Assessing the Impact of Bromine and Iodine Chemistry on Tropospheric Composition

Halogens (chlorine, bromine and iodine) have been studied extensively in the stratosphere, where they catalytically destroy ozone, but more recently there has been growing interest in their effect on tropospheric composition and oxidative capacity. Of particular interest is how they affect ozone, a powerful greenhouse gas and air pollutant at the Earth’s surface. Reactive halogen species such as  BrO and IO can impact ozone concentrations directly through catalytic cycles and indirectly via affecting the partitioning of HO_X and NO_X, resulting in important effects on tropospheric composition that are not yet fully understood.

Here, we explore how bromine and iodine chemistry can impact tropospheric ozone concentrations. Using the FRSGC/UCI CTM, we expand the existing tropospheric chemistry scheme (that includes a comprehensive description of iodine chemistry) by including a detailed bromine chemistry scheme. This encompasses major gas-phase and heterogeneous reactions, including reactions occurring on ice particles, alongside physical processes like wet and dry deposition. Sources of bromine include (1) CH₃Br and emissions of five short-lived bromocarbons (CHBr₃, CH₂Br₂, CH₂BrCl, CHBr₂Cl and CHBrCl₂), (2) debromination of sea-salt aerosol from the open ocean and from blowing snow in polar regions, and (3) transport from the stratosphere. We explore different approaches to representing open ocean sea-salt debromination, comparing a ‘depletion factor’ based parameterisation with sea-salt debromination arising from a series of heterogeneous reactions. The impact of these different parameterisations on the resulting bromine budget is shown.

A detailed evaluation of the model performance is presented using ground-based, aircraft and satellite observations of key radicals (e.g. halogen oxides) with the results comparing reasonably well with observations. We also explore the spatial and temporal variation of natural halogens in the troposphere and the importance of different sources, and further quantify the impact of reactive halogens on present-day ozone concentrations and tropospheric oxidising capacity.