Measurements and modelling of molecular iodine emissions, transport and photodestruction in the coastal region around Roscoff

Iodine emissions from the dominant six macroalgal species in the coastal regions around Roscoff, France, have been modelled to support the Reactive Halogens in the Marine Boundary Layer Experiment (RHaMBLe) undertaken in September 2006. A two-dimensional model is used to explore the relationship between geographically resolved regional emissions (based on maps of seaweed beds in the area and seaweed I[subscript 2] emission rates previously measured in the laboratory) and in situ point and line measurements of I[subscript 2] performed respectively by a broadband cavity ringdown spectroscopy (BBCRDS) instrument sited on the shoreline and a long-path differential optical absorption spectroscopy (LP-DOAS) instrument sampling over an extended light path to an off-shore island. The modelled point and line I[subscript 2] concentrations compare quantitatively with BBCRDS and LP-DOAS measurements, and provide a link between emission fields and the different measurement geometries used to quantify atmospheric I[subscript 2] concentrations during RHaMBLe. Total I[subscript 2] emissions over the 100 km[superscript 2] region around Roscoff are calculated to be 1.7×10[superscript 19] molecules per second during the lowest tides. During the night, the model replicates I[subscript 2] concentrations up to 50 pptv measured along the LP-DOAS instrument's line of sight, and predicts spikes of several hundred pptv in certain conditions. Point I[subscript 2] concentrations up to 50 pptv are also calculated at the measurement site, in broad agreement with the BBCRDS observations. Daytime measured concentrations of I[subscript 2] at the site correlate with modelled production and transport processes. However substantial recycling of the photodissociated I[subscript 2] is required for the model to quantitatively match measured concentrations. This result corroborates previous modelling of iodine and NO[subscript x] chemistry in the semi-polluted marine boundary layer which proposed a mechanism for recycling I[subscript 2] via the formation, transport and subsequent reactions of the IONO[subscript 2] reservoir compound. The methodology presented in this paper provides a tool for linking spatially distinct measurements to inhomogeneous and temporally varying emission fields.