We conducted a detailed investigation of the evolution of methyl bromide concentrations, degradation rates, and ventilation rates for 26 days in a naturally contained, warm-core eddy of the North Atlantic Ocean. This is the first study of the oceanic cycling of methyl bromide in a natural, contained system with a complete suite of supporting measurements of physical and chemical variables. Methyl bromide concentrations in the mixed layer ranged from 2.3 to 4.2 nmol m-3, degradation rates ranged from 0.1 to 0.9 nmol m-3 d-1, net sea-to-air exchange rates ranged from 0 to 0.5 nmol m-3 d-1, and net loss rates through the thermocline were less than 0.1 nmol m-3 d-1. From a mass balance for methyl bromide in the mixed layer, we calculated production rates ranging from <0.1 to 1.3 nmol m-3 d-1. The median of this range, 0.48 nmol m-3 d-1, is higher than the ∼0.15 nmol m-3 d-1 necessary to maintain the reported global oceanic emission of 56 Gg yr-1. This is reasonable, because our study area was supersaturated in methyl bromide, whereas the ocean as a whole is undersaturated.

We conducted a detailed investigation of the evolution of methyl bromide concentrations, degradation rates, and ventilation rates for 26 days in a naturally contained, warm-core eddy of the North Atlantic Ocean. This is the first study of the oceanic cycling of methyl bromide in a natural, contained system with a complete suite of supporting measurements of physical and chemical variables. Methyl bromide concentrations in the mixed layer ranged from 2.3 to 4.2 nmol m⁻³, degradation rates ranged from 0.1 to 0.9 nmol m⁻³ d⁻¹, net sea-to-air exchange rates ranged from 0 to 0.5 nmol m⁻³ d⁻¹, and net loss rates through the thermocline were less than 0.1 nmol m⁻³ d⁻¹. From a mass balance for methyl bromide in the mixed layer, we calculated production rates ranging from <0.1 to 1.3 nmol m⁻³ d⁻¹. The median of this range, 0.48 nmol m⁻³ d⁻¹, is higher than the ∼0.15 nmol m⁻³ d⁻¹ necessary to maintain the reported global oceanic emission of 56 Gg yr⁻¹. This is reasonable, because our study area was supersaturated in methyl bromide, whereas the ocean as a whole is undersaturated.

Measurements of trace gases in air trapped in polar firn (unconsolidated snow) demonstrate that natural sources of chlorofluorocarbons, halons, persistent chlorocarbon solvents and sulphur hexafluoride to the atmosphere are minimal or non-existent. Atmospheric concentrations of these gases, reconstructed back to the late nineteenth century, are consistent with atmospheric histories derived from anthropogenic emission rates and known atmospheric lifetimes. The measurements confirm the predominance of human activity in the atmospheric budget of organic chlorine, and allow the estimation of atmospheric histories of halogenated gases of combined anthropogenic and natural origin. The pre-twentieth-century burden of methyl chloride was close to that at present, while the burden of methyl bromide was probably over half of today's value.