Stratospheric Chlorine Processing After the Unprecedented Hunga Tonga Eruption

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. Plain Language Summary After the eruption of the Hunga Tonga–Hunga Ha'apai (HTHH) volcano in January 2022, there was a substantial decrease in stratospheric hydrochloric acid (HCl) in the Southern Hemisphere mid‐latitudes in the latter part of 2022. This decrease suggests that a chemical reaction involving chlorine processing might have happened. This study aims to comprehensively understand the significant loss of HCl following the HTHH eruption, utilizing satellite measurements and a global chemistry‐climate model for analysis. The anomalies in 2022 show remarkable agreement between the modeled and measured data. By comparing the levels of HCl with another gas called nitrous oxide (N2O), we discover that a lot of the HCl loss was due to chemical reactions, especially during the Southern Hemisphere winter. Upon further investigation into the role of chlorine gas‐phase and heterogeneous chemistry, heterogeneous chemistry emerges as a primary driver for the chemical loss of HCl. The reaction between hypobromous acid (HOBr) and HCl on sulfate aerosols is identified as the dominant loss process. Key Points Analysis using both model and satellites suggest a significant role of chemical processing in the observed HCl reduction at 20 km Analysis of gas‐ and heterogeneous‐phase reactions indicates that heterogeneous chemistry is the main driver for the chemical loss of HCl The dominant heterogeneous loss for HCl is via HOBr + HCl, with HCl + OH also sig