Atmospheric impacts of chlorinated very short-lived substances over the recent past - Part 1: Stratospheric chlorine budget and the role of transport

Impacts of chlorinated very short-lived substances (Cl-VSLS) on stratospheric chlorine budget over the first two decades of the 21st century are assessed using the Met Office’s Unified Model coupled to the United Kingdom Chemistry and Aerosol (UM-UKCA) chemistry–climate model; this constitutes the most up-to-date assessment and 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 and two pairs of integrations where the meteorology was “nudged” to either ERA5 or ERA-Interim reanalysis.

The stratospheric chlorine source gas injection due to Cl-VSLS estimated from the free-running integrations doubled from ∼40 ppt Cl injected into the stratosphere in 2000 to ∼80 ppt Cl injected in 2019. Combined with chlorine product gas injection, the integrations showed a total of ∼130 ppt Cl injected into the stratosphere in 2019 due to Cl-VSLS. The use of the nudged model significantly increased the abundance of Cl-VSLS simulated in the lower stratosphere relative to the free-running model. Averaged over 2010–2018, simulations nudged to ERAI-Interim and ERA5 showed 20 ppt (i.e. a factor of 2) and 10 ppt (i.e. ∼50 %) more Cl, respectively, in the tropical lower stratosphere at 20 km in the form of Cl-VSLS source gases compared to the free-running case. These differences can be explained by the corresponding differences in the speed of the large-scale circulation. The results illustrate the strong dependence of the simulated stratospheric Cl-VSLS levels on the model dynamical fields. In UM-UKCA, this corresponds to the choice between free-running versus nudged set-up, and to the reanalysis dataset used for nudging.

Temporal changes in Cl-VSLS are found to have significantly impacted recent HCl and COCl2 trends in the model. In the tropical lower stratosphere, the inclusion of Cl-VSLS reduced the magnitude of the negative HCl and COCl2 trends (e.g. from ~ -8%(HCl)/decade and ~ -4 ppt(COCl2)/decade at ∼20 km to ~ -6%(HCl)/decade and ∼ −2 ppt(COCl2)/decade in the free running simulations) and gave rise to positive tropospheric trends in both tracers. In the tropics, both the free-running and nudged integrations with Cl-VSLS included compared much better to the observed trends from the ACE-FTS satellite record than the analogous simulations without Cl-VSLS. Since observed HCl trends provide information on the evolution of total stratospheric chlorine and, thus, the effectiveness of the Montreal Protocol, our results demonstrate that Cl-VSLS are a confounding factor in the interpretation of such data and should be factored into future analysis. Unlike the nudged model runs, the ensemble mean free-running integrations did not reproduce the hemispheric asymmetry in the observed mid-latitude HCl and COCl2 trends related to short-term dynamical variability. The individual ensemble members also showed a considerable spread of the diagnosed tracer trends, illustrating the role of natural interannual variability in modulating the diagnosed responses and the need for caution when interpreting both model and observed tracer trends derived over a relatively short time period.