Abstract
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-a (chl-a) 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.
Highlights
Combined reactive bromine and iodine species can alter the oxidative capacity of the troposphere through a number of processes
Yamamoto et al (2001) observed depth profiles of iodocarbons (CH2I2 and CH2ICl) very similar to those of chl-a in seawater collected from the Bay of Bengal under tropical stratified conditions, suggesting production by phytoplankton followed by rapid decay in seawater
We have shown that the use of the same chl-a dependent parameterisation for all very short-lived (VSL) halocarbons, with elevated emission fluxes within 20◦ N–20◦ S, enables the model to capture the maximum atmospheric mixing ratios of bromocarbons observed in the tropics
Summary
Combined reactive bromine and iodine species can alter the oxidative capacity of the troposphere through a number of processes. The most abundant brominated VSL species are predominantly of natural oceanic origin: bromoform (CHBr3), dibromomethane (CH2Br2), bromochloromethane (CH2BrCl), bromodichloromethane (CH2BrCl2), dibromochloromethane (CHBr2Cl) Identified sources for these compounds include macroalgae, ice algae and phytoplankton Warwick et al (2006a) created a number of bromoform emission data sets, with different geographic distributions, to test flux predictions against atmospheric measurements and assess how well these observations constrain oceanic and coastal emissions They scaled the emission fields of the other bromocarbons to the CHBr3 emission field. We have derived an emission inventory for very short-lived bromocarbons (CHBr3, CH2Br2, CH2BrCl, CHBrCl2, CHBr2Cl) and iodocarbons (CH2ICl, CH2IBr, CH2I2) using a compilation of aircraft campaigns and some observations available in the MBL, respectively
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