Reactivity of Organic Aerosol Proxies to HCl and ClONO2 Under Stratospheric Conditions

Stratospheric aerosols have long been known to impact stratospheric chemistry, including ozone concentrations, through heterogeneous chemistry involving halogen species. While these aerosols are commonly assumed to consist entirely of sulfate, in situ measurements have revealed a significant organic fraction in the ambient lower stratosphere. Additionally, recent large wildfires, such as the 2019-2020 Australian New Year fires (ANY), have injected large quantities of organic aerosol (OA) and gas-phase organics into the stratosphere. Satellite observations following ANY found that chlorine species and ozone were markedly perturbed. These perturbations cannot be explained based on the heterogeneous chemistry of sulfate aerosol alone, demonstrating the need for an improved understanding of stratospheric heterogeneous chemistry. Despite the widespread presence of OA in the stratosphere, virtually no laboratory experiments have been performed to constrain the interaction of OA with halogen species under stratospheric conditions. Using a coated wall reactor, we report our findings on the reactivity of organic surfaces to the two largest chlorine reservoir species in the stratosphere, HCl and ClONO₂. These results will help to constrain the effects of OA on stratospheric chlorine chemistry and, by extension, ozone chemistry, which is critical as large wildfires are expected to increase in frequency with climate change.