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

Abstract. Discrete in situ atmospheric measurements of molecular iodine (I₂) were carried out at Mace Head and Mweenish Bay on the west coast of Ireland using diffusion denuders in combination with a gas chromatography-mass spectrometry (GC-MS) method. I₂, IO and OIO were also measured by long-path differential optical absorption spectroscopy (LP-DOAS). The simultaneous denuder and LP-DOAS I₂ measurements were well correlated (R²=0.80) but the denuder method recorded much higher concentrations. This can be attributed to the fact that the in situ measurements were made near to macroalgal sources of I₂ in the intertidal zone, whereas the LP-DOAS technique provides distance-averaged mixing ratios of an inhomogeneous distribution along the light-path. The observed mixing ratios of I₂ at Mweenish Bay were significantly higher than that at Mace Head, which is consistent with differences in local algal biomass density and algal species composition. Above algal beds, levels of I₂ were found to correlate inversely with tidal height and positively with the concentrations of O₃ in the surrounding air, indicating a role for O₃ in the production of I₂ from macroalgae, as has been previously suggested from laboratory studies. However, measurements made ~150 m away from the algal beds showed a negative correlation between O₃ and I₂ during both day and night. We interpret these results to indicate that the released I₂ can also lead to O₃ destruction via the reaction of O₃ with I atoms that are formed by the photolysis of I₂ during the day and via the reaction of I₂ with NO₃ radicals at night. The results show that the concentrations of daytime IO are correlated with the mixing ratios of I₂, and suggest that the local algae sources dominate the inorganic iodine chemistry at Mace Head and Mweenish Bay.