Atmospheric iodine levels influenced by sea surface emissions of inorganic iodine

Abstract

Naturally occurring bromine- and iodine-containing compounds substantially reduce regional, and possibly even global, tropospheric ozone levels1,2,3,4. As such, these halogen gases reduce the global warming effects of ozone in the troposphere5, 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 chemistry2. So far, organic iodine compounds have been assumed to serve as the main source of oceanic iodine emissions1,6,7,8,9. However, known organic sources of atmospheric iodine cannot account for gas-phase iodine oxide concentrations in the lower troposphere over the tropical oceans3,4. 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.