Ongoing large ozone depletion in the polar lower stratospheres: the role of increased water vapour

The very low temperatures of the polar lower stratosphere lead to the efficient seasonal depletion of ozone following the formation of polar stratospheric clouds (PSCs) and heterogeneous chlorine-activating reactions on their surfaces. The Montreal Protocol has controlled the production of major chlorine- (and bromine-) containing Ozone Depleting Substances (ODSs) and the stratospheric Cl and Br loadings have been slowly decreasing for over two decades. However, we are still experiencing very large (by some measures record) ozone depletion in the Antarctic and cold Arctic springs. There are a variety of factors involved but here we focus on the possible role of increased stratospheric water vapour, for example as occurred due to the eruption of the underwater volcano Hunga Tonga-Hunga Ha'apai in January 2022. We perform idealised TOMCAT three-dimensional chemical transport model experiments to investigate the impacts of a Hunga-like eruption being followed by conditions such as the very cold Arctic winter of 2019/2020; and contrast the impact of the cold Antarctic spring of 2020 with the previous warmer, more disturbed year of 2019. In the Antarctic, efficient dehydration by sedimenting ice PSCs limits the impact of a 1 ppmv increase in H₂O to a maximum additional depletion of 16 Dobson Units (DU) in 2020 and 11 DU in 2019 at the vortex edge in late September. A 1 ppmv H₂O increase in the cold Arctic vortex of 2019/2020 causes a maximum additional depletion of 16 DU at the vortex edge in mid March. The direct chemical impact of water vapour from a Hunga-like eruption on polar ozone is therefore modest in any given year, given natural variability. However, regular increased H₂O injection or production from increased CH₄ oxidation could represent an important factor in gradual long-terms trends.