Stratospheric chlorine processing after the 2020 Australian wildfires derived from satellite data

The 2019 to 2020 Australian summer wildfires injected an amount of organic gases and particles into the stratosphere unprecedented in the satellite record since 2002, causing large unexpected changes in HCl and ClONO₂. These fires provided a novel opportunity to evaluate heterogeneous reactions on organic aerosols in the context of stratospheric chlorine and ozone depletion chemistry. It has long been known that heterogeneous chlorine (Cl) activation occurs on the polar stratospheric clouds (PSCs; liquid and solid particles containing water, sulfuric acid, and in some cases nitric acid) that are found in the stratosphere, but these are only effective for ozone depletion chemistry at temperatures below about 195 K (i.e., largely in the polar regions during winter). Here, we develop an approach to quantitatively assess atmospheric evidence for these reactions using satellite data for both the polar (65 to 90°S) and the midlatitude (40 to 55°S) regions. We show that heterogeneous reactions apparently even happened at temperatures at 220 K during austral autumn on the organic aerosols present in 2020 in both regions, in contrast to earlier years. Further, increased variability in HCl was also found after the wildfires, suggesting diverse chemical properties among the 2020 aerosols. We also confirm the expectation based upon laboratory studies that heterogeneous Cl activation has a strong dependence upon water vapor partial pressure and hence atmospheric altitude, becoming much faster close to the tropopause. Our analysis improves the understanding of heterogeneous reactions that are important for stratospheric ozone chemistry under both background and wildfire conditions.