Tropospheric Bromine Chemistry: Implications for Present and Pre-Industrial Ozone and Mercury

• Published in: Atmospheric Chemistry and Physics
• Main Authors: Parrella, J. P., Jacob, Daniel J., Liang, Q., Zhang, Y., Mickley, Loretta J., Miller, Benjamin Franklin, Evans, M. J., Yang, X., Pyle, J. A., Theys, N., Van Roozendael, M.
• Format: Article in Journal/Newspaper
• Language: English
• Published: European Geosciences Union 2012
• Subjects: Arctic; North Atlantic
• Online Access: http://nrs.harvard.edu/urn-3:HUL.InstRepos:11916599; https://doi.org/10.5194/acp-12-6723-2012

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. Engineering and Applied Sciences Version of Record