Strong Evidence of Heterogeneous Processing on Stratospheric Sulfate Aerosol in the Extrapolar Southern Hemisphere Following the 2022 Hunga Tonga-Hunga Ha'apai Eruption

The January 2022 eruption of Hunga Tonga-Hunga Ha'apai (HT-HH) caused the largest enhancement in stratospheric aerosol loading in decades and produced an unprecedented enhancement in stratospheric water vapor, leading to strong stratospheric cooling that in turn induced changes in the large-scale circulation. Here we use satellite measurements of gas-phase constituents together with aerosol extinction to investigate the extent to which the thick aerosol, excess moisture, and strong cooling enabled heterogeneous chemical processing. In the southern tropics, unambiguous signatures of substantial chlorine and nitrogen repartitioning appear over a broad vertical domain almost immediately after the eruption, with depletion of N2O5, NOx, and HCl accompanied by enhancement of HNO3, ClO, and ClONO2. After initially rising steeply, HNO3 and ClO plateau, maintaining fairly constant abundances for several months. These patterns are consistent with the saturation of N2O5 hydrolysis, suggesting that this reaction is the primary mechanism for the observed composition changes. The southern midlatitudes and subtropics show similar but weaker enhancements in ClO and ClONO2. In those regions, however, effects of anomalous transport dominate the evolution of HNO3 and HCl, obscuring the signs of heterogeneous processing. Perturbations in chlorine species are considerably weaker than those measured in the southern midlatitude stratosphere in 2020 following the Australian New Year's fires. The moderate HT-HH-induced enhancements in reactive chlorine seen throughout the southern middle and low-latitude stratosphere, far smaller than those in typical winter polar vortices, do not lead to appreciable chemical ozone loss; rather, extrapolar lower-stratospheric ozone remains primarily controlled by dynamical processes. Plain Language Summary The January 2022 eruption of the Hunga Tonga-Hunga Ha'apai (HT-HH) volcano caused the largest increase in stratospheric aerosol in decades and produced an unprecedented increase in stratospheric water vapor, which led to strong stratospheric cooling that in turn altered stratospheric winds. Here we use satellite measurements to investigate the extent to which the thick aerosol, excess moisture, and strong cooling enabled heterogeneous chemical processing. In the southern tropics, unambiguous signatures of substantial changes in several chlorine and nitrogen species appear over a broad vertical range almost immediately after the eruption, with decreases in some species mirrored by increases in others. The southern midlatitudes and subtropics show similar but weaker evidence of chemical processing. In those regions, however, effects of anomalous wind patterns obscure the signs of heterogeneous processing in some species. Perturbations in chlorine are considerably weaker than those measured in the southern midlatitude stratosphere following the Australian New Year's fires in 2020. The moderate HT-HH-induced enhancements in reactive chlorine seen throughout the southern middle and low-latitude stratosphere, far smaller than those in typical winter polar vortices, do not lead to appreciable chemical ozone loss; rather, outside the polar regions, lower-stratospheric ozone remains primarily controlled by dynamical processes.