Rapid degradation of atmospheric methyl bromide in soils

Abstract

METHYL bromide (CH₃Br), a widely used agricultural fumigant, may be an important source of atmospheric bromine radicals, which destroy stratospheric ozone^1–10. In current models of this compound's atmospheric behaviour, the main sinks are taken to be oxidation by hydroxyl radicals, photolysis and uptake by the oceans^1–5,7,9,10. But there is also evidence that CH₃Br is consumed in soils^8,11–13. Here we report laboratory and field experiments which show that, when exposed to a variety of soil types at low mixing ratios, CH₃Br is rapidly and irreversibly removed to below the levels found in the global atmosphere. We show that the uptake process is bacterially mediated. We estimate the global annual soil sink to be 42 ± 32 Gg; coupled with other removal mechanisms, this suggests an atmospheric lifetime for CH₃Br of about 0.8 yr, just half the previous best estimate⁹, and an ozone depletion potential that is about 30% smaller than the previous estimate¹.