High abundances of iodine monoxide (IO) are known to exist and to participate in local photochemistry of the marine boundary layer. IO participates in depletion episodes of O3 and in the removal of mercury in the Arctic polar spring. IO and higher-order iodine oxides are involved in the formation of new particles in coastal marine environments. IO has also been shown to affect the oxidizing capacity of the troposphere by altering the partitioning of NO2/NO and HO2/HO and by activating chlorine and bromine in sea salt aerosols. In the stratosphere, these same processes can lead to enhanced ozone loss rates. Detailed photochemical models that include iodine photochemistry, however, are hampered by the lack of observational data. The distribution of IO in vertical, horizontal, and temporal coordinates is unknown, so the impact of IO on global photochemistry cannot be predicted. The resolution of these important scientific issues requires an in situ IO sensor.