The potential effect of oceanic biological degradation on the lifetime of atmospheric CH₃Br

Abstract

Abstract. We use a global, coupled, ocean-atmosphere box model to examine the potential effect that biological degradation and its distribution can have on the lifetime of atmospheric CH3Br. The results of this study show that both the value of the oceanic degradation rate constant and its geographic distribution are important in determining the calculated atmospheric lifetime. The best estimate of the partial lifetime of atmospheric CH3Br with respect to oceanic loss now comes to 1.8 -1.9 y with a full possible range of 1.1 -3.9 y, which, together with other, non-oceanic losses, yields a total atmospheric lifetime of 0.7 y (0.6 -0.9 y). A subsequent revision of the budget for atmospheric CH3Br indicates that estimated sinks of CH3Br today exceed estimated sources by about 70 Gg y-1.

Atmospheric methyl halides and dimethyl sulfide from cattle

“…Equation (1) is more accurate for gases with Tr greater than the time constant for exchange between the Northern and Southern Hemispheres, which is about 0.8 year [Prather et al, 1987]. The most reliable quantity in (1) is the atmospheric burden: 145 Gg of CH3Br (1 Gg = 10 9 g) and 5000 Gg of CH3C1 (these data are from Butler [1994], Khalil et al [1993], and Singh et al [1983] and are probably accurate to within Gg yr -x [Yvon-Lewis and Butler, 1997]. Alternatively, current sink estimates may be too large; for relevant data and discussion of individual sources and sinks, see Yvon and Butler [1996] and Butler and Rodriguez [1996].…”

“…Briefly, major sources are thought to be biomass burning and emissions from fumigated agricultural soils. The burning of leaded gasoline as a source is highly uncertain, and the world's oceans appear to be a net sink for atmospheric CH3Br [Lobeft et al, 1995; Yvon and Butler, 1996; Yvon-Lewis and Butler, 1997].…”

A perspective on time: loss frequencies, time scales and lifetimes

“…Methyl bromide is a naturally occurring gas with oceanic sources. [45] Its production and use in agriculture and fumigation has increased its atmospheric abundance, and the bromine radicals it releases in the stratosphere (Br Y ) cause ozone depletion. Formal treatment of the CH 3 Br system identified lifetimes and time scales that were not directly recognisable from the local loss frequencies.…”

“…During the peak of anthropogenic emissions in the 1990s, ocean saturation was about 85 %, [45,47] and following mitigation the abundance dropped to ,8 ppt with average ocean saturation of 100 %. [48] Note that this drop of 20 % in CH 3 Br tropospheric abundance requires reduction of more than 50 % in tropospheric emissions.…”

Distribution of methyl chloride, methyl bromide, and methyl iodide in the marine boundary air over the western Pacific and southeastern Indian Ocean.

“…The ocean (Singh et al, 1983), biomass burning (Lobert et al, 1991) and fungal activity in rotten woods (Harper, 1985) are known sources for this compound. Recent studies have provided evidence that terrestrial systems, especially coastal areas can contribute significantly to CH 3 Cl in the global atmosphere (Yokouchi et al, 2000a; Rhew ing to be accounted for (Yvon and Butler, 1997). Methyl iodide (CH 3 I), being a major natural source of iodine in the atmosphere, is much more reactive than CH 3 Br or CH 3 Cl.…”