Contrasting Chlorine Chemistry on Volcanic and Wildfire Aerosols in the Southern Mid‐Latitude Lower Stratosphere

Volcanic eruptions and wildfires can impact stratospheric chemistry. We apply tracer‐tracer correlations to satellite data from Atmospheric Chemistry Experiment—Fourier Transform Spectrometer and the Halogen Occultation Experiment at 68 hPa to consistently compare the chemical impact on HCl after multiple wildfires and volcanic eruptions of different magnitudes. The 2020 Australian New Year (ANY) fire displayed an order of magnitude less stratospheric aerosol extinction than the 1991 Pinatubo eruption, but showed similar large changes in mid‐latitude lower stratosphere HCl. While the mid‐latitude aerosol loadings from the 2015 Calbuco and 2022 Hunga volcanic eruptions were similar to the ANY fire, little impact on HCl occurred. The 2009 Australian Black Saturday fire and 2021 smoke remaining from 2020 yield small HCl changes, at the edge of the detection method. These observed contrasts across events highlight greater reactivity for smoke versus volcanic aerosols at warm temperatures. Plain Language Summary An unprecedented change in HCl was observed in the lower stratosphere after the 2020 Australian New Year (ANY) wildfire using satellite records since 2004. In this study, we conduct a consistent analysis of HCl impacts using an additional satellite product with measurements since 1991 to examine effects of the catastrophic 1991 Pinatubo volcanic eruption and smaller eruptions, and compare them to the 2020 ANY fire (and remaining smoke in 2021), as well as the much smaller 2009 Australian Black Saturday (ABS) bushfire. This allows analysis of different types of particles (smoke vs. volcanic) and the extremes of each observed to date. While the Pinatubo eruption displayed 10 times greater aerosol loading in the stratosphere than the ANY fire, these two events led to similar net chemical changes in HCl. In contrast, no significant changes in HCl were observed in the lower stratosphere following the Hunga and Calbuco volcanic eruptions, which displayed similar levels of extinction to ANY. Small effects were observed from ABS fire and in 2021, allowing identification of the lower limit of the amount of smoke affecting HCl. These contrasts between events indicate that the wildfire smoke aerosols must be more reactive insofar as chlorine chemistry is concerned. Key Points The tracer‐tracer correlation method reveals chemical perturbations associated with volcanic aerosols or smoke The 1991 Pinatubo eruption and the 2020 Australian wildfire markedly perturbed H