Two-years of stratospheric chemistry perturbations from the 2019–2020 Australian wildfire smoke

The very large pyrocumulonimbus events that occurred during the Australian summer of 2019–2020 caused extremely unusual partitioning of stratospheric chlorine in the Southern Hemisphere midlatitude and Antarctic regions not only in 2020 but also in 2021. This was likely caused by enhanced HCl solubility in organic species that increased heterogeneous chemistry. Here, we show that observed HCl and ClONO2 values remain outside the pre-wildfire satellite range, measured from 2005 until just prior to the event, in both the Southern Hemisphere midlatitude and Antarctic regions in 2021. Through model simulations, we replicate this prolonged multiyear chemical perturbation, in good agreement with observations. This was achieved by calculating the HCl solubility in mixed wildfire and sulfate aerosols consistent with assumptions of (1) liquid–liquid phase separation and (2) linear dependence on organic and sulfate composition. The model simulations also suggest that the Australian pyrocumulonimbus organic aerosols contributed to low midlatitude ozone values in 2020 and 2021. A marked, photochemically controlled seasonality of the chemical perturbations and ozone depletion is also observed and simulated, and its underlying chemical drivers are identified. This work highlights that lower concentrations of smoke still had profound effects on stratospheric heterogeneous chemistry more than a year after the 2019–2020 wildfire event.