Towards a quantitative link between tidal macroalgal exposure and iodine-mediated CCN formation

In recent years the phenomenon of coastal ultrafine particle formation has been extensively reported and a qualitative linkage to molecular iodine emissions from macroalgae at low tide has now been well-established. In September 2006, within the Reactive Halogens in the Marine Boundary Layer (RHaMBLe) programme, an extensive payload of instrumentation was deployed in Roscoff, Brittany, to characterise particle formation and reactive halogen chemistry at a coastal location rich in intertidal macroalgae. Aerosol measurements included particle number concentration (>2 nm and > 10 nm diameter) and size distributions from 3 nm. Particle and ozone fluxes were measured by Eddy Correlation. Optical absorption instruments were used to investigate the path integrated and in situ concentrations of iodine oxide radicals. Halocarbon concentrations were measured online by GC-MS and a Relaxed Eddy Accumulation system was deployed to measure halocarbon and molecular iodine fluxes. A digitised map of speciated macroalgal distribution was superimposed on local bathymetry, to enable areal exposure to be calculated for each species as a function of time through the tidal cycle. Molecular iodine source functions were established for each species and footprints calculated at the measurement “receptor” site. With transport driven by local meteorology and photochemistry by measured spectral radiometry, the predicted molecular iodine concentration was compared with in situ measurements. A 1-D model of halogen cycling through a size-resolved aerosol distribution, driven by remote boundary conditions and local iodine emissions was used to predict the temporal and vertical variability of I2 and IO. The predictions were compared with long-path and in situ measurements aiming to resolve discrepancies resulting from spatial heterogeneity. Correlations were investigated between the measured particle concentrations and in situ IO concentrations as was the relationship between measured ozone deposition fluxes and particle emission fluxes. This combination of analyses was used to relate integrated tidal molecular iodine fluxes in the footprint of the measurement site to the observed I2, IO and particle concentrations and to the apparent particle emission flux and ozone deposition flux. Growth of particles to CCN sizes was observed under certain conditions; these events will be discussed in terms of the local geography and prevailing meteorological, pollution and tidal conditions.