Natural methyl bromide and methyl chloride emissions from coastal salt marshes

Atmospheric methyl bromide (CH3Br) and methyl chloride (CH3Cl), compounds t hat are involved in stratospheric ozone depletion, originate from both natu ral and anthropogenic sources. Current estimates of CH3Br and CH3Cl emissio ns from oceanic sources, terrestrial plants and fungi, biomass burning and anthropogenic inputs do not balance their losses owing to oxidation by hydr oxyl radicals, oceanic degradation, and consumption in soils, suggesting th at additional natural terrestrial sources may be important(1). Here we show that CH3Br and CH3Cl are released to the atmosphere from all vegetation zo nes of two coastal salt marshes. We see very large fluxes of CH3Br and CH3C l per unit area: up to 42 and 570 mu mol m(-2) d(-1), respectively. The flu xes show large diurnal, seasonal and spatial variabilities, but there is a strong correlation between the fluxes of CH3Br and those of CH3Cl, with an average molar flux ratio of roughly 1:20. If our measurements are typical o f salt marshes globally, they suggest that such ecosystems, even though the y constitute less than 0.1% of the global surface area(2), may produce roug hly 10% of the total fluxes of atmospheric CH3Br and CH3Cl.