Stratospheric Heterogeneous Chemistry after the 2019–2020 Australian Wildfires

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• Dr. Kane Stone, Research Scientist, Department of Earth Atmospheric and Planetary Sciences, Massachusetts Institute of Technology
• Tuesday 05 November 2024, 15:00-16:00
• Chemistry Dept, Unilever Lecture Theatre and Zoom.

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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 is 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. 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.

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