A box model study on photochemical interactions between VOCs and reactive halogen species in the marine boundary layer

K. Toyota (1) , Y. Kanaya (1) , M. Takahashi (1) , H. Akimoto (1)

1 FRSGC - Frontier Research System for Global Change

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 organic 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 treatment of oxidative degradation of up to C₃-hydrocarbons CH₄, C₂H₆, C₃H₈, C₂H₄, C₃H₆, and C₂H₂) initiated by reactions with OH radicals, 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 halogen atoms with alkenes will result in the formation of 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 emissions 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 hydroperoxides, bromoacetaldehyde, and bromoacetone, which sequester bromine from reactive inorganic pool. The total mixing ratio of brominated organic species thus produced is likely to reach 10-20% or more of that of inorganic gaseous bromine species over wide regions over the ocean. On the other hand, the reaction between Br atoms and C₂H₂ is unimportant for determining the degree of bromine activation in the remote MBL. It is suggested that peroxyacetic acid formed via CH₃CHO oxidation is one of the important chemical agents for triggering autocatalytic halogen release from sea-salt aerosols. 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, NO_x, and O₃ in the MBL.

Domaines: Océan, Atmosphère

K. Toyota, Y. Kanaya, M. Takahashi, H. Akimoto. A box model study on photochemical interactions between VOCs and reactive halogen species in the marine boundary layer. Atmospheric Chemistry and Physics Discussions, 2003, 3 (5), pp.4549-4632. ⟨hal-00301233⟩