Timescales in atmospheric chemistry: CH 3 Br, the ocean, and ozone depletion potentials

Methyl bromide (CH3Br) supplies about half of the chemically active bromine (Bry) in the stratosphere. Efforts to control Bry-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, Bry} 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 Bry) 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 Bry 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