Coastal iodine emissions: Part 2. chamber experiments of particle formation from laminaria digitata-derived and laboratory-generated I₂

Laboratory studies into particle formation from Laminaria digitata macroalgae were undertaken to elucidate aerosol formation for a range of I ₂ (0.3-76 ppb_v) and O₃ (<3-96 ppb _v) mixing ratios and light levels (E_PAR = 15, 100, and 235 μmol photons m^-2 s^-1). No clear pattern was observed for I₂ or aerosol parameters as a function of light levels. Aerosol mass fluxes and particle number concentrations, were, however, correlated with I₂ mixing ratios for low O₃ mixing ratios of <3 ppb_v (R² = 0.7 and 0.83, respectively for low light levels, and R² = 0.95 and 0.98, respectively for medium light levels). Additional experiments into particle production as a function of laboratory-generated I₂, over a mixing ratio range of 1-8 ppb _v, were conducted under moderate O₃ mixing ratios (∼24 ppb_v) where a clear, 100-fold or greater, increase in the aerosol number concentrations and mass fluxes was observed compared to the low O ₃ experiments. A linear relationship between particle concentration and I₂ was found, in reasonable agreement with previous studies. Scaling the laboratory relationship to aerosol concentrations typical of the coastal boundary layer suggests a I₂ mixing ratio range of 6-93 ppt_v can account for the observed particle production events. Aerosol number concentration produced from I₂ is more than a factor of 10 higher than that produced from CH₂I₂ for the same mixing ratios.