The Influence of Stratospheric Hydration From the Hunga Eruption on Chemical Processing in the 2023 Antarctic Vortex.

We use measurements of trace gases from the Microwave Limb Sounder and polar stratospheric clouds (PSCs) from the Cloud‐Aerosol Lidar with Orthogonal Polarization to investigate how the extraordinary stratospheric water vapor enhancement from the 2022 Hunga eruption affected polar processing during the 2023 Antarctic winter. Although the dynamical characteristics of the vortex itself were generally unexceptional, the excess moisture initially raised PSC formation threshold temperatures above typical values. Cold conditions, especially in early July, prompted ice PSC formation and unusually severe irreversible dehydration at higher levels (500–700 K), while atypical hydration occurred at lower levels (380–460 K). Heterogeneous chemical processing was more extensive, both vertically (up to 750–800 K) and temporally (earlier in the season), than in prior Antarctic winters. The resultant HCl depletion and ClO enhancement redefined their previously observed ranges at and above 600 K. Albeit unmatched in the satellite record, the early‐winter upper‐level chlorine activation was insufficient to induce substantial ozone loss. Chlorine activation, denitrification, and dehydration processes ran to completion by July/August, with trace gas evolution mostly following the climatological mean thereafter, but with chlorine deactivation starting slightly later than usual. While cumulative ozone losses at 410–550 K were relatively large, probably because of the delayed chlorine deactivation, they were not unprecedented. Thus, ozone depletion was unremarkable throughout the lower stratosphere. Although Hunga enhanced PSC formation and chemical processing in early winter, saturation of lower stratospheric denitrification, dehydration, and chlorine activation (as is typical in the Antarctic) prevented an exceptionally severe ozone hole in 2023. Plain Language Summary: The 2022 eruption of the Hunga volcano produced an increase in stratospheric water vapor unmatched in the satellite record. Excess moisture pervaded the Antarctic stratosphere at the beginning of the 2023 Southern Hemisphere winter. Cold moist conditions prompted formation of ice clouds in the Antarctic stratosphere at higher altitudes than usual. Falling ice particles caused unusually severe permanent removal of water vapor from these levels and deposited it at lower altitudes. Chemical processing on polar stratospheric cloud particles occurred both earlier in the season and over a broader vertical domain than typical, resulting in exceptional early‐winter upper‐level conversion of chlorine from nonreactive into ozone‐destroying forms. Chemical processing ran its course by midwinter (i.e., July/August), with trace gases, including ozone, mostly following average behavior thereafter. Deactivation (reconversion of chlorine into nonreactive forms) started slightly later than in most years. While cumulative seasonal ozone losses were relatively large at some levels, probably because of the delayed chlorine deactivation, they were not unprecedented. Thus, although Hunga accelerated and expanded the vertical extent of chemical processing in early winter, that chemical processing reached completion as usually happens in Antarctic midwinter, preventing an extraordinarily severe ozone hole in 2023. Key Points: Hydration from Hunga led to unusually early and vertically extensive polar stratospheric cloud activity in the 2023 Antarctic vortexAs a result, heterogeneous chlorine activation occurred weeks earlier and at higher altitudes and dehydration removed more H2O than typicalDespite exceptional early‐winter conditions, cumulative ozone losses were unremarkable because stratospheric chemical processing saturated [ABSTRACT FROM AUTHOR]