Iodine chemistry and its role in halogen activation and ozone loss in the marine boundary layer: A model study

A detailed set of reactions treating the gas and aqueous phase chemistry of the most important iodine species in the marine boundary layer (MBL) has been added to a box model which describes Br and Cl chemistry in the MEL. While Br and Cl originate from seasalt, the I compounds are largely derived photochemically from several biogenic alkyl iodides, in particular CH2I2, CH2ClI, C2H5I, C3H7I, or CH3I which are released from the sea. Their photodissociation produces some inorganic iodine gases which can rapidly react in the gas and aqueous phase with other halogen compounds. Scavenging of the iodine species HI, HOI, INO2, and IONO2 by aerosol particles is not a permanent sink as assumed in previous modeling studies. Aqueous-phase chemical reactions can produce the compounds IBr, ICI, and I-2, which will be released back into the gas phase due to their low solubility. Our study, although highly theoretical, suggests that almost all particulate iodine is in the chemical form of IO3-. Other aqueous-phase species are only temporary reservoirs and can be re-activated to yield gas phase iodine. Assuming release rates of the organic iodine compounds which yield atmospheric concentrations similar to some measurements, we calculate significant concentrations of reactive halogen gases. The addition of iodine chemistry to our reaction scheme has the effect of accelerating photochemical Br and Cl release from the seasalt. This causes an enhancement in ozone destruction rates in the MBL over that arising from the well established reactions O(D-1)+H2O --> 2OH, HO2+O-3 --> OH+2O(2), and OH+O-3 --> HO2+O-2. The given reaction scheme accounts for the formation of particulate iodine which is preferably accumulated in the smaller sulfate aerosol particles.