A box model study on photochemical interactions between VOCs and reactive halogen species in the marine boundary layer
A new chemical scheme is developed for the multiphase photochemical box model SEAMAC (size- SEgregated Aerosol model for Marine Air Chemistry) to investigate photochemical interactions between volatile or- ganic compounds (VOCs) and reactive halogen species in the marine boundary layer (MBL). Based primarily on critically evaluated kinetic and photochemical rate parameters as well as a protocol for chemical mechanism development, the new scheme has achieved a near-explicit description of oxidative degradation of up to C3-hydrocarbons (CH4, C2H6, C3H8, C2H4, C3H6, and C2H2) initiated by reactions with OH rad- icals, Cl- and Br-atoms, and O3. Rate constants and product yields for reactions involving halogen species are taken from the literature where available, but the majority of them need to be estimated. In particular, addition reactions of halo- gen atoms with alkenes will result in forming halogenated organic intermediates, whose photochemical loss rates are carefully evaluated in the present work. Model calculations with the new chemical scheme reveal that the oceanic emis- sions of acetaldehyde (CH3CHO) and alkenes (especially C3H6) are important factors for regulating reactive halogen chemistry in the MBL by promoting the conversion of Br atoms into HBr or more stable brominated intermediates in the organic form. The latter include brominated hydroper- oxides, bromoacetaldehyde, and bromoacetone, which se- quester bromine from a reactive inorganic pool. The total mixing ratio of brominated organic species thus produced is likely to reach 1020% or more of that of inorganic gaseous bromine species over wide regions over the ocean. The reac- tion between Br atoms and C2H2 is shown to be unimportant for determining the degree of bromine activation in the re- mote MBL. These results imply that reactive halogen chemistry can mediate a link between the oceanic emissions of VOCs and the behaviors of compounds that are sensitive to halogen chemistry such as dimethyl sulfide, NOx, and O3 in the MBL.