Abstract
Abstract. Based on the results of a pilot study in 2007, which found high mixing ratios of molecular iodine (I2) 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 I2 mixing ratio found above the macroalgae beds at nine different locations ranged from 104 to 393 ppt, implying a high source strength of I2. Such mixing ratios are sufficient to result in photochemically driven coastal new-particle formation events. Mixing ratios above the Ascophyllum nodosum and Fucus vesiculosus 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 Laminaria digitata, where most I2 was emitted within the first half hour of exposure. Discrete in situ measurements (offline) of I2 emission from ambient air-exposed chamber experiments of L. digitata, A. nodosum and F. vesiculosus substantially supported the field observations. Further online and time-resolved measurements of the I2 emission from O3-exposed macroalgal experiments in a chamber confirmed the distinct I2 emission characteristics of A. nodosum and F. vesiculosus compared to those of L. digitata. The emission rates of A. nodosum and F. vesiculosus were comparable to or even higher than L. digitata after the initial exposure period of ~20–30 min. We suggest that A. nodosum and F. vesiculosus 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.
Highlights
Sciences tios were one order of magnitude higher than those initially present
Efforts have been made to study the I2 emission profiles of macroalgae through laboratory incubation experiments (Dixneuf et al, 2009; Ball et al, 2010; Nitschke et al, 2011; Kundel et al, 2012b; AshuAyem et al, 2012) in order to understand the emission mechanism and to better estimate the flux of I2 from macroalgae. These studies have mainly focused on L. digitata as this species accumulates iodine at up to around 5 % of its dry weight (Kupper et al, 1998; Gall et al, 2004) and emits large amounts of I2 when exposed to ambient air; the results showed a high variability of I2 emission rates, with values ranging from 3 to 2500 pmol min−1 g fresh weight−1 (FW−1) (Bale et al, 2008; Ball et al, 2010; Ashu-Ayem et al, 2012)
Based on model study predictions that 80–100 ppt I2 is required for iodine oxide particle bursts (Saiz-Lopez et al, 2006a), the mixing ratios of I2, ranging from 104 ppt to 393 ppt, observed in all sampling sites would be sufficient to result in photochemically driven new-particle formation events
Summary
Sciences tios were one order of magnitude higher than those initially present. This contrasts with the emission characteristics of Laminaria digitata, where most I2 was emitted within the first half hour of exposure. Discrete in situ measurements (offline) of I2 emission from ambient air-exposed chamber experiments of L. digitata, A. nodosum and F. vesiculosus substantially supported the field observations. Further online and time-resolved measurements of the I2 emission from O3-exposed macroalgal experiments in a chamber confirmed the distinct I2 emission characteristics of A. nodosum and. Brown algae include kelps of the genus Laminaria, the sLtaromnigneasrtiabisoploecgiiecsal(aLcacOmuimcnauerlaiaatonsrspSpo.f)ciaoiredeinnaeckceeuyrrbeniotlgyeokcnhoewmn-. Ical pump for the transfer of iodine from the sea to the atmosphere (Kupper et al, 2011). HSowoelviedr, Etheairotdhine metabolism of other brown algae and their role in marine–atmospheric halogen transfer is much less well understood.
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