Chlorine Processing by Organic Aerosol from the 2020 Australian Wildfires and Implications for Future Ozone Recovery

The monitoring of inorganic chlorine species in the stratosphere, particularly hydrogen chloride (HCl), has been a critical measure for the success of the 1987 Montreal Protocol. As the most abundant chlorinated reservoir, HCl levels also reflect stratospheric variability caused by transient events, such as large wildfires. During December 2019 and January 2020, the Australian wildfires injected an unprecedented amount of smoke, containing organic aerosol, into the stratosphere. These particles provided surfaces for heterogeneous chemical reactions, altering the partitioning of chlorine species as a result. To investigate these effects, we used the TOMCAT 3-D chemical transport model to analyse the transport and chemical impact of smoke in the stratosphere in 2020. By incorporating an organic tracer (hexanoic acid) into our simulations, we modelled the evolution of smoke-related aerosol and its observed impact on the HCl distribution and variability.

Output from TOMCAT was evaluated using remote sensing data from the Atmospheric Chemistry Experiment - Fourier Transform Spectrometer (ACE-FTS) solar occultation instrument, along with data from the Aura Microwave Limb Sounder (MLS). ACE-FTS measurements show that HCl concentrations decreased to half their climatological values following the Australian wildfires. Reactivation processes on sulfate and organic aerosol particles contributed to this reduction, accompanied by an increase in active inorganic chlorine species and, in particular, approximately a 4% depletion of southern mid-latitude total column ozone.  

Ongoing work explores the potential impact of similar wildfire smoke injections under future conditions, in an atmosphere with less chlorine (and increased methane and nitrous oxide), by performing TOMCAT simulations for the year 2050. These experiments provide insights into inorganic chlorine processing and ozone layer recovery under conditions of increasing wildfire frequency and intensity driven by climate change.

Overall, our findings highlight the role of smoke organic aerosol in perturbing stratospheric chlorine chemistry and ozone. With wildfires expected to become more frequent and severe due to climate change, understanding these processes is essential for attribution of observed trace gas variability and ensuring the underlying recovery of the ozone layer from halogenated ozone-depleting substances.