Contrasting stratospheric chlorine processes on volcanic and wildfire aerosols

Combining satellite data from HALOE (The Halogen Occultation Experiment, available from 1991-2005) and ACE-FTS (Atmospheric Chemistry Experiment - Fourier Transform Spectrometer, available from 2004-present), we quantified the stratospheric chlorine processes after the 2020 Australian wildfire and major volcanic eruptions (1991 Pinatubo, 2015 Calbuco, and 2022 Tonga). The 2020 Australian wildfire was the largest wildfire since the satellite era. This wildfire released of the order of 1 Tg of aerosols into the stratosphere, comparable to small-scale volcanic eruptions. Despite this rather small amount of stratospheric aerosol loading, its impact on the stratospheric chlorine reservoirs (HCl and ClONO₂) was enormous. In contrast to volcanic eruptions, most of the aerosols from wildfires are organics, which could lead to different chemical processes from inorganic sulfates. We use these observations to demonstrate that wildfire aerosols uptake HCl much more efficiently than volcanic aerosols, especially at temperatures warmer than 200 K. Furthermore, while the 1991 Pinatubo eruption injected an order of magnitude more aerosol into the stratosphere than the 2020 Australian wildfire, we show that the two events led to a similar amount of HCl decrease in the mid-latitude and polar region. Most of the decrease in HCl after the 2020 Australian wildfire was balanced by an increase in ClONO₂; whereas calculated ClONO₂ remained unchanged after the 1991 Pinatubo eruption (indicated by model simulations). With current climate change projections, we are expected to have more frequent wildfires in the future, and this more efficient HCl loss pathway poses new threats to the recovery of the stratospheric ozone layer.