Reactive Halogen Chemistry Research Articles

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

Abstract. 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 description 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 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 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 a 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. The reaction between Br atoms and C2H2 is shown to be unimportant for determining the degree of bromine activation in the remote 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.

Ground‐based measurements of halogen oxides at the Hudson Bay by active longpath DOAS and passive MAX‐DOAS

Intensive field measurements were carried out on the southeast coast of the Hudson Bay (55°N, 75°W) in spring 2001. The study focussed on reactive halogen chemistry and ozone/mercury depletion in the Hudson Bay region/Canadian lower Arctic. Several events of enhanced bromine oxide (BrO) coinciding with ozone depletion in the boundary layer (BL) were simultaneously measured by active longpath differential optical absorption spectroscopy (LP‐DOAS) and passive multi axis differential optical absorption spectroscopy (MAX‐DOAS). Significant differences in intensity and duration of ozone depletion events compared to high latitudes can be explained considering the daily alternation of daylight/nighttime which prevents complete ozone depletion within one day. First simultaneous measurements of active LP‐DOAS and passive MAX‐DOAS were carried out and compared. While LP‐DOAS monitored precise concentration values near the surface, MAX‐DOAS also captured BrO layers elevated from the surface which could not be seen by LP‐DOAS.

Reactive gaseous mercury formation in the North Pacific Ocean's marine boundary layer: A potential role of halogen chemistry

Reactive gaseous mercury (RGHg), atmospheric elemental mercury (Hg0), and ozone (O3) along with other ancillary parameters have been measured simultaneously in the marine boundary layer (MBL) over the North Pacific Ocean during the 2002‐IOC cruise between Osaka, Japan, and Honolulu, Hawaii. Atmospheric Hg0 concentrations varied from 1.6 to 4.7 ng m−3 with an average of 2.5 ng m−3 and did show significant variations along the cruise track with a slight diel cycle in Hg0 concentrations. RGHg concentrations varied from 0.15 to 92.4 pg m−3 with an average of 9.5 pg m−3. The data strongly suggest photochemical in situ formation of RGHg with increasing concentrations in the tropical regions concomitant to a dramatic decrease in O3 (from over 50 to less than 5 ppbv). A distinct diurnal variation in RGHg concentrations with maxima at midday suggests a photochemically driven oxidation of marine boundary layer Hg0. In the tropical area, enhanced RGHg formation and daily variation in O3 could both be related to reactive halogen chemistry. Flux calculations also emphasize the role of the tropical marine boundary layer in mercury cycling over the North Pacific Ocean.