Global Modelling of the Atmospheric Methyl Bromide Budget |
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| Authors: | N J Warwick J A Pyle D E Shallcross |
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| Institution: | (1) Centre for Atmospheric Science, Chemistry Department, University of Cambridge, Lensfield Road, Cambridge, CB2 1EW, UK.;(2) NCAS Atmospheric Chemical Modelling Support Unit, Cambridge, UK;(3) School of Chemistry, University of Bristol, Bristol, UK |
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| Abstract: | A global three-dimensional chemical transport model has been used to identify and evaluate possible candidates for the `missing' surface source required to balance the atmospheric budget of methyl bromide. Both natural and anthropogenic emissions of methyl bromide are `coloured' in the model, thus allowing the global CH3Br distribution to be broken-down into its source components. These coloured CH3Br tracers are then combined in various ways to create one base-line emission scenario and five further plausible scenarios. The additional emission scenarios are specifically designed to test whether the geographical distribution and seasonal cycles of additional vegetation and/or increased biomass burning emissions are consistent with atmospheric observations of methyl bromide mixing ratios. Due to an imbalance in our current understanding of the methyl bromide budget, simulated CH3Br mixing ratios from the base-line emission scenario are significantly lower than atmospheric measurements. Both the inclusion of a vegetation source in the tropics and a double strength biomass burning source substantially improve the agreement between model simulations and atmospheric measurements compared with the base-line emission scenario. While measurement data provides useful information on global fluxes and regional CH3Br seasonal cycles, small differences between the simulated seasonal cycles of different emission scenarios makes it difficult to distinguish between the relative likelihoods of model scenarios containing a tropical vegetation source or an increased biomass burning source. Further measurements performed in continental mid-to-high northern latitudes, central-southern Africa and South America would be of particular benefit in future attempts to constrain the location and magnitude of the natural terrestrial sources of methyl bromide. |
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| Keywords: | budget emissions methyl bromide modelling |
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