Stratospheric chlorine activation after the unprecedented water-rich Hunga Tonga eruption

Following the eruption of Hunga Tonga–Hunga Ha’apai (HTHH) in January 2022, a significant reduction in stratospheric hydrochloric acid (HCl) was observed in the Southern Hemisphere mid-latitudes during the austral winter of 2022. This eruption injected sulfur dioxide and unprecedented amounts of water vapor into the stratosphere. The objective of this study is to comprehensively understand the substantial loss of HCl in the aftermath of HTHH. Satellite measurements from the Atmospheric Chemistry Experiment-Fourier Transform Spectrometer (ACE) and Microwave Limb Sounder (MLS), along with data from the global chemistry-climate model Whole Atmosphere Community Climate Model (WACCM), are employed for the analysis. We first compare the modeled 2022 anomalies of HCl and N2O with observations from ACE and MLS, and find noteworthy agreement between the model outputs and the measured data. We then utilize the observed tracer-tracer relations between N2O and HCl to distinguish HCl chemical processing from dynamical transport. The results indicate a significant role of chemical processing in the observed HCl reduction. The chemical changes in HCl derived from ACE and MLS align with the changes calculated from nudged model simulations, where dynamics are fixed to reanalysis. Further delving into the WACCM’s detailed chemistry, we examine individual chlorine gas-phase and heterogeneous reactions. Heterogeneous chemistry emerges as the primary driver for the chemical loss of HCl, with the reaction between HOBr and HCl on sulfate aerosols identified as the dominant loss process.