Potential influence of iodine-containing compounds on the chemistry of the troposphere in the polar spring. I. Ozone depletion

Iodine in the atmosphere, identified largely by the presence of IO, is a ubiquitous component of the troposphere in coastal and oceanic areas. The role, if any, that iodine chemistry plays in the polar ozone depletion episodes is not known. These events are rationalized today largely in terms of Br2- and BrCl-initiated reactions. The potential for enhancement of ozone depletions through the presence of iodine-containing molecules (I2, IBr, ICl, CH2I2, CH2IBr, CH2ICl, and CH3I) is investigated in this study. Computer simulations of the homogeneous chemistry are made using a reasonably complete reaction mechanism for Br-, Cl- and I-containing species together with representative chemistry of trace gases in the clean troposphere. The extent of uncertain alternative pathways and efficiencies for OIO and I2O2 photolyses are varied over a range of possible values to establish the sensitivity of the depletion events to these variables. The study shows that significant enhancements of the polar ozone depletion are expected when small amounts of iodine-containing compounds such as CH2I2, IBr, or ICl are present in a polar air mass containing representative Br2–BrCl-trace gas mixtures. The synergistic effect of the iodine compounds results from additional halogen-atom formation from IO–IO, IO–BrO, and IO–ClO reactions. Measurements of IO and precursor iodine-containing compounds are encouraged for future polar spring studies, as well as currently acknowledged important trace species (O3, CH2O, BrO, Br2, and BrCl).