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The eruption of the Hunga Tonga‐Hunga Ha'apai volcano on 15 January 2022 was one of the most explosive eruptions of the last decades. The amount of water vapor injected into the stratosphere was unprecedented in the observational record, increasing the stratospheric water vapor burden by about 10%. Using model runs from the ATLAS chemistry and transport model and Microwave Limb Sounder (MLS) satellite observations, we show that while 20%–40% more water vapor than usual was entrained into the Antarctic polar vortex in 2023 as it formed, the direct chemical effect of the increased water vapor on Antarctic ozone depletion in June through October was minor (less than 4 DU). This is because low temperatures in the vortex, as occur every year in the Antarctic, limit water vapor to the saturation pressure and thus reset any anomalies through the process of dehydration before they can affect ozone loss. Plain Language Summary: The eruption of the Hunga Tonga‐Hunga Ha'apai volcano on 15 January 2022 was one of the most explosive eruptions of the last decades. An amount of water vapor unprecedented in the observational record was injected into the stratosphere, increasing the total stratospheric water vapor mass by about 10%. Using model runs and satellite observations, we show that while the dispersion of the plume increased water vapor in the Antarctic in 2023 by 20%–40% at the beginning of the ozone hole season compared to earlier years, the effect of the increased water vapor on the Antarctic ozone hole was minor. This is because low temperatures in the vortex, as occur every year in the Antarctic, limit water vapor due to condensation and thus reset any anomalies before they can affect ozone loss. Key Points: The Hunga Tonga‐Hunga Ha'apai eruption increased water vapor in the emerging Antarctic vortex in 2023 by 20%–40% compared to earlier yearsThe increased water vapor from Hunga Tonga had a minor effect on Antarctic ozone depletion through the end of October (less than 4 DU)This minor effect is due to low, but not unusual, vortex temperatures that reset water vapor anomalies before they could impact ozone loss [ABSTRACT FROM AUTHOR]

Abstract The eruption of the Hunga Tonga‐Hunga Ha'apai volcano on 15 January 2022 was one of the most explosive eruptions of the last decades. The amount of water vapor injected into the stratosphere was unprecedented in the observational record, increasing the stratospheric water vapor burden by about 10%. Using model runs from the ATLAS chemistry and transport model and Microwave Limb Sounder (MLS) satellite observations, we show that while 20%–40% more water vapor than usual was entrained into the Antarctic polar vortex in 2023 as it formed, the direct chemical effect of the increased water vapor on Antarctic ozone depletion in June through October was minor (less than 4 DU). This is because low temperatures in the vortex, as occur every year in the Antarctic, limit water vapor to the saturation pressure and thus reset any anomalies through the process of dehydration before they can affect ozone loss.

We propose a novel mechanism for isotopic exchange between CO2 and O3 via O(1D) + CO2 --> CO3* followed by CO3* --> CO2 + O(3P). A one-dimensional model calculation shows that this mechanism can account for the enrichment in 18O in the stratospheric CO2 observed by Gamo et al. [1989], using the heavy O3 profile observed by Mauersberger [1981]. The implications of this mechanism for other stratospheric species and as a source of isotopically heavy CO2 in the troposphere are briefly discussed.