Effects of CO and Smoke Optical Properties on Stratospheric Ozone After the 2019/2020 Australian PyroCb Outbreak

In late December 2019 and early January 2020, a large pyrocumulonimbus (pyroCb) outbreak occurred in southeastern Australia, injecting around 0.9 Tg of smoke aerosols into the Upper Troposphere and Lower Stratosphere (UTLS). This event, known as the Australian New Year Super Outbreak (ANYSO) was the largest pyroCb outbreak on record and resulted in profound changes to the temperature, composition, and chemistry of the stratosphere. Using the Chemical Transport Model (CTM) GEOS-Chem (GC), we apply a three-dimensional emission inventory that captures the mass, injection heights, locations, and times of each pyroCb smoke emission during the ANYSO using empirical emission factors of each species for biomass burning. We use this model to examine the effects of CO and the effects of smoke induced changes in ultraviolet (UV) actinic flux on stratospheric ozone concentrations. We also conduct a sensitivity study on the impact of the changes in the smoke optical properties, such as single-scattering albedo, due to coating of the smoke particles on stratospheric ozone. Heterogeneous chemistry on smoke particles is not considered, but this will have little impact on CO chemistry or photochemistry in the context of this study. We find that the ANYSO CO emissions result in a prolonged increase in the UTLS 20°S-80°S domain averaged concentration of ozone that peaks at ~1% roughly 80 days after the ANYSO and the local concentration within the plume by as much as 6%. This increase in ozone is significant to compete with the ozone depletion mechanisms related to heterogeneous chemistry discussed in previous modeling studies. The effects of the smoke induced changes in UV actinic flux on ozone, when considered in isolation, are much smaller, causing a short-term domain averaged ozone concentration decrease of ~0.8% peaking roughly 20 days after the ANYSO.