Detection of iodine over the continental United States: Implications for ozone and mercury oxidation

Most of our knowledge about the global distribution of atmospheric iodine is based on measurements of iodine monoxide (IO) radicals in the marine boundary layer and at high latitudes. Recent evidence of IO in the free troposphere is limited to few available aircraft measurements over oceans. We report the first ground-based Multi-AXis Differential Optical Absorption Spectroscopy (MAX-DOAS) observations of tropospheric IO radicals over the central continental United States. IO vertical columns measured at Storm Peak Laboratory, CO (40.455°N, 106.744°W, 3220 m.a.s.l.) are similar to those over oceans and vary significantly. A priori sensitivity studies indicate that IO mixing ratios increase with altitude. Back trajectory modeling indicates that IO is consistently observed in air masses quickly transported from over the Pacific Ocean. We compare the IO columns and their variability with predictions by a global model and use the observations to constrain a chemical box model to assess the atmospheric implications for ozone and mercury oxidation. Iodine-induced mercury oxidation is currently missing in atmospheric models, understudied, and helps to partially explain the elevated concentrations of oxidized mercury measured at SPL.