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

[摘要] A new chemical scheme is developed for the multiphase photochemical boxmodel SEAMAC (size-SEgregated Aerosol model for Marine Air Chemistry)to investigate photochemical interactions betweenvolatile organic compounds (VOCs) and reactive halogen speciesin the marine boundary layer (MBL).Based primarily on critically evaluated kinetic and photochemical rateparameters as well as a protocol for chemical mechanism development,the new scheme has achieved a near-explicit description of oxidativedegradation of up to C₃-hydrocarbons (CH₄, C₂H₆,C₃H₈, C₂H₄, C₃H₆, and C₂H₂) initiated byreactions with OH radicals, Cl- and Br-atoms,and O₃.Rate constants and product yields for reactions involving halogen speciesare taken from the literature where available, but the majority of themneed to be estimated.In particular, addition reactions of halogen atoms with alkenes willresult in forming halogenated organic intermediates,whose photochemical loss rates are carefully evaluated in the present work.Model calculations with the new chemical scheme reveal thatthe oceanic emissions of acetaldehyde (CH₃CHO)and alkenes (especially C₃H₆)are important factors for regulating reactive halogen chemistry in the MBLby promoting the conversion of Br atoms into HBror more stable brominated intermediates in the organic form.The latter include brominated hydroperoxides, bromoacetaldehyde,and bromoacetone, which sequester bromine from a reactive inorganic pool.The total mixing ratio of brominated organic species thus produced is likelyto reach 10-20% or more of that of inorganic gaseous bromine speciesover wide regions over the ocean.The reaction between Br atoms and C₂H₂is shown to be unimportant for determining the degree of bromine activationin the remote MBL.These results imply that reactive halogen chemistry can mediatea link between the oceanic emissions of VOCs and the behaviors ofcompounds that are sensitive to halogen chemistry such as dimethyl sulfide,NO_x, and O₃ in the MBL.

[学科分类] 大气科学