Estimating the sources and sinks of methyl iodide in the springtime North Atlantic Ocean

Methyl iodide is a naturally produced compound that can be used to identify transport of marine air masses. Since methyl iodide is primarily produced in the ocean and has a short lifetime (on the order of 3-5 days), high concentrations suggest the recent influence of the ocean on an air mass. However, interest in methyl iodide is not limited to its use as a tracer. Methyl iodide is a major supplier of iodine for chemical reactions in the marine boundary layer, and it could contribute to ozone removal in the troposphere and stratosphere. Rapid vertical advection, particularly in the tropics, could transport methyl iodide to the lower stratosphere, releasing iodine radicals that participate in ozone depletion. This transport is expected to be most important in the tropics, but methyl iodide could also be delivered to the stratosphere at mid-latitudes. The cycling of methyl iodide in the water and the atmosphere is not well understood, which makes it difficult to predict how its concentrations will respond to global climate change. To address this, we use data collected for 30 days in a warm-core eddy (at about 46° N, -21° E) during the GasEx 98 cruise in the North Atlantic Ocean to study the oceanic cycling of methyl iodide. During this cruise methyl iodide concentrations were measured in both air and seawater samples. Methyl iodide seawater concentrations ranged from roughly 5-25 pM (corresponding to saturation anomalies of about 1000-8000%), although changes were generally gradual, taking place over the course of several days. We use surface irradiance measurements to test the assumption that methyl iodide production in the surface ocean is primarily photochemical. Methyl iodide air concentrations were more variable, increasing from about 0.8 ppt to about 3 ppt in a day or two on more than one occasion. The episodic increases in atmospheric values corresponded to increased wind speed events and also are strongly anticorrelated with two anthropogenic compounds (perchloroethylene and dichloromethane). We use flux calculations and air mass trajectories to identify the relative contributions from the local oceanic flux and from more distant transport of methyl iodide-rich air masses.