Kane Stone (Research scientist in the Department of Earth Atmosphere and Planetary Science at MIT)

The 2019–2020 Australian wildfires injected an estimated 1 Tg of smoke into the Southern Hemisphere stratosphere, significantly increasing aerosol surface area for heterogeneous chemistry to occur. Unlike a volcanic eruption which typically only enhances N₂O₅ hydrolysis in the midlatitudes, the Australian wildfires also caused large perturbations in chlorine containing species. Large decreases of HCl, large increases in ClONO₂, and enhanced activated chlorine were observed in both the Southern Hemisphere midlatitude and polar regions. This in indicative of HCl based heterogeneous chemistry occurring at a faster rate. Previous work has shown that heightened HCl solubility in aerosols with elevated organic content is a likely driver of the enhanced heterogeneous chemistry. In this seminar, we will discuss how we modelled this enhanced heterogeneous chemistry in CESM-CARMA for both 2020, when very large amounts of wildfire organics are present, and 2021, when most of the wildfire aerosols have been removed from the stratosphere. The importance of all currently modelled heterogeneous reactions will be discussed. A clear seasonality in the perturbations of chlorine species and ozone is observed. This seasonality is captured very well in our model simulations and is photochemically driven. Similarly to the midlatitudes, very unusual perturbations in HCl and ClONO₂ occurred in the Antarctic polar region in both 2020 and 2021 before the polar night. For a normal year without wildfire smoke, models cannot adequately replicate the observed rapid HCl decline that occurs in early austral winter. The current cause of this is unknown. However, in both 2020 and 2021, total HCl loss occurred months earlier than normal due the presence of wildfire organic aerosols. Our model simulations replicated the early onset of HCl loss seen in the observations very well. In addition to wildfire organics, the lower stratosphere typically has a large amount of “background” organics originating from non-pyrocumulonimbus biomass burning. Both the background and Australian wildfire aerosols are expected to be in mixed aerosol with sulfate. Based on our model simulations, the expected phases of organic-nonorganic mixtures will be discussed, including any potential differences between the background and Australian wildfire organics. Finally, we will discuss how unique we think the 2019–2020 Australian wildfire event was regarding both its effects on stratospheric chemistry and its size in comparison to other past pyroCB injections of wildfire smoke and what this could mean for the future.

Kane Stone is a research scientist in the Department of Earth Atmosphere and Planetary Science at MIT. He completed his PhD in Australia at The University of Melbourne in 2016 and afterwards moved to Boston to begin a post-doc at MIT. His work focuses on stratospheric chemistry and dynamics and stratosphere-troposphere coupling. His recent focus has been on wildfire organic chemistry in the stratosphere. In his spare time I like to rock climb.