Tropical cyclones drive enhanced inorganic iodine in the mid-latitude upper troposphere

Halogens deplete tropospheric and stratospheric ozone, but the role of iodine remains elusive. Nevertheless, recent research has demonstrated iodine’s wide-ranging impact on tropospheric photochemistry. We report airborne measurements of atmospheric iodine oxide (IO) concentrations up to 15 km altitude from two flights of the WISE (Wave-driven ISentropic Exchange) campaign over the mid-Atlantic in September and October 2017. IO was retrieved from limb-scattered skylight in the upper troposphere (UT) using the airborne mini-DOAS instrument onboard the German High Altitude and Long Range research aircraft (HALO). Up to sixfold elevated IO mixing ratios (up to 0.6 ± 0.1 ppt) above background levels were observed in the UT in air masses transported by the category 5 hurricanes Irma and Maria, as indicated by CLaMS back-trajectory analyses. Atmospheric iodine predominantly originates from marine inorganic (I₂, HOI) and organic (CH₃I, CH₂I₂, CH₂IBr, and CH₂ICl) emissions. Our findings suggest that enhanced IO mixing ratios are likely driven by enhanced marine iodine emissions associated with high surface wind speeds in the vicinity of hurricanes, photochemical conversion of source gases into reactive iodine and efficient vertical transport of these iodine-rich air masses by tropical cyclones. Furthermore, our observations imply a potentially significant role of iodine-driven chemistry in air masses affected by tropical storms.