Inorganic bromine in the marine boundary layer: a critical review
1Air Chemistry Department, Max-Planck Institute of Chemistry, P.O. Box 3060, 55020 Mainz, Germany
2Department of Environmental Sciences, University of Virginia, Charlottesville, VA 22903, USA
3Center for Global Change Science, Massachusetts Institute of Technology, Cambridge, MA 02139, USA.
Now at: Climate Change Research Center, Institute for the Study of Earth, Oceans, and Space, University of New Hampshire, Durham, NH 03824, and Mount Washington Observatory, P.O. Box 2310, North Conway, NH 03860, USA
4CEMRC/New Mexico State University, 1400 University Drive, Carlsbad, NM 88220, USA
5CSIRO Atmospheric Research, Private Bag No. 1, Aspendale 3195, Australia
6Cape Grim Baseline Air Pollution Station, 159 Nelson Street, Smithton, Tasmania 7330, Australia
7Depts. of Oceanography and Atmospheric Sciences, Texas A&M University, TAMU-3146, College Station, TX 77843-3146, USA
8Institut für Umweltphysik, Universität Heidelberg, INF 229, 69120 Heidelberg, Germany. Now at: Meteorological Service of Canada, 4905 Dufferin Street, Toronto, Ont. M3H 5T4, Canada
9Department of Oceanography, University of Hawaii, 1000 Pope Road, Honolulu, HI 96822, USA
10Ghent University, Department of Analytical Chemistry, Institute for Nuclear Sciences, Proeftuinstraat 86, B-9000 Gent, Belgium
11Environmental Chemical Processes Laboratory, Department of Chemistry, University of Crete, P.O. Box 1470, 714090 Heraklion, Greece
12National Academy of Sciences, 2101 Constitution Ave. NW, Washington, DC 20418, USA
13European Commission, DG Joint Research Centre, Institute for Environment and Sustainability, T.P 290, I-21020 Ispra (VA), Italy
Abstract. The cycling of inorganic bromine in the marine boundary layer (mbl) has received increased attention in recent years. Bromide, a constituent of sea water, is injected into the atmosphere in association with sea-salt aerosol by breaking waves on the ocean surface. Measurements reveal that supermicrometer sea-salt aerosol is substantially depleted in bromine (often exceeding 50%) relative to conservative tracers, whereas marine submicrometer aerosol is often enriched in bromine. Model calculations, laboratory studies, and field observations strongly suggest that the supermicrometer depletions reflect the chemical transformation of particulate bromide to reactive inorganic gases that influence the processing of ozone and other important constituents of marine air. Mechanisms for the submicrometer enrichments are not well understood. Currently available techniques cannot reliably quantify many Br containing compounds at ambient concentrations and, consequently, our understanding of inorganic Br cycling over the oceans and its global significance are uncertain. To provide a more coherent framework for future research, we have reviewed measurements in marine aerosol, the gas phase, and in rain. We also summarize sources and sinks, as well as model and laboratory studies of chemical transformations. The focus is on inorganic bromine over the open oceans outside the polar regions. The generation of sea-salt aerosol at the ocean surface is the major tropospheric source producing about 6.2 Tg/a of bromide. The transport of Br from continents (as mineral aerosol, and as products from biomass-burning and fossil-fuel combustion) can be of local importance. Transport of degradation products of long-lived Br containing compounds from the stratosphere and other sources contribute lesser amounts. Available evidence suggests that, following aerosol acidification, sea-salt bromide reacts to form Br2 and BrCl that volatilize to the gas phase and photolyze in daylight to produce atomic Br and Cl. Subsequent transformations can destroy tropospheric ozone, oxidize dimethylsulfide (DMS) and hydrocarbons in the gas phase and S(IV) in aerosol solutions, and thereby potentially influence climate. The diurnal cycle of gas-phase Br and the corresponding particulate Br deficits are correlated. Higher values of Br in the gas phase during daytime are consistent with expectations based on photochemistry. We expect that the importance of inorganic Br cycling will vary in the future as a function of both increasing acidification of the atmosphere (through anthropogenic emissions) and climate changes. The latter affects bromine cycling via meteorological factors including global wind fields (and the associated production of sea-salt aerosol), temperature, and relative humidity.