Atmospheric Chemistry of Organic Bromine and Iodine Compounds

In this chapter, the atmospheric sources, sinks, distributions, trends, and impacts of organic bromine and iodine compounds are reviewed. Most studies of bromine in the atmosphere have been driven by its well-characterized contribution to stratospheric ozone depletion. Most organic bromine can be grouped into three classes-methyl bromide, the man-made Halons, and a group of shorter-lived, naturally occurring species (e.g., CH₂Br₂, CHBr₃, etc.). Methyl bromide, which originates from an array of natural and anthropogenic sources, constitutes the major source of bromine to the stratosphere, contributing about half of the 20 ppt Br believed to be present there. The Halons, a group of long-lived compounds of strictly anthropogenic origin, are believed to contribute currently about 35% to this present-day stratospheric bromine burden, while the shorter-lived species (which emanate primarily from the oceans) contribute about 15%. Due to their link to ozone depletion, regulations are now in place (in the case of the Halons) or are soon to be in place (in the case of methyl bromide) to eliminate the production and sales of these species. Thus, the ensuing decades should see a reduction in the stratospheric burden of organic bromine. Most organic iodine in the atmosphere appears to originate from the ocean, though anthropogenic sources (rice paddies, biomass burning) also appear to contribute. Methyl iodide appears to be the largest contributor to the overall budget, though other iodinated methanes and higher alkanes (e.g., CH₂I₂, CH₂ICl, CH₂IBr, CH₃CH₂I, CH₃CHICH₃, CH₃CH₂CH₂I) also play a role. The lifetimes of iodinated species are short (of the order of a few days or less) due to their rapid photolysis in the troposphere. Thus, the impact of these species is largely restricted to the boundary layer, though a contribution to ozone depletion in the lower stratosphere cannot be entirely ruled out.