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.