Inorganic bromine in the marine boundary layer: a critical review

[摘要] 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 inassociation with sea-salt aerosol by breaking waves on the ocean surface. Measurements reveal that supermicrometer sea-salt aerosol issubstantially depleted in bromine (often exceeding 50%) relative to conservative tracers, whereas marine submicrometer aerosol is oftenenriched in bromine. Model calculations, laboratory studies, and field observations strongly suggest that the supermicrometer depletionsreflect the chemical transformation of particulate bromide to reactive inorganic gases that influence the processing of ozone and otherimportant constituents of marine air. Mechanisms for the submicrometer enrichments are not well understood. Currently available techniquescannot reliably quantify many Br containing compounds at ambient concentrations and, consequently, our understanding ofinorganic 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 andlaboratory studies of chemical transformations. The focus is on inorganic bromine over the open oceans outside the polar regions. Thegeneration of sea-salt aerosol at the ocean surface is the major tropospheric source producing about6.2 Tg/a of bromide. The transport of Br from continents (as mineral aerosol, and asproducts from biomass-burning and fossil-fuel combustion) can be of local importance. Transport of degradation products of long-livedBr 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 andBrCl that volatilize to the gas phase and photolyze in daylight to produce atomicBr and Cl. Subsequent transformations can destroy tropospheric ozone, oxidizedimethylsulfide (DMS) and hydrocarbons in the gas phase and S(IV) in aerosol solutions, and thereby potentially influenceclimate. The diurnal cycle of gas-phase Br and the corresponding particulateBr deficits are correlated. Higher values of Br in the gas phase during daytime are consistentwith expectations based on photochemistry. We expect that the importance of inorganicBr cycling will vary in the future as a function of both increasing acidification of the atmosphere (throughanthropogenic emissions) and climate changes. The latter affects bromine cycling via meteorological factors including global windfields (and the associated production of sea-salt aerosol), temperature, and relative humidity.

[学科分类] 大气科学