Chloro‐iodomethane in N. Atlantic waters: A potentially significant source of atmospheric iodine

Measurements have been made of chloroiodomethane (CH 2 ClI) in seawater samples collected in the N.W. Atlantic. Elevated concentrations (> 3 ng/ L) were found in surface waters at a number of locations in the pelagic zone. Background surface concentrations in the pelagic zone were 0.1–0.4 ng/L. Coastal/shelf regions averaged 0.1 ng/L off Labrador and Greenland, and 0.8 ng/L on the Grand Banks. We suggest that its distribution may be due to a phytoplankton source, either direct or indirect ( i.e. via an intermediate compound). Using measurements of methyl iodide (CH 3 I) made on the same samples, we make a first order estimate of the comparative fluxes of the two compounds from ocean to atmosphere. This indicates that the iodine carried by CH 2 ClI may be comparable with that carried as CH 3 I. Refinement of the flux estimates requires measurements of atmospheric concentrations of CH 2 ClI, and determination of its Henry's Law constant.