Chlorine activation and enhanced ozone depletion induced by wildfire aerosol

Remarkable perturbations in the stratospheric abundances of chlorine species and ozone were observed over Southern Hemisphere mid-latitudes following the 2020 Australian wildfires(1,2). These changes in atmospheric chemical composition suggest that wildfire aerosols affect stratospheric chlorine and ozone depletion chemistry. Here we propose that wildfire aerosol containing a mixture of oxidized organics and sulfate(3-7) increases hydrochloric acid solubility(8-11) and associated heterogeneous reaction rates, activating reactive chlorine species and enhancing ozone loss rates at relatively warm stratospheric temperatures. We test our hypothesis by comparing atmospheric observations to model simulations that include the proposed mechanism. Modelled changes in 2020 hydrochloric acid, chlorine nitrate and hypochlorous acid abundances are in good agreement with observations(1,2). Our results indicate that wildfire aerosol chemistry, although not accounting for the record duration of the 2020 Antarctic ozone hole, does yield an increase in its area and a 3-5% depletion of southern mid-latitude total column ozone. These findings increase concern(2,12,13) that more frequent and intense wildfires could delay ozone recovery in a warming world.

Automated summary

Remarkable changes in stratospheric chlorine species and ozone were observed after the 2020 Australian wildfires. Wildfire aerosols containing oxidized organics and sulfate increase hydrochloric acid solubility and reaction rates, activating reactive chlorine species and enhancing ozone loss rates. Comparisons between atmospheric observations and model simulations show agreement, indicating that wildfire aerosol chemistry can lead to an increase in the area of the ozone hole and a depletion of mid-latitude total column ozone. These findings raise concerns about the potential delay in ozone recovery due to more frequent and intense wildfires in a warming world.