Lower Atmosphere – Modelling
Tropospheric Halogen Chemistry MOdel (THAMO)
Modelling of Halogen Chemistry in the Marine Boundary Layer (MBL)
(In collaboration with Alfonso Saiz-Lopez’s AC2 group at CSIC)
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 1 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.2).
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.3,4 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.5,6 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.
Testing a laboratory-derived parameterisation of iodine emisions with field data.
Parameterised expressions for HOI and I2 fluxes from the sea surface resulting from the reaction between ozone and sea surface iodine have been generated from our laboratory results.7 The scarce concurrent measurements of sea surface iodide and temperature available in the literature were then used to parameterise the iodide concentration as a function of temperature, which enables easier inclusion in atmospheric models. The adapted expressions were then input into the Tropospheric HAlogen chemistry MOdel to compare with latitudinal MAX-DOAS measurements of IO and IOx performed during the HaloCAST-P cruise in the Eastern Pacific ocean, spanning a wide range of SST, wind speed and O3 mixing ratios. The modelled IO and IOx matches well with the observations when the predicted fluxes are lower, however, there is an over-prediction in the model at low wind speeds. The inorganic iodine flux contributions to IO and IOx are found to be comparable with or larger than the contribution of organoiodine compounds.8
- Saiz-Lopez, A., et al. Bromine oxide in the mid-latitude marine boundary layer. Geophys. Res. Lett., 2004, 31, L03111.
- Saiz-Lopez, A., et al. Measurements and modelling of I2, IO, OIO, BrO and NO3 in the mid-latitude marine boundary layer. Atmos. Chem. Phys., 2006, 6, 1513.
- Saiz-Lopez, A., et al., Absolute absorption cross-section and photolysis rate of I2. Atmos. Chem. Phys., 2004, 4, 1443.
- Saiz-Lopez, A., and Plane, J.M.C., Novel iodine chemistry in the marine boundary layer. Geophys. Res. Lett., 2004, 31, L04112
- McFiggans, G., et al., A modeling study of iodine chemistry in the marine boundary layer. J. Geophys. Res. [Atmos.], 2000,105, 14371.
- Saiz-Lopez, A., et al.Modelling molecular iodine emissions in a coastal marine environment: the link to new particle formation. Atmos. Chem. Phys., 2006, 6, 883.
- Carpenter, L.J. et al., Atmospheric iodine levels influenced by sea surface emissions of inorganic iodine, Nature Geosci., 2013, 6, 108-111.
- MacDonald, et al., A laboratory characterisation of inorganic iodine emissions from the sea surface: dependence on oceanic variables and parameterisation for global modelling, Atmos. Chem. Phys. Disc., 2013, 13, 31445-31477.