Department of Earth System Science

Methyl bromide (CH₃Br) supplies about half of the chemically active bromine (Br_y) in the stratosphere. Efforts to control Br_y-catalyzed ozone depletion by phasing out, for example, agricultural use of CH₃Br may be thwarted by a lack of understanding of how the varied biogeochemical processes interact as a coupled system: in addition to the chemical industry, large natural sources come from the ocean; and losses occur in the atmosphere, ocean, and soils. A simplified one-dimensional stratosphere-troposphere-ocean model for {CH³Br, Br_y} that fits current understanding of sources and sinks is analyzed in terms of natural modes. Surface and ocean sources have effectively different steady state lifetimes (1.0 and 0.5 years, respectively), but the natural-mode decay times of the system (1.8 years for CH₃Br and 4.5 years for stratospheric Br_y) do not depend on the location of sources. The cumulative ozone depletion resulting from a single atmospheric release of CH₃Br integrates over the consequent slow rise and fall of Br_y in the lower stratosphere. Thus, in spite of the 1-year lifetime of CH₃Br, only half of the anticipated ozone recovery occurs in the first 7 years.