Chapter 5 of the Hunga Impacts Report presents the current state of knowledge of the immediate and longer-term effects of the Hunga volcanic eruption of January 2022 on global stratospheric composition based on observational and modelling studies. The extraordinary water vapour enhancement from the eruption triggered both heterogeneous and gas-phase chlorine activation, leading to rapid ozone loss in the fresh plume within one week. Longer-term impacts include altering stratospheric chemical composition, mostly in the southern tropical and middle latitudes, however, changes in stratospheric ozone observed were largely due to transport anomalies.

Overall, Hunga did not strongly perturb total column ozone. The minor effects on ozone from Hunga will diminish over the next decade as the excess water vapour is removed from the stratosphere through Antarctic dehydration and stratosphere-to-troposphere exchange. The observed anomalous chemical mechanisms following the eruption, on both short and longer-term timescales, resulted from well-known chemical reactions occurring on volcanic sulfate aerosol under conditions of highly enhanced stratospheric humidity unique in the observational record. No evidence of novel or previously unknown chemical mechanisms has been found.