Bromine and iodine chemistry in a global chemistry-climate model: description and evaluation of very short-lived oceanic sources

Abstract

The global chemistry-climate model CAM-Chem has been extended to incorporate an expanded bromine and iodine chemistry scheme that includes natural oceanic sources of very short-lived (VSL) halocarbons, gas-phase photochemistry and heterogeneous reactions on aerosols. Ocean emissions of five VSL bromocarbons (CHBr₃, CH₂Br₂, CH₂BrCl, CHBrCl₂, CHBr₂Cl) and three VSL iodocarbons (CH₂ICl, CH₂IBr, CH₂I₂) have been parameterised by a biogenic chlorophyll-<i>a</i> (chl-<i>a</i>) dependent source in the tropical oceans (20° N–20° S). Constant oceanic fluxes with 2.5 coast-to-ocean emission ratios are separately imposed on four different latitudinal bands in the extratropics (20°–50° and above 50° in both hemispheres). Top-down emission estimates of bromocarbons have been derived using available measurements in the troposphere and lower stratosphere, while iodocarbons have been constrained with observations in the marine boundary layer (MBL). Emissions of CH₃I are based on a previous inventory and the longer lived CH₃Br is set to a surface mixing ratio boundary condition. The global oceanic emissions estimated for the most abundant VSL bromocarbons – 533 Gg yr⁻¹ for CHBr₃ and 67.3 Gg yr⁻¹ for CH₂Br₂ – are within the range of previous estimates. Overall the latitudinal and vertical distributions of modelled bromocarbons are in good agreement with observations. Nevertheless, we identify some issues such as the reduced number of aircraft observations to validate models in the Southern Hemisphere, the overestimation of CH₂Br₂ in the upper troposphere – lower stratosphere and the underestimation of CH₃I in the same region. Despite the difficulties involved in the global modelling of the shortest lived iodocarbons (CH₂ICl, CH₂IBr, CH₂I₂), modelled results are in good agreement with published observations in the MBL. Finally, sensitivity simulations show that knowledge of the diurnal emission cycle for these species, in particular for CH₂I₂, is key to assess their global source strength.