1School of Engineering and Applied Sciences, Harvard University, Cambridge, MA, USA
2Universities Space Research Association GESTAR, Columbia, MD, USA
3NASA Goddard Space Flight Center, Greenbelt, MD, USA
4Department of Atmospheric Sciences, University of Washington, Seattle, Washington, USA
5Department of Chemistry, University of York, York, UK
6National Centre for Atmospheric Sciences (NCAS), York, UK
7National Centre for Atmospheric Sciences (NCAS), Cambridge, UK
8Centre for Atmospheric Sciences, Department of Chemistry, University of Cambridge, Cambridge, CB2 1EW, UK
9Belgian Institute for Space Aeronomy (IASB-BIRA), Brussels, Belgium
Received: 05 Apr 2012 – Discussion started: 16 Apr 2012
Abstract. We present a new model for the global tropospheric chemistry of inorganic bromine (Bry) coupled to oxidant-aerosol chemistry in the GEOS-Chem chemical transport model (CTM). Sources of tropospheric Bry include debromination of sea-salt aerosol, photolysis and oxidation of short-lived bromocarbons, and transport from the stratosphere. Comparison to a GOME-2 satellite climatology of tropospheric BrO columns shows that the model can reproduce the observed increase of BrO with latitude, the northern mid-latitudes maximum in winter, and the Arctic maximum in spring. This successful simulation is contingent on the HOBr + HBr reaction taking place in aqueous aerosols and ice clouds. Bromine chemistry in the model decreases tropospheric ozone mixing ratios by <1–8 nmol mol−1 (6.5% globally), with the largest effects in the northern extratropics in spring. The global mean tropospheric OH concentration decreases by 4%. Inclusion of bromine chemistry improves the ability of global models (GEOS-Chem and p-TOMCAT) to simulate observed 19th-century ozone and its seasonality. Bromine effects on tropospheric ozone are comparable in the present-day and pre-industrial atmospheres so that estimates of anthropogenic radiative forcing are minimally affected. Br atom concentrations are 40% higher in the pre-industrial atmosphere due to lower ozone, which would decrease by a factor of 2 the atmospheric lifetime of elemental mercury against oxidation by Br. This suggests that historical anthropogenic mercury emissions may have mostly deposited to northern mid-latitudes, enriching the corresponding surface reservoirs. The persistent rise in background surface ozone at northern mid-latitudes during the past decades could possibly contribute to the observations of elevated mercury in subsurface waters of the North Atlantic.
Revised: 06 Jul 2012 – Accepted: 09 Jul 2012 – Published: 01 Aug 2012
Parrella, J. P., Jacob, D. J., Liang, Q., Zhang, Y., Mickley, L. J., Miller, B., Evans, M. J., Yang, X., Pyle, J. A., Theys, N., and Van Roozendael, M.: Tropospheric bromine chemistry: implications for present and pre-industrial ozone and mercury, Atmos. Chem. Phys., 12, 6723-6740, doi:10.5194/acp-12-6723-2012, 2012.