Upper limits of stratospheric IO and OIO inferred from center-to-limb-darkening-corrected balloon-borne solar occultation visible spectra:: Implications for total gaseous iodine and stratospheric ozone -: art. no. 4455

We report upper limits of lower stratospheric IO ( inferred lowest values: 0.10 ppt, 0.07 ppt, and 0.06 ppt at 20, 15, and 12.5 km, respectively) and OIO ( inferred lowest values: 0.10 ppt, 0.06 ppt, and 0.04 ppt at 20, 15, and 12.5 km, respectively) inferred from balloon-borne solar occultation UV/visible spectroscopy. The spectra were recorded during a series of Laboratoire de Physique Moleculaire et Applications/differential optical absorption spectroscopy (LPMA/DOAS) balloon flights that were conducted at different geophysical conditions, i.e., inside the Arctic winter vortex, at midlatitudes, and at high latitudes in spring, summer, and fall. Photochemical modeling that accounts for the iodine partitioning during the observations allows us to infer upper limits of total inorganic gas-phase iodine (I-y), i.e., I-y (<20 km) of <= 0.10 +/- 0.02 ppt and <= 0.07 +/- 0.01 ppt, taking into account and neglecting OIO photolysis, respectively. For the middle stratosphere, the inferred upper limits of total Iy are larger because of the smaller detection sensitivity there. These observations suggest that either much less iodine enters the stratosphere than the amount expected from the stratospheric entry level iodine concentrations ( primarily the tropical upper troposphere) or it resides in other minor gaseous species or, eventually, in a nongaseous, i.e., particulate, form in the stratosphere. The implications of our iodine measurements for stratospheric ozone are also briefly assessed using a two-dimensional model. For the assumed loading of 0.1 ppt I-y the modeled ozone reduction was small compared to a model run without iodine, with a maximum decrease of around 1% in midlatitudes to high latitudes when OIO photolysis is included.