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Modelling of Halogen Chemistry in the Marine Boundary Layer (MBL) |
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Bromine Bromine chemistry is likely to play an important role in a number of processes in the lower troposphere. Attention has focused on the bromine-catalysed destruction of O3 in the polar boundary layer during springtime. In this environment, two sources of reactive halogens have been proposed: acidified sea-salt surfaces such as aerosol or newly-formed sea ice with associated frost flowers; and photodegradable halocarbon compounds from anthropogenic or natural origin. In the marine boundary layer (MBL), the major source of gas-phase bromine is the release of species such as IBr, Br2 and BrCl from sea-salt aerosol, following the uptake from the gas phase, and subsequent aqueous-phase reactions of hypohalous acids (HOX, where X = Br, Cl, I). To interpret our DOAS observations of the BrO radical [Saiz-Lopez et al., 2004a] we use a photochemical model of bromine chemistry containing gas-phase reactions, photochemistry, and heterogeneous uptake processes (see more details in Saiz-Lopez et al., 2006a). |
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Iodine The relevance of iodine in the chemistry of the lower troposphere has been the subject of numerous studies over the past two decades. These investigations have concentrated on the potential of iodine to affect the oxidizing capacity of the MBL in a number of ways: catalytic destruction of O3 by cycles involving the iodine species IO, HOI and OIO; altering the partitioning of NOx and HOx; and activating chlorine and particularly bromine from sea-salt aerosol. In addition, the role of higher order iodine oxides in the formation of new particles in coastal marine environments has also been widely discussed. Our DOAS detection of I2 and the experimental determination of its short photolytic lifetime have demonstrated that the molecule is a major source of reactive iodine in the atmosphere [Saiz-Lopez and Plane, 2004; Saiz-Lopez et al., 2004b]. We have used a photochemical model to investigate the impact of iodine chemistry, in particular that of I2 emissions, in the MBL. The model contains a full treatment of gas-phase iodine chemistry, combined with a description of the nucleation and growth, by condensation and coagulation, of iodine oxide nano-particles [McFiggans et al., 2000; Saiz-Lopez et al., 2006b]. The combination of simultaneous measurements of enhanced I2 emissions and particle bursts at Mace Head (Ireland) have shown that I2 is almost certainly the main precursor of new particles at this location. |
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References McFiggans, G., et al. (2000) A modeling study of iodine chemistry in the marine boundary layer. J. Geophys. Res.-Atmos, 105, 14371. Saiz-Lopez, A., et al. (2004a) Bromine oxide in the mid-latitude marine boundary layer. Geophys. Res. Lett., 31, L03111, doi:10.1029/2003GL018956. Saiz-Lopez, A., et al. (2004b) Absolute absorption cross-section and photolysis rate of I2. Atmos. Chem. Phys., 4, 1443. Saiz-Lopez, A., and J.M.C. Plane (2004) Novel iodine chemistry in the marine boundary layer. Geophys. Res. Lett., 31, L04112, doi: 04110.01029/02003GL019215. Saiz-Lopez, A., et al. (2006a) Measurements and modelling of I2, IO, OIO, BrO and NO3 in the mid-latitude marine boundary layer. Atmos. Chem. Phys., 6, 1513. Saiz-Lopez, A., et al. (2006b) Modelling molecular iodine emissions in a coastal marine environment: the link to new particle formation. Atmos. Chem. Phys., 6, 883. |
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For information related to this project Go to > Troposphere > Modelling > DOAS |
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