Based on the results of a pilot study in 2007, which found high mixing ratios of molecular iodine (I<sub>2</sub>) above the intertidal macroalgae (seaweed) beds at Mweenish Bay (Ireland), we extended the study to nine different locations in the vicinity of Mace Head Atmospheric Research Station on the west coast of Ireland during a field campaign in 2009. The mean values of I<sub>2</sub> mixing ratio found above the macroalgae beds at nine different locations ranged from 104 to 393 ppt, implying a high source strength of I<sub>2</sub>. Such mixing ratios are sufficient to result in photochemically driven coastal new-particle formation events. Mixing ratios above the </i>Ascophyllum nodosum</i> and <i>Fucus vesiculosus</i> beds increased with exposure time: after 6 h exposure to ambient air the mixing ratios were one order of magnitude higher than those initially present. This contrasts with the emission characteristics of <i>Laminaria digitata</i>, where most I<sub>2</sub> was emitted within the first half hour of exposure. Discrete in situ measurements (offline) of I<sub>2</sub> emission from ambient air-exposed chamber experiments of <i>L. digitata</i>, <i>A. nodosum</i> and <i>F. vesiculosus</i> substantially supported the field observations. Further online and time-resolved measurements of the I<sub>2</sub> emission from O<sub>3</sub>-exposed macroalgal experiments in a chamber confirmed the distinct I<sub>2</sub> emission characteristics of <i>A. nodosum</i> and <i>F. vesiculosus</i> compared to those of <i>L. digitata</i>. The emission rates of <i>A. nodosum</i> and <i>F. vesiculosus</i> were comparable to or even higher than <i>L. digitata</i> after the initial exposure period of ~20–30 min. We suggest that <i>A. nodosum</i> and <i>F. vesiculosus</i> may provide an important source of photolabile iodine in the coastal boundary layer and that their impact on photochemistry and coastal new-particle formation should be reevaluated in light of their longer exposure at low tide and their widespread distribution.