospheric Chemistry Experiment), which uses infrared measurements to determine the concentration of atmospheric gases, they investigated the influence of Australia’s “black summer” on the chemical composition of the atmosphere and in particular on the ozone layer in the lower stratosphere would have.
During the bush fires, large, smoke-laden thunderclouds, so-called pyrocumulonimbus clouds (pyroCbs), formed due to the heat, which threw the smoke particles high into the atmosphere. “These introduced smoke particles have led to unexpected and extreme perturbations of the stratospheric gases that have not been observed in measurements in the last 15 years,” the authors describe. Among other things, increases in formaldehyde, chlorine nitrate, chlorine monoxide and hypochlorous acid were observed, with a simultaneous decrease in ozone, nitrogen dioxide and hydrochloric acid. “These perturbations in stratospheric composition have the potential to affect ozone chemistry in unexpected ways,” the researchers said.
criticism from peers
From the point of view of Bernath and his colleagues, forest fires could play an important role for the global ozone balance in addition to direct human emissions – especially since the number of forest fires is very likely to increase due to human-induced climate change. “This could delay the recovery of stratospheric ozone,” say the researchers. Colleagues take a critical view of this conclusion. “In my opinion, one cannot clearly see that the anomalies in the concentrations of chlorine nitrate, hydrochloric acid, hypochlorous acid and chlorine monoxide in 2020 caused by the fires had a particular influence on the ozone depletion in the stratosphere in spring,” says Martin Dameris, professor at the institute for Atmospheric Physics (IPA) at the German Aerospace Center (DLR) in Oberpfaffenhofen.
According to him, the relatively low measured ozone values are mainly due to the fact that the stratospheric polar vortex was very pronounced in winter and spring 2020, so that it reached to mid-latitudes and the low temperatures created good conditions for the expected seasonal ozone depletion . “The results suggest that the chlorine-containing gases have an impact on ozone chemistry, but not in such a succinct way that they affect the ozone layer significantly in the spring – i.e. can delay the recovery of the ozone layer and the closure of the ozone hole.”
Further research needed
Johannes Laube, group leader at the Institute for Energy and Climate Research at Forschungszentrum Jülich, also considers the informative value of the current study to be limited. He points out that many of the chemicals studied are so reactive that their concentration is difficult to determine and any measurement is subject to uncertainty. However, the study does not indicate how great this uncertainty is. “For this reason, in my opinion, many of the statements in the article are on very shaky ground, since it is not possible to see to what extent the concentration-height profiles in 2020 actually differ significantly from those of other years,” says Laube.
Basically, the realization that wildfires can unbalance the chemistry of the stratosphere is not new, he explains. “However, the current study shows that in the case of the Australian bushfires of 2019 this happened to an extent previously unknown.” From his point of view, further research is necessary. “Since the probability of such fires increases with increasing anthropogenic climate change, it is important to better study and understand these processes and their precise effects,” he says. “Since there is a high degree of variability in the composition of such fire particles and other factors – such as the exact geographical location of the fire and the time it took place – also play an important role, the potential range of future consequences cannot be derived from this spectacular individual case.”
Source: Peter Bernath (Old Dominion University, Norfolk, Virginia, USA) et al., Science, doi: 10.1126/science.abm5611