HR: 0830h
AN: A11F-0054    [PDF]
TI: Formation of Gaseous Bromine From the Photolysis of Nitrate and Hydrogen Peroxide in Sea-Salt Solutions
AU: * George, I
EM: ijgeorge@ucdavis.edu
AF: Department of Land, Air and Water Resources, University of California, Davis, One Shields Ave., Davis, CA 95616-8627 United States
AU: Anastasio, C
EM: canastasio@ucdavis.edu
AF: Department of Land, Air and Water Resources, University of California, Davis, One Shields Ave., Davis, CA 95616-8627 United States
AB: The formation of reactive halogen gases in sea-salt particles can play a significant role in chemistry within the marine boundary layer (MBL). While there have been several proposed mechanisms for the formation of reactive halogens in sea-salt particles, we were interested in whether hydroxyl radical ($^{\cdot}$OH) generated within the particles themselves could lead to release of gaseous reactive bromine species (Br$^{\ast}$(g)). To examine this question, we illuminated (313 nm) solutions of chloride and bromide that contained either nitrate or hydrogen peroxide as a photochemical source of $^{\cdot}$OH. Experiments were performed either in concentrated seawater or in laboratory solutions with halide concentrations that in most experiments were typical for deliquesced, aged sea-salt particles (e.g., 3.5 M Cl$^{-}$, 5.0 mM Br$^{-}$, 100 mM NO$_{3}$$^{-}$, and pH 4). During illumination we bubbled air through the solutions and collected the released gaseous reactive bromine in downstream bubblers or denuders. We found that gaseous bromide was nearly always released during the illumination of solutions containing either nitrate (100 mM) or hydrogen peroxide (1.0 mM). In order to gain insight into the mechanisms for these reactions, we studied the effects of several factors on the rate of release of gaseous bromine. For example, the release of Br$^{\ast}$(g) was enhanced in solutions at lower pH, and this pH-dependent trend was similar in both the nitrate and hydrogen peroxide systems. In solutions containing bromide and nitrate at a fixed ionic strength, the addition of chloride showed little effect on the Br$^{\ast}$(g) release, while increasing ionic strength significantly slowed down its release. In hydrogen peroxide solutions, however, the presence of chloride strongly enhanced Br$^{\ast}$(g) release, while the ionic strength of the solution was shown to have no effect on the reaction. Furthermore, the Br$^{\ast}$(g) formation rate increased rapidly with bromide concentration in nitrate solutions containing 0 - 2 mM Br$^{-}$, but was constant in the concentration range more typical in sea-salt particles (2 - 6 mM Br$^{-}$). Finally, we found that Br$^{\ast}$(g) release was independent of temperature in the range of 5 to $35^{\circ}$C in concentrated seawater with added nitrate. The dependence of Br$^{\ast}$(g) release on bromide concentration and temperature in hydrogen peroxide solutions are currently being investigated. These findings suggest that OH-initiated reactions in sea-salt particles may be a source of gaseous reactive bromine species in the MBL, especially in polluted areas. The implications of this study for chemistry in the MBL, and possible mechanisms for the formation of gaseous bromine, will be discussed.
DE: 0315 Biosphere/atmosphere interactions
DE: 0317 Chemical kinetic and photochemical properties
DE: 0365 Troposphere--composition and chemistry
SC: Atmospheric Sciences [A]
MN: 2003 Fall Meeting