Modelling multi-phase halogen chemistry in the coastal marine boundary layer: investigation of the relative importance of local chemistry vs. long-range transport

Abstract. Measurements of significant concentrations of IO, I₂ and BrO in a semi-polluted coast environment at Roscoff, in North-West France, have been made as part of the Reactive Halogens in the Marine Boundary Layer (RHaMBLe) campaign undertaken in September 2006. We use a one-dimensional column model, with idealised I₂ emissions predicted using macroalgael maps and tidal data from the littoral area surrounding Roscoff, to investigate the probable causes for these observations. The coupled microphysical and chemical aerosol model simulates mixed-phase halogen chemistry using two separate particle modes, seasalt and non-seasalt, each comprising of eight size-sections. This work confirms the finding of a previous study that the BrO measurements are most likely caused by unknown, local sources. We find that the remote observations of IO and I₂ are best replicated using the I₂ recycling mechanism suggested by previous studies, but that such a mechanism is not wholly necessary. However in-situ measurements of I₂ can only be explained by invoking an I₂ recycling mechanism. We suggest that focussed observations of the changes in NO_x and NO_y concentrations, as well as changes in the nitrate fraction of the non-seasalt aerosol mode, in the presence of I₂ bursts could be used to determine the atmospheric relevance of the predicted I₂ recycling mechanism.