How sensitive is the recovery of stratospheric ozone to changes in concentrations of very short-lived bromocarbons?

X Yang,A T Archibald,P Braesicke,N L Abraham,N J Warwick,P J Telford,J A Pyle,J Keeble

doi:10.5194/acp-14-10431-2014

Abstract

Abstract. Naturally produced very short-lived substances (VSLS) account for almost a quarter of the current stratospheric inorganic bromine, Bry. Following VSLS oxidation, bromine radicals (Br and BrO) can catalytically destroy ozone. The extent to which possible increases in surface emissions or transport of these VSLS bromocarbons to the stratosphere could counteract the effect of halogen reductions under the Montreal Protocol is an important policy question. Here, by using a chemistry–climate model, UM-UKCA, we investigate the impact of a hypothetical doubling (an increase of 5 ppt Bry) of VSLS bromocarbons on ozone and how the resulting ozone changes depend on the background concentrations of chlorine and bromine. Our model experiments indicate that for the 5 ppt increase in Bry from VSLS, the ozone decrease in the lowermost stratosphere of the Southern Hemisphere (SH) may reach up to 10% in the annual mean; the ozone decrease in the Northern Hemisphere (NH) is smaller (4–6%). The largest impact on the ozone column is found in the Antarctic spring. There is a significantly larger ozone decrease following the doubling of the VSLS burden under a high stratospheric chlorine background than under a low chlorine background, indicating the importance of the inter-halogen reactions. For example, the decline in the high-latitude, lower-stratospheric ozone concentration as a function of Bry is higher by about 30–40% when stratospheric Cly is ~ 3 ppb (present day), compared with Cly of ~ 0.8 ppb (a pre-industrial or projected future situation). Bromine will play an important role in the future ozone layer. However, even if bromine levels from natural VSLS were to increase significantly later this century, changes in the concentration of ozone will likely be dominated by the decrease in anthropogenic chlorine. Our calculation suggests that for a 5 ppt increase in Bry from VSLS, the Antarctic ozone hole recovery date could be delayed by approximately 6–8 years, depending on Cly levels.

Highlights

It is very well established that stratospheric ozone loss since the 1970s has occurred as a consequence of emissions of halogenated species into the atmosphere
Produced very short-lived substances (VSLS) bromocarbons account for almost a quarter of the current stratospheric inorganic bromine and about half of that in the pre-industrial era
Fundamental emissions processes for the VSLS are not well understood, so that we are unsure how emissions might have changed in the past or how they will respond to future climate change

Summary

Introduction

It is very well established that stratospheric ozone loss since the 1970s has occurred as a consequence of emissions of halogenated species into the atmosphere. While anthropogenic halogen loading to the stratosphere should decrease in response to the Montreal Protocol, we do not know how the concentration of the VSLS bromocarbons will change in the future Since they are mainly produced by biochemical processes, mostly in the ocean, their emissions could change with climate, perhaps depending on sea surface temperature, on ocean pH or on surface wind stress (Gschwend et al, 1985; Keppler et al, 2000). Each experiment is run twice, first with a VSLS contribution of 5 ppt and with an additional increase of 5 ppt These experiments allow us to explore how ozone loss due to short-lived bromocarbons could vary with background chlorine and bromine loading.

Model and experiments

Result and discussions

Findings

Conclusions and discussions