Enhanced inorganic iodine in the upper troposphere potentially driven by elevated emission and/or fast vertical transport by tropical cyclone

Halogens are known to deplete ozone both in the troposphere and stratosphere. As the source distribution and thus the local contribution of chlorine and bromine containing species to atmospheric ozone depletion is reasonably well known, the respective role of iodine containing species is subject of current research. In contrast to the major sources of chlorine in the stratosphere which derive from man-made chlorinated hydrocarbons, about half of stratospheric bromine stems from bromocarbons of natural origin while iodine predominantly originates from inorganic species (I₂, and HOI) emitted from the oceans. It has been indicated previously that tropical cyclones emit elevated amounts of brominated species (e.g. CHBr₃ and CH₂Br₂) which are efficiently transported into the extratropical upper troposphere and eventually into the lower stratosphere. Here we provide evidence that also significant amounts of inorganic iodine emitted from the ocean surface and/or through sea-spray are transported to the extratropical upper troposphere through tropical cyclone driven fast vertical transport.

Our finding is based on the simultaneous detection of elevated amounts of brominated very short-lived substances (VSLS) and iodine oxide (IO, ~0.3 pptv) while simultaneously relatively low mixing ratios of the anthropogenically emitted CH₂Cl₂ were measured in the upper troposphere (~13 km, θ~360 K) of the mid-Atlantic on October 1^st,2017. CLaMS back-trajectory calculations driven by ECMWF ERA-Interim reanalysis suggest that these iodine-rich air masses originate from marine surface air masses being uplifted by the category 5 hurricane Maria.

The measurements were performed from aboard HALO (High Altitude and LOng range Aircraft) during the WISE (Wave-driven ISentropic Exchange) campaign over the mid-Atlantic in September and October 2017. IO was detected in limb scattered skylight using the miniDOAS instrument, while the organic chlorinated and brominated species were detected by the HAGAR-V and GhOST GC/MS instruments.

Our findings suggest that tropical storms lead to elevated emissions of inorganic iodine rapidly transported from the tropical marine boundary layer into the upper troposphere. This mechanism implies a potentially significant role of iodine in ozone destruction in the remnant air of tropical storms and a possible pathway for iodine to enter the lower stratosphere in significant amounts.