Unexpected Repartitioning of Stratospheric Inorganic Chlorine After the 2020 Australian Wildfires

Abstract

The inorganic chlorine (Cly) and odd nitrogen (NOy) chemical families influence stratospheric O3. In January 2020 Australian wildfires injected record-breaking amounts of smoke into the southern stratosphere. Within 1–2 months ground-based and satellite observations showed Cly and NOy were repartitioned. By May, lower stratospheric HCl columns declined by ∼30% and ClONO2 columns increased by 40%–50%. The Cly perturbations began and ended near the equinoxes, increased poleward, and peaked at the winter solstice. NO2 decreased from February to April, consistent with sulfate aerosol reactions, but returned to typical values by June - months before the Cly recovery. Transport tracers show that dynamics not chemistry explains most of the observed O3 decrease after April, with no significant transport earlier. Simulations assuming wildfire smoke behaves identically to sulfate aerosols couldn't reproduce observed Cly changes, suggesting they have different composition and chemistry. This undermines our ability to predict ozone in a changing climate.