School of Earth and Environment

Short-Lived Halogenated Species

Ryan Hossaini, Martyn Chipperfield, Hannah Mantle

Background

Bromine chemistry has a significant impact on atmospheric composition. In the troposphere, bromine monoxide (BrO) is a sink for dimethyl sulphide (DMS) - an important climatic gas associated with cloud formation (Breider et al. 2010). In the stratosphere, bromine (along with chlorine) is largely responsible for the depletion of stratospheric ozone (O₃) which results in the Antarctic O₃ hole each Spring.

Biogenic Bromine Sources

Anthropogenic emissions account for approximately 75% of the total bromine in the stratosphere. The remainder is thought to arise from biogenic very short-lived substances (VSLS) such as bromoform (CHBr₃), although there is significant uncertainty regarding their emission, chemical transformation and transport through the atmosphere. It has been suggested that intensive seaweed farming in tropical regions may result in an increase in total VSLS emissions. Furthermore, changes to atmospheric circulation and chemistry due to global warming may impact the amount of VSLS reaching the stratosphere - where they contribute to O₃ destruction.

<b>Figure 1:</b> Modelled CHBr<sub>3</sub> increase in the lower stratosphere by 2100. The tropical West Pacific is an important region for transporting VSLS from the surface to higher altitudes.

Global Model Simulations

Simulations have been performed using the state-of-the-art United Kingdom Chemistry and Aerosol (UKCA) chemistry-climate model (CCM). This model is able to simulate the complex coupling between climate and chemistry and has been used here to assess how climate change may impact the transport of biogenic bromine species to the stratosphere. The results of this research were recently published in geophysical research letters (GRL).

Summary

Short-lived gases of natural marine origin are an important source of bromine in the stratosphere. As anthropogenic sources are now regulated under the Montreal Protocol, biogenic gases will become more important. Chemistry-climate model simulations show the amount of these gases reaching the stratosphere will likely increase in response to climate change. This is due to a range of complex and competing chemical/dynamical processes. If biogenic emissions were to increase in the future, it is possible that bromine chemistry could impact the timescale for recovery of the global ozone layer.

Publications

Breider, T., Chipperfield, M. P., Richards, N. A. D., Carslaw, K. S., Mann, G. W., and Spracklen, D. V.: The impact of BrO on dimethylsulfide in the remote marine boundary layer, Geophys. Res. Lett., 37, L02807, doi:10.1029/2009GL040868, 2010.

Hossaini, R., Chipperfield, M. P., Dhomse, S., Ordonez, C., Saiz-Lopez, A., Abraham, N. L., Archibald, A., Braesicke, P., Warwick, N., Yang, X., and Pyle, J.: Modelling future changes to the stratospheric source gas injection of biogenic bromocarbons, Geophys. Res. Lett., 39, L20813, doi:10.1029/2012GL053401, 2012.

Hossaini, R., Chipperfield, M. P., Feng, W., Breider, T. J., Atlas, E., Montzka, S. A., Miller, B. R., Moore, F., and Elkins, J.: The contribution of natural and anthropogenic very short-lived species to stratospheric bromine, Atmos. Chem. Phys., 12, 371-380, doi:10.5194/acp-12-371-2012, 2012.

Hossaini, R., Chipperfield, M. P., Monge-Sanz, B. M., Richards, N. A. D., Atlas, E., and Blake, D. R.: Bromoform and dibromomethane in the tropics: a 3-D model study of chemistry and transport, Atmos. Chem. Phys., 10, 719-735, doi:10.5194/acp-10-719-2010, 2010.