[1] Theoretical studies have predicted that concentrations of gaseous I 2 and IO of the order of 80 -100 ppt and 40-50 ppt, respectively, are required in coastal air to account for photochemically-driven coastal new-particle formation events to occur. However, measurements reported to date (i.e., $20 ppt I 2 , 10 ppt IO) have not supported the required model predictions. Here, we present measurements of high concentrations of I 2 and IO in N.E. Atlantic marine air on the west coast of Ireland. The maximum mixing ratios of daytime I 2 and IO over the seaweed beds during low tide were 302 ppt and 35 ppt, respectively. The I 2 distribution was rather inhomogeneous, even at the inter-tidal zone, but closely related to the macroalgae biomass abundance. New particle formation bursts were frequently observed during daytime hours with the concentrations up to 4.5 Â 10 5 particles cm À3 during low-tide conditions, and the concentrations of ultra-fine particles were positively correlated with the IO concentrations. Considering the constraints set out in theoretical studies for new particle formation via condensation of condensable iodine oxide vapours, the results reported here clearly demonstrate that the molecular iodine and iodine monoxide concentrations in coastal air are sufficient to meet the theoretical precursor concentrations required to drive intensive coastal newparticle formation from higher order condensable iodine oxides. Citation: Huang, R. J., K. Seitz, T. Neary, C. D. O'Dowd, U. Platt, and T. Hoffmann (2010), Observations of high concentrations of I 2 and IO in coastal air supporting iodineoxide driven coastal new particle formation, Geophys. Res. Lett., 37, L03803,

Abstract. We present investigations of the reactive iodine species (RIS) IO, OIO and I 2 in a coastal region from a field campaign simultaneously employing active long path differential optical absorption spectroscopy (LP-DOAS) as well as passive multi-axis differential optical absorption spectroscopy (MAX-DOAS). The campaign took place at the Martin Ryan Institute (MRI) in Carna, County Galway at the Irish West Coast about 6 km south-east of the atmospheric research station Mace Head in summer 2007. In order to study the horizontal distribution of the trace gases of interest, we established two almost parallel active LP-DOAS light paths, the shorter of 1034 m length just crossing the intertidal area, whereas the longer one of 3946 m length also crossed open water during periods of low tide. In addition we operated two passive Mini-MAX-DOAS instruments with the same viewing direction. While neither OIO nor I 2 could be unambiguously identified with any of the instruments, IO could be detected with active as well as passive DOAS. The IO column densities seen at both active LP-DOAS light paths are almost the same. Thus it can be concluded that coastal IO is almost exclusively located in the intertidal area, where we detected mixing ratios of up to 29±8.8 ppt (equivalent to pmol/mol). Nucleation events with particle concentrations of 10 6 cm −3 particles were observed each day correlating with high IO mixing ratios. Therefore we feel that our detected IO concentrations confirm the results of model studies, which state that in order to explain such particle bursts, IO mixing ratios of 50 to 100 ppt in so called "hot-spots" are required.