OBSERVATIONS IN EASTERN ENGLAND OF ELEVATED METHYL-IODIDE CONCENTRATIONS IN AIR OF ATLANTIC ORIGIN

Atmospheric methyl iodide (CH3I) has been measured at a ground-based site in eastern England for two consecutive summers. Maximum values of 43.1 pptv and 28.9 pptv were recorded in 1989 and 1990, respectively. CH3I was not detectable in the autumn and winter months. Episodes of elevated concentration persisted for periods ranging from a few hours to several days. The origin of much of the observed CH3I would appear to be the Atlantic Ocean, indicating the presence of large source areas, possibly phytoplankton blooms, in ocean waters. If so, this work provides the first evidence of long-range transport of an important iodine-bearing species at concentrations of hemispheric significance. Estimates are made of the dry deposition velocity of CH3I, and the potential impact of elevated tropospheric levels on the human uptake of iodine.