Chlorine activation on volcanic and wildire aerosols
Heterogeneous chlorine activation is a major driver for stratospheric ozone depletion and is understood to happen mainly on polar stratospheric clouds (PSCs) at temperatures below about 195 K. The 2020 Australian wildfire released large amounts of organic aerosols, whose chemical properties under stratospheric conditions are virtually unknown.
We combined the past 30 years of satellite data to estimate chlorine activation after a series of volcanic eruptions and wildfires of different magnitudes. We found that chlorine activation after major wildfires can happen at warm mid-latitude temperatures even above 220 K. Model incorporating such mechanism also shows remarkable agreement with the observations.
Heterogeneous chlorine activation is a major driver for stratospheric ozone depletion and is understood to happen on polar stratospheric clouds (PSCs) at temperatures below about 195 K. The 2020 Australian wildfire released large amounts of organic aerosols, whose chemical properties under stratospheric conditions are virtually unknown. Here, we developed an approach to analyze the temperature dependency of atmospheric chlorine heterogeneous chemistry using satellite data. We found that such reactions can happen at temperatures even at 220 K on wildfire aerosols in 2020. Organic aerosols are present to some degree in the lower stratosphere even under background conditions. This result indicates that Cl processes on organic aerosols likely need to be considered in future stratospheric ozone simulations. The 2019 to 2020 Australian summer wildfires injected an amount of organic gases and particles into the stratosphere unprecedented in the satellite record since 2002, causing large unexpected changes in HCl and ClONO2. These fires provided a novel opportunity to evaluate heterogeneous reactions on organic aerosols in the context of stratospheric chlorine and ozone depletion chemistry. It has long been known that heterogeneous chlorine (Cl) activation occurs on the polar stratospheric clouds (PSCs; liquid and solid particles containing water, sulfuric acid, and in some cases nitric acid) that are found in the stratosphere, but these are only effective for ozone depletion chemistry at temperatures below about 195 K (i.e., largely in the polar regions during winter). Here, we develop an approach to quantitatively assess atmospheric evidence for these reactions using satellite data for both the polar (65 to 90°S) and the midlatitude (40 to 55°S) regions. We show that heterogeneous reactions apparently even happened at temperatures at 220 K during austral autumn on the organic aerosols present in 2020 in both regions, in contrast to earlier years. Further, increased variability in HCl was also found after the wildfires, suggesting diverse chemical properties among the 2020 aerosols. We also confirm the expectation based upon laboratory studies that heterogeneous Cl activation has a strong dependence upon water vapor partial pressure and hence atmospheric altitude, becoming much faster close to the tropopause. Our analysis improves the understanding of heterogeneous reactions that are important for stratospheric ozone chemistry under both background and wildfire conditions.
@article{wang_stratospheric_2023,title={Stratospheric chlorine processing after the 2020 {Australian} wildfires derived from satellite data},volume={120},number={11},urldate={2024-08-17},journal={Proceedings of the National Academy of Sciences},author={Wang, Peidong and Solomon, Susan and Stone, Kane},month=mar,year={2023},pages={e2213910120},}
2024
GRL
Contrasting Chlorine Chemistry on Volcanic and Wildfire Aerosols in the Southern Mid-Latitude Lower Stratosphere
Abstract Volcanic eruptions and wildfires can impact stratospheric chemistry. We apply tracer-tracer correlations to satellite data from Atmospheric Chemistry Experiment—Fourier Transform Spectrometer and the Halogen Occultation Experiment at 68 hPa to consistently compare the chemical impact on HCl after multiple wildfires and volcanic eruptions of different magnitudes. The 2020 Australian New Year (ANY) fire displayed an order of magnitude less stratospheric aerosol extinction than the 1991 Pinatubo eruption, but showed similar large changes in mid-latitude lower stratosphere HCl. While the mid-latitude aerosol loadings from the 2015 Calbuco and 2022 Hunga volcanic eruptions were similar to the ANY fire, little impact on HCl occurred. The 2009 Australian Black Saturday fire and 2021 smoke remaining from 2020 yield small HCl changes, at the edge of the detection method. These observed contrasts across events highlight greater reactivity for smoke versus volcanic aerosols at warm temperatures.
@article{wang_contrasting_2024,title={Contrasting {Chlorine} {Chemistry} on {Volcanic} and {Wildfire} {Aerosols} in the {Southern} {Mid}-{Latitude} {Lower} {Stratosphere}},volume={51},number={18},journal={Geophysical Research Letters},author={Wang, Peidong and Solomon, Susan},year={2024},keywords={wildfire, stratospheric chemistry, volcanic eruption, satellite},pages={e2024GL110412},}
2023
Nature
Chlorine activation and enhanced ozone depletion induced by wildfire aerosol
Susan Solomon, Kane Stone, Pengfei Yu, D. M. Murphy, Doug Kinnison, A. R. Ravishankara, and Peidong Wang
Remarkable perturbations in the stratospheric abundances of chlorine species and ozone were observed over Southern Hemisphere mid-latitudes following the 2020 Australian wildfires. 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 sulfate increases hydrochloric acid solubility 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 observations. 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 concern that more frequent and intense wildfires could delay ozone recovery in a warming world.
@article{solomon_chlorine_2023,title={Chlorine activation and enhanced ozone depletion induced by wildfire aerosol},volume={615},issn={1476-4687},number={7951},journal={Nature},author={Solomon, Susan and Stone, Kane and Yu, Pengfei and Murphy, D. M. and Kinnison, Doug and Ravishankara, A. R. and Wang, Peidong},month=mar,year={2023},pages={259--264},}
2024
GRL
Stratospheric Chlorine Processing After the Unprecedented Hunga Tonga Eruption
Jun Zhang, Peidong Wang, Douglas Kinnison, Susan Solomon, Jian Guan, Kane Stone, and Yunqian Zhu
Abstract Following the Hunga Tonga?Hunga Ha’apai (HTHH) eruption in January 2022, significant reductions in stratospheric hydrochloric acid (HCl) were observed in the Southern Hemisphere mid-latitudes during the latter half of 2022, suggesting potential chlorine activation. The objective of this study is to comprehensively understand the loss of HCl in the aftermath of HTHH. Satellite measurements and a global chemistry-climate model are employed for the analysis. We find strong agreement of 2022 anomalies between the modeled and the measured data. The observed tracer-tracer relations between nitrous oxide (N2O) and HCl indicate a significant role of chemical processing in the observed HCl reduction, especially during the austral winter of 2022. Further examining the roles of chlorine gas-phase and heterogeneous chemistry, we find that heterogeneous chemistry emerges as the primary driver for the chemical loss of HCl, and the reaction between hypobromous acid (HOBr) and HCl on sulfate aerosols is the dominant loss process.
@article{zhang_stratospheric_2024,title={Stratospheric {Chlorine} {Processing} {After} the {Unprecedented} {Hunga} {Tonga} {Eruption}},volume={51},issn={0094-8276},number={17},urldate={2025-06-05},journal={Geophysical Research Letters},author={Zhang, Jun and Wang, Peidong and Kinnison, Douglas and Solomon, Susan and Guan, Jian and Stone, Kane and Zhu, Yunqian},month=sep,year={2024},pages={e2024GL108649},}
