Stratospheric ozone depletion inside the volcanic plume shortly after the 2022 Hunga Tonga eruption

Abstract. In-plume ozone depletion was observed for about ten days by Microwave Limb Sounder (Aura/MLS) right after the January 2022 Hunga Tonga-Hunga Ha’apai (HTHH) eruption. This work analyzes the dynamic and chemical causes of this ozone depletion. The results show that the large water injection (~150 Tg) from the HTHH eruption, with ~0.0013 Tg injection of ClO (or ~0.0009 Tg of HCl), causes ozone loss due to strongly enhanced HOx and ClOx cycles and their interactions. Aside from the gas phase chemistry, the heterogeneous reaction rate for HOCl+HCl→Cl2+H2O increases to 104 cm-3 sec-1 and is a major cause of chlorine activation, making this event unique compared with the springtime polar ozone depletion where HCl+ClONO2 is more important. The large water injection causes relative humidity over ice to increase to 70 %–100 %, decreases the H2SO4/H2O binary solution weight percent to 35 % compared with the 70 % ambient value, and decreases the plume temperature by 2–6 K. These changes lead to high heterogeneous reaction rates. Plume lofting of ozone-poor air is evident during the first two days after the eruption, but ozone concentrations quickly recover because its chemical lifetime is short at 20 hPa. With such a large seawater injection, we expect that ~5 Tg Cl was lifted into the stratosphere by the HTHH eruption in the form of NaCl, but only ~0.02 % of that remained as active chlorine in the stratosphere. lightning NOx changes are not the reason for the HTHH initial in-plume O3 loss.