Extremes in stratospheric abundances of HCl (record low) 1,2 , ClONO2 2 (record high), and ozone (record low) 1 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 3-6 . Laboratory data indicate that HCl is extremely soluble in such species at relatively warm stratospheric temperatures 7-10 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.Massive wildfires in Australia during austral summer of 2019-2020 (December-January) produced pyrocumulonimbus (pyroCb) towers that released about 0.9 Tg of wildfire smoke into the stratosphere 11,12 . Wildfires are also sometimes denoted as bushfires, wildland fires, and forest fires; here we refer to them as wildfires. Wildfire smoke can be expected to be primarily composed of organic material, but its stratospheric chemistry is virtually unknown.Airborne mass spectrometry data 13,14,15 showed that carbonaceous compounds were also frequently present in about 30-40% of individual particles in the background northern hemisphere tropopause and lowermost stratosphere region.The carbonaceous fraction was largely internally mixed with sulfate (i.e., both