Abstract

Abstract. Very short-lived substances (VSLS) contribute as source gases significantly to the tropospheric and stratospheric bromine loading. At present, an estimated 25 % of stratospheric bromine is of oceanic origin. In this study, we investigate how climate change may impact the ocean–atmosphere flux of brominated VSLS, their atmospheric transport, and chemical transformations and evaluate how these changes will affect stratospheric ozone over the 21st century. Under the assumption of fixed ocean water concentrations and RCP6.0 scenario, we find an increase of the ocean–atmosphere flux of brominated VSLS of about 8–10 % by the end of the 21st century compared to present day. A decrease in the tropospheric mixing ratios of VSLS and an increase in the lower stratosphere are attributed to changes in atmospheric chemistry and transport. Our model simulations reveal that this increase is counteracted by a corresponding reduction of inorganic bromine. Therefore the total amount of bromine from VSLS in the stratosphere will not be changed by an increase in upwelling. Part of the increase of VSLS in the tropical lower stratosphere results from an increase in the corresponding tropopause height. As the depletion of stratospheric ozone due to bromine depends also on the availability of chlorine, we find the impact of bromine on stratospheric ozone at the end of the 21st century reduced compared to present day. Thus, these studies highlight the different factors influencing the role of brominated VSLS in a future climate.

Highlights

  • Ozone is an important trace gas in the Earth’s atmosphere

  • The Very short-lived substances (VSLS) lifetime due to reaction with OH has been fixed to monthly mean values from the National Centre for Meteorological Research (CNRM) (Michou et al, 2011; Morgenstern et al, 2016) model calculations, while photolysis rates are computed within the ECHAM/Modular Earth Submodel System (MESSy) Atmospheric Chemistry (EMAC) submodel jval (Sander et al, 2014)

  • We have investigated long-term changes in emission and transport of brominated VSLS under a changing climate (RCP6.0)

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Summary

Introduction

Ozone is an important trace gas in the Earth’s atmosphere. The stratospheric layer with its highest abundance, the ozone layer, absorbs harmful ultraviolet radiation threatening all life forms on the Earth’s surface and acts as a potent greenhouse gas (GHG). Long-lived, anthropogenically emitted, halogenated source gases (SG), e.g., CH3Br and halons, have been restricted by the Montreal Protocol and its amendments Their atmospheric concentrations have started to decline globally PG are produced through photochemical decomposition of VSLS and provide reactive bromine (Bry, from Br, Br2, HBr, BrO, BrONO2, BrNO2, BrCl, and HOBr) to the stratosphere As shown by Lennartz et al (2015), the bottom-up fluxes based on the oceanic water concentrations of Ziska et al (2013) are in good agreement with available atmospheric VSLS observations. Ziska et al (2017) have investigated the future evolution of the ocean–atmosphere fluxes of VSLS through the 21st century based on Coupled Model Intercomparison Project (CMIP) 5 model output and fixed atmospheric VSLS concentrations.

Model and experiments
Long-term trends in oceanic emission fluxes
Global
Stratospheric bromine loading
Implications of a rising tropopause on VSLS mixing ratio profiles
Implications on ozone depletion
Findings
Conclusions
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