Atmospheric iodine levels influenced by sea surface emissions of inorganic iodine

Naturally occurring bromine-and iodine-containing compounds substantially reduce regional, and possibly even global, tropospheric ozone levels(1-4). As such, these halogen gases reduce the global warming effects of ozone in the troposphere(5), and its capacity to initiate the chemical removal of hydrocarbons such as methane. The majority of halogen-related surface ozone destruction is attributable to iodine chemistry(2). So far, organic iodine compounds have been assumed to serve as the main source of oceanic iodine emissions(1,6-9). However, known organic sources of atmospheric iodine cannot account for gas-phase iodine oxide concentrations in the lower troposphere over the tropical oceans(3,4). Here, we quantify gaseous emissions of inorganic iodine following the reaction of iodide with ozone in a series of laboratory experiments. We show that the reaction of iodide with ozone leads to the formation of both molecular iodine and hypoiodous acid. Using a kinetic box model of the sea surface layer and a one-dimensional model of the marine boundary layer, we show that the reaction of ozone with iodide on the sea surface could account for around 75% of observed iodine oxide levels over the tropical Atlantic Ocean. According to the sea surface model, hypoiodous acid-not previously considered as an oceanic source of iodine-is emitted at a rate ten-fold higher than that of molecular iodine under ambient conditions.