Methyl iodide (CH3I) is the dominant organic iodine species in the marine atmosphere,and could be a source of iodine radicals potential for ozone destruction.The seasonality of atmospheric CH3I at different latitudes was studied by periodical monitoring at Alert,Canada in the Arctic (82.5oN; 62.3oW),at Cape Ochiishi (43.2oN,145.5oE),at Hateruma Island (24.1oN,123.7oE),at San Cristobal Island in the tropics (1.0oS,89.4oW),at Cape Grim (40.41oS,144.6oE),at Showa Station in the Antarctic (69.0oS,39.6oE),over northwest Pacific Ocean,and over western Pacific Ocean.The observed CH3I concentration was highly variable,and raged from <0.02 ppt to 5 ppt.Highest annual average (1.1 ppt) was observed at San Cristobal Island in the tropics.At mid-latitudes,atmospheric CH3I showed a seasonal variation positively correlated with surface sea temperature.At high latitudes (polar regions),however,its highest concentration (0.5ppt) was observed in winter.Taking a photochemical lifetime of CH3I (about 4 days in the tropics,10 days at 60o latitude at the equinox) into consideration,the observed global distribution of CH3I suggests much higher emission at lower latitudes.