Radiative forcing and stratospheric ozone changes due to major forest fires and recent volcanic eruptions including Hunga Tonga

Using the chemistry-climate model EMAC with nudged tropospheric meteorology, we show that organic carbon injected into the stratosphere through forest fire-related pyro-cumulonimbi enhances heterogeneous chlorine activation due to enhanced solubility of HCl in particles containing organic acids and a larger aerosol surface area. After the 2019/2020 Australian mega-bushfires, the upward transport of the pollution plume led to enhanced ozone depletion in the Southern hemispheric lower stratosphere, in agreement with AURA-MLS satellite observations. It reduced total ozone in 2020 and 2021 by up to 28 DU around 70^oS, accompanied by a dynamic reduction in August 2020 from the lofting of smoke-filled vortices, reaching 24 DU (total about 40 DU near 65^oS). The eruption of Hunga Tonga in January 2022 led to a reduction of total ozone in the entire Southern hemisphere, exceeding 10 DU south of about 55^oS in Austral spring of 2022 and 2023. The water vapor injection by the volcano modified only the vertical distribution of ozone loss.
The absorbing aerosol from the combined Australian and Canadian forest fire emissions in 2019/2020 caused the largest perturbation in stratospheric optical depth (e.g., seen in OSIRIS data) since the eruption of Pinatubo. It changed the instantaneous stratospheric aerosol forcing -derived at the top of  the atmosphere- from -0.2 W/m² to +0.3 W/m² in January 2020. In January 2022, the remaining effect was about 0.05 W/m², reducing the negative forcing by volcanoes. The computed global aerosol radiative forcing caused by the Hunga Tonga eruption in 2022 was about -0.12 W/m², decreasing to -0.06 W/m² by December 2023, dominated by the change in stratospheric sulfate aerosols. The positive forcing of the injected water vapor was small (in agreement with other models).