The Impacts of Short-Lived Chlorinated Species on Stratospheric Chlorine Budget and Ozone, and the Role of Transport

The emissions of most long-lived halogenated ozone-depleting substances (ODSs) are now decreasing, owing to controls on their production introduced by Montreal Protocol and its amendments. However, short-lived halogenated compounds can also have substantial impact on atmospheric chemistry, including stratospheric ozone, particularly if emitted near climatological uplift regions. It has recently become evident that emissions of some chlorinated very short-lived species (Cl-VSLS), such as chloroform (CHCl3) and dichloromethane (CH2Cl2), could be larger than previously believed and increasing, particularly in Asia. While these may exert a significant influence on atmospheric chemistry and climate, their impacts remain poorly characterised.

We address this issue using the UM-UKCA chemistry-climate model. We assess the impacts of Cl-VSLS on stratospheric chlorine budget and ozone over the first two decades of the 21st century; this constitutes the most up-to-date assessment as well as the first study to simulate Cl-VSLS impacts using a whole atmosphere chemistry-climate model. We examine the Cl-VSLS responses using a small ensemble of free-running simulations as well as two pairs of integrations where the meteorology was 'nudged' to either ERA5 or ERA-Interim reanalysis. This comparison between the free-running and nudged approaches provides insight into and demonstrates the role of atmospheric dynamics and model dynamical fields in modulating the atmospheric responses to Cl-VSLS. It also demonstrates the importance of the choice of model set-up for the inferred responses. Finally, we quantify the Cl-VSLS impacts during the recent Arctic winter of 2019/2020, where the formation of strong and cold polar vortex led to one of the largest Arctic stratospheric ozone depletion episodes in the observational record.