In situ measurements of molecular iodine in the marine boundary layer: the link to macroalgae and the implications for O₃, IO, OIO and NO_x

"Single-point" in situ measurements of molecular iodine (I₂) were carried out in the coastal marine boundary layer (MBL) using diffusion denuders in combination with a gas chromatography-mass spectrometry (GC-MS) method. Comparison measurements were taken at Mace Head and Mweenish Bay, on the West Coast of Ireland. The observed mixing ratios of I₂ at Mweenish Bay are much higher than that at Mace Head, indicating the emissions of I₂ are correlated with the local algal biomass density and algae species. The concentration levels of I₂ were found to correlate inversely with tidal height and correlate positively with the concentration levels of O₃ in the surrounding air. However, the released I₂ can also lead to O₃ destruction via the reaction of O₃ with iodine atoms that are formed by the photolysis of I₂ during the day and via the reaction of I₂ with NO_x at night. IO and OIO were measured by long-path differential optical absorption spectroscopy (LP-DOAS). The results show that the concentrations of both daytime and nighttime IO are correlated with the mixing ratios of I₂. OIO was observed not only during the day but also, for the first time at both Mace Head and Mweenish Bay, at night. In addition, I₂ was measured simultaneously by the LP-DOAS technique and compared with the "single-point" in situ measurement. The results suggest that the local algae sources dominate the inorganic iodine chemistry at Mace Head and Mweenish Bay.