Iodine volatilisation from the ocean

Overview
The presence of halogens (Cl, Br I) in the troposphere markedly changes the chemistry of this part of the atmosphere [1]. In particular, halogens affect the concentration of OH radicals (the most important oxidant in the troposphere), alter the partitionning in chemical families (NOx, HOx etc), and act as sinks for tropospheric ozone. A robust understanding of halogen chemistry and the source rates of halogens into the atmosphere is therefore crucial for understanding trace gas composition and the oxidising capacity of the troposphere [2].

In the case of iodine, the major souce was initailly thought to be organic compounds (CH3I, CH2I2 etc) emitted by plankton. Subsequently very large inputs of molecular iodine (I2) were observed in coastal regions from seaweeds exposed to the air around low tides [3]. Recent attention has switched to non-biological iodine inputs occuring over the open ocean [4]. Here gas-phase ozone reacts iodide ions dissolved in the surface layer of the ocean to release molecular iodine and hypoiodous acid (HOI). The reaction is slow because ozone concentrations are typically only 30 ppbv (mixing ratio = 30 x 10-9) and the concentration of iodide in seawater is only around 0.1 micro-Molar. However the ocean is a big place, and so aggregated over a large area, even a slow reaction can introduce large fluxes of iodine to the atmosphere. Additionally, the reaction of ozone at the sea surface consitutes a significant sink for tropospheric ozone. This sink partically offsets the net global sources of tropospheric ozone from the chemistry of nitrogen oxides (NOx is mainly a manmade pollutant).