Impact of fire smoke vortices on stratospheric ozone chemistry: case of the Australian fires in 2019-2020

The 2019-2020 Australian wildfires marked a significant event characterized by an unprecedented injection of biomass burning products into the stratosphere. This study focuses on the unique atmospheric phenomena that occurred during these fires, i.e. the formation of vortex-like structures in the stratosphere which had profound effects on stratospheric chemistry.

The wildfires triggered severe pyrocumulonimbus thunderstorms which propelled combustion products into the upper troposphere and lower stratosphere. These emissions could self-organize into a high vorticity anticyclonic structure, a phenomenon observed by Khaykin et al. (2020). This vortex effectively confined the mixture of gases and aerosols from biomass combustion products in the stratosphere for an extended period, leading to specific chemical reactions and interactions.

This study mainly utilizes data from the Microwave Limb Sounder (MLS) and the Atmospheric Chemistry Experiment Fourier Transformer Spectrometer (ACE-FTS) to investigate the behaviour of ozone-depleting species and the impact of biomass burning products within this most striking vortex structure. The data reveal a significant increase in water vapour and biomass burning products, such as CO, CH₃Cl, CH₃CN, and HCN. Concurrently, there were marked depletions in key stratospheric chemicals like the HNO₃, ClONO₂ and HCl reservoirs with an increase of the ClO radical as an indicator of chlorine activation over the lifetime of the vortex i.e. until 3 months starting in early January 2020.

The core objective of this study is to elucidate the specific reactivity of inorganic atmospheric species inside this smoke vortex which led to the formation of a localized ozone hole.  More investigations would be necessary to highlight the role of organic compounds on the observed ozone depletion.

The frequency of these dynamic structures is currently not well established in a context of global warming and may follow the increasing frequency and intensity of forest fires. As a result, smoke vortices could become a recurrent and important disturbance of the stratospheric chemistry in the future.