Chemical impacts of wildfire smoke on stratospheric chlorine and ozone depletion

Abstract Extremes in stratospheric abundances of HCl (record low), ClONO2 (record high), and ozone (record low) were observed over southern hemisphere mid-latitudes following the 2020 Australian wildfires by satellite records spanning 15–20 years. These unprecedented changes suggest that wildfire aerosols pose a challenge to stratospheric chlorine and ozone depletion chemistry. Here we propose a wildfire smoke chemical mechanism linking data from the field and the laboratory, and test it by comparison of these extraordinary observations to model simulations. Aged wildfire aerosols likely contain a mix of oxidized organics. Laboratory data indicate that HCl is extremely soluble in such species at relatively warm stratospheric temperatures in mid-latitudes and Antarctic autumn. We find that the HCl solubility in oxidized organics, and associated enhancements in heterogeneous reaction rates in/on the aerosols, yield remarkable modelled changes in 2020 HCl, ClONO2, and HOCl abundances, in good agreement with observations. Our results indicate that wildfire smoke does not explain the record duration of the 2020 Antarctic ozone hole, but does yield an increase in its area, as well as 3-5% depletion of southern mid-latitude total column ozone. These findings increase concern that more frequent and intense wildfires could delay ozone recovery in a warming world.