Modelling the chemistry of ozone, halogen compounds and hydrocarbons during Arctic Spring

The box model MoccaIce has been developed to study the chemistry of the arctic boundary layer. It treats chemical reactions in the gas phase and in the aerosol, as well as exchange between the 2 phases. Photolysis rates vary according to the solar declination during polar sunrise. Apart from the standard tropospheric chemistry of ozone, hydrocarbons, and nitrogen species, the reaction mechanism includes sulfur and the halogens Cl, Br, and I. Modeling an ozone depletion event, we found that iodine species contribute to the chemical destruction of ozone significantly if IO mixing ratios are about 1 pmol/mol. The reactions of BrO with BrO and IO are the main pathways of the ozone destruction cycle. Hydrocarbon concentrations decrease during ozone depletion events due to reaction with halogen atoms. The rate of ozone destruction depends on whether the addition of Br to C2H4 and C2H2 yields inert products or intermediates from which Br can be regenerated. Bromine and HCHO are positively correlated. The model produces HCHO during ozone depletion events, though not as much as reported from field observations. After the destruction of ozone has been competed, the halogen species are converted to halides and subsequently scavenged by aerosol particles.