Identification of coastal emissions of methyl chloride and methyl bromide based on high-frequency measurements on Hateruma Island

Abstract

Hourly measurements of atmospheric methyl chloride (CH3Cl) and methyl bromide (CH3Br), as well as some other halocarbons, were performed by using automated preconcentration gas chromatography/mass spectrometry at Hateruma Island, the southernmost inhabited island in the Japanese archipelago. Our data indicate that CH3Cl and CH3Br concentrations were occasionally increased without corresponding increases of anthropogenic halocarbons, such as HCFC-22. Those nonpollution high-CH3Cl and high-CH3Br events were observed mostly when the wind was blowing at rather low speed along the shore (a partially vegetated raised coral beach). Presuming that these methyl halides were accumulated while the air mass traveled over the beach, the emission strengths of CH3Cl and CH3Br from the beach were roughly estimated to be 900 ± 350 μg m-2h-1 and 21 ± 10 μg m-2h-1, respectively. Coastal beach is likely a new important source for CH3Cl and CH3Br on a global scale.