Iodine speciation and deposition fluxes from the marine atmosphere

The concentration and speciation of iodine have been determined in wet and dry deposition at a coastal site over a 15-month period. Deposition fluxes in rain (2.7 μmol m−2 yr−1) and aerosol (3.6–6.5 μmol m−2 yr−1) are the major routes for removal of iodine from the marine atmosphere onto the Earth's surface, with only a minor contribution from direct deposition of methyl iodide (0.003–0.17 μmol m−2 yr−1). Iodate (IO3−) is often considered to be the only species of iodine that is permanently removed to the aerosol phase, and IO3− may therefore be expected to be the dominant form of iodine in precipitation. However, iodide (I−) was found to constitute a significant fraction (5–100%) of iodine in both rain and aerosol. This implies that the rates of iodate formation and iodide volatilization (through reaction with hypohalous acids) are relatively slow. A third pool of aerosol iodine (nonvolatile organic compounds) may also contribute to removal of iodine from the atmosphere in dry or wet deposition.