Natural methyl bromide and methyl chloride emissions from coastal salt marshes

Atmospheric methyl bromide (CH 3 Br) and methyl chloride (CH 3 Cl), compounds that are involved in stratospheric ozone depletion, originate from both natural and anthropogenic sources. Current estimates of CH 3 Br and CH 3 Cl emissions from oceanic sources, terrestrial plants and fungi, biomass burning and anthropogenic inputs do not balance their losses owing to oxidation by hydroxyl radicals, oceanic degradation, and consumption in soils, suggesting that additional natural terrestrial sources may be important 1 . Here we show that CH 3 Br and CH 3 Cl are released to the atmosphere from all vegetation zones of two coastal salt marshes. We see very large fluxes of CH 3 Br and CH 3 Cl per unit area: up to 42 and 570 µmol m -2 d -1 , respectively. The fluxes show large diurnal, seasonal and spatial variabilities, but there is a strong correlation between the fluxes of CH 3 Br and those of CH 3 Cl, with an average molar flux ratio of roughly 1:20. If our measurements are typical of salt marshes globally, they suggest that such ecosystems, even though they constitute less than 0.1% of the global surface area 2 , may produce roughly 10% of the total fluxes of atmospheric CH 3 Br and CH 3 Cl.