Measurement of methyl halides in the marine atmosphere

An analytical method for measuring atmospheric methyl halides was established based on canister sampling and capillary GC/MS. Stability tests for air samples collected in two kinds of canisters (electro-chemical buffing and fused-silica lined) with smooth inner surfaces, showed that both provided stable storage for CH3Cl, CH3Br, and CH3I. The method was applied to the measurement of methyl halides at a remote island (Okinawa, Japan) in August 1996, where nocturnal ozone depletion had been observed in summer. We found that atmospheric CH3Cl increased during stable nights and was negatively correlated with surface ozone concentration. The highest CH3Cl concentration amounted to as high as 1400 pptv and indicated that CH3Cl emitted from the surrounding coastal areas had accumulated in the boundary layer under a stable atmosphere at that time. A positive correlation was observed between CH3Br and CH3I, suggesting a common source in the area.