ABSTRACT

Methyl chloride (CH₃Cl), methyl bromide (CH₃Br), and methyl iodide (CH₃I) in marine boundary air were measured over the western Pacific and the southeastern Indian Ocean during the period of December 1996–February 1997. The mean concentrations of CH₃Cl, CH₃Br, and CH₃I were 623, 10.3, and 1.1 pptv, respectively, and their highest concentrations were observed in the tropics. The enhancement of CH₃Cl concentration in the tropics, particularly near islands, was consistent with the recently reported finding on its high emissions from tropical coastal lands. The enhancement of CH₃Br in the tropics was not related to the closeness of the sampling sites to islands, suggesting that land sources were not so important for CH₃Br as for CH₃Cl. The atmospheric CH₃I concentrations showed a little higher concentration in the tropics and in the southern hemisphere (SH) than in the northern hemisphere (NH). This finding suggests that its higher emission from the tropics and SH in austral summer compensates the higher rate of photolytic decay of this compound in the regions.