Remarkable perturbations in the stratospheric abundances of chlorine species and ozone were observed over Southern Hemisphere mid-latitudes following the 2020 Australian wildfires1,2. These changes in atmospheric chemical composition suggest that wildfire aerosols affect stratospheric chlorine and ozone depletion chemistry. Here we propose that wildfire aerosol containing a mixture of oxidized organics and sulfate3,4,5,6,7 increases hydrochloric acid solubility8,9,10,11 and associated heterogeneous reaction rates, activating reactive chlorine species and enhancing ozone loss rates at relatively warm stratospheric temperatures. We test our hypothesis by comparing atmospheric observations to model simulations that include the proposed mechanism. Modelled changes in 2020 hydrochloric acid, chlorine nitrate and hypochlorous acid abundances are in good agreement with observations1,2. Our results indicate that wildfire aerosol chemistry, although not accounting for the record duration of the 2020 Antarctic ozone hole, does yield an increase in its area and a 3–5% depletion of southern mid-latitude total column ozone. These findings increase concern2,12,13 that more frequent and intense wildfires could delay ozone recovery in a warming world.
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All data used in this study are publicly available. MLS data: https://disc.gsfc.nasa.gov/datasets?page=1&source=Aura%20MLS; ACE-FTS data: http://www.ace.uwaterloo.ca (with registration: https://databace.scisat.ca/l2signup.php); CESM1-CARMA: https://doi.org/10.7910/DVN/GHNJQA.
The model used in this study can be accessed at https://www2.cesm.ucar.edu/models/cesm1.2/cesm/doc/usersguide/x290.html. The changes described herein for the kinetics parameterization are available at https://doi.org/10.7910/DVN/GHNJQA.
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S.S. and K.S. are partly supported by NSF 1848863. D.K. was financed in part by NASA grant 80NSSC19K0952. P.Y. is supported by the National Natural Science Foundation of China (42175089, 42121004). D.M.M. is supported by NOAA base and climate funding. The CESM project is supported by the National Science Foundation and the Office of Science (BER) of the U.S. Department of Energy. We gratefully acknowledge high-performance computing support from Cheyenne (https://doi.org/10.5065/D6RX99HX) provided by NCAR’s Computational and Information Systems Laboratory, sponsored by the National Science Foundation.
The authors declare no competing interests.
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Extended data figures and tables
Extended Data Fig. 1 Modelled and observed aerosol extinction at 18.5 km.
The time evolution of aerosol extinction (km−1) is shown at 18.5 km for Ozone Mapping and Profiler Suite (OMPS) observations (for 745 nm, a) and in the model (for 675 nm, b) in 2020.
Extended Data Fig. 2 Observed and modelled 2020 absolute abundances for chemical species from 30–50° S at 68 hPa.
Grey shaded regions show the ranges of 24-h averaged satellite data from the climatologies of satellite observations (in mixing ratio units) before 2020 (daily O3, HCl and ClO from MLS and monthly ClONO2 from ACE) and the grey line shows their averages, whereas black lines show the observed values for 2020. Other coloured lines show model-calculated abundances for the no organics control run (blue line) and for three model test cases: including only N2O5 hydrolysis on the aerosols (dashed brown line), considering the added organic material as a dilution factor (green dashed line) and considering the adopted solubility of HCl in organic acid particles (red line). Corresponding anomalies are shown in Fig. 2.
Extended Data Fig. 3 Observed and modelled monthly averaged anomalies (a) and mixing ratios (b) for HOCl (from ACE) for 30–50° S at 68 hPa.
Grey shaded regions show the ranges of 24-h averaged satellite data from the climatology before 2020, whereas black lines show the observed values for 2020. Other coloured lines show calculated values for 2020 for the no organics control run (blue line) and for three model test cases: including only N2O5 hydrolysis on the aerosols (dashed brown line), considering the added organic material as a dilution factor (green dashed line) and considering the adopted solubility of HCl in organic acid particles (red line).
Extended Data Fig. 4 Observed and modelled anomalies (a) and mixing ratios (b) for HNO3 (from MLS) for 30–50° S at 68 hPa.
Grey shaded regions show the ranges of 24-h daily averaged satellite data from the climatology before 2020, whereas black lines show the observed values for 2020. Other coloured lines show calculated values for 2020 for the no organics control run (blue line) and for three model test cases: including only N2O5 hydrolysis on the aerosols (brown dashed line), considering the added organic material as a dilution factor (dashed green line) and considering the adopted solubility of HCl in organic acid particles (red line).
Extended Data Fig. 5 Percent ozone anomalies for 30–50° S on coincident days of measurement for ACE and MLS during June–July 2020.
Data for each satellite have been normalized by their respective climatologies. Note that there are differences in spatial and temporal sampling between the two instruments. Black line shows MLS data while grey line shows ACE data interpolated onto the MLS pressure grid.
Extended Data Fig. 6 Observed and modelled vertical profile absolute abundances for chemical species from 30–50° S in June–July of 2020.
Grey shaded regions show the ranges of 24-h averaged satellite anomalies (in number density units) in years before 2020 (daily O3 and ClO from MLS and HCl and ClONO2 from ACE) and the grey line shows their averages, whereas black lines show observed abundances for 2020. Other coloured lines show calculated values for 2020 for the no organics control run (blue line) and for three model test cases: including only N2O5 hydrolysis on the aerosols (brown dashed line), considering the added organic material as a dilution factor (green dashed line) and considering the adopted solubility of HCl in organic acid particles (red line). Corresponding anomalies are shown in Fig. 3.
Extended Data Fig. 7 Distribution of calculated ozone loss in September 2020.
Percentage change in model-calculated ozone as a function of latitude and height for the oxidized organics solubility model case is shown, as compared with the no organics control run.
Extended Data Fig. 8 Contour maps of monthly mean HCl abundances (ppbv) at 68 hPa for observations and models.
The modelled no organics control case is shown (left column), along with MLS-measured climatological average from 2005–2019 (second from left), modelled oxidized organics solubility case (second from right) and MLS measurements for 2020 (right).
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Solomon, S., Stone, K., Yu, P. et al. Chlorine activation and enhanced ozone depletion induced by wildfire aerosol. Nature 615, 259–264 (2023). https://doi.org/10.1038/s41586-022-05683-0
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