Atmospheric impacts of chlorinated very short-lived substances over the recent past – Part 1: Stratospheric chlorine budget and the role of transport
Authors: Bednarz Ewa M. , Hossaini Ryan , Chipperfield Martyn P. , Abraham N. Luke , Braesicke Peter 
Published: August 2022

Abstract. 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.