Sophie Dixneuf,^1,2 Albert A. Ruth^1,2 and Dean S. Venables^2,3

¹ Department of Physics, University College Cork, Cork, Ireland

² Environmental Research Institute, University College Cork, Cork, Ireland

³ Department of Chemistry, University College Cork, Cork, Ireland

Introduction

The transport and transformation of substances within the atmosphere, hydrosphere, biosphere and lithosphere are known as biogeochemical cycles. These cycles apply to trace elements, like iodine, as well as abundant elements like carbon, nitrogen and phosphorus. The sea surface is a particularly important interface in biogeochemical cycling, with the exchange of vast amounts of material between the sea and atmosphere. For example, sulfur-containing molecules are produced by micro-organisms in the sea and eventually released to the atmosphere. These molecules are transformed by atmospheric processes to sulfate particles, which affect the formation of clouds and ultimately have an important influence on the Earth’s climate.¹ The reverse transfer ‰ÛÒ from air to sea ‰ÛÒ also is important. The deposition of Saharan dust to the Atlantic Ocean is one such instance in which the dust stimulates phytoplankton growth by providing a source of iron to iron-deficient parts of the sea.²

Many aspects of sea-air exchange are poorly understood. The total amount and composition of chemical compounds transferred across this interface must be known in order to accurately model biogeochemical cycles. For the atmosphere, the iodine cycle is of particular importance. Iodine is involved in several key atmospheric reactions that impact the concentrations of other atmospheric constituents. Because iodine is a trace nutrient required for the synthesis of thyroid hormones and its deficiency can affect brain development, the supply of this element in our food chain is also an important public health issue.³

Although iodine is found at low concentrations throughout the ocean waters, many types of seaweed accumulate the element in very high levels. It has been suggested that seaweeds could be an important route to transfer iodine to the atmosphere. In the following, we examine the role of iodine in the lower atmosphere, its release from seaweeds, and outline techniques and strategies developed in the Centre for Research into Atmospheric Chemistry at University College Cork, together with colleagues from Galway, to study these transformations.

Iodine and Seaweeds

Seaweeds are among the most potent accumulators of iodine. However, the extent to which seaweeds release iodine to the atmosphere is still unclear. This is an intriguing and important question for atmospheric scientists who are familiar with the way in which other halogens can catalytically destroy ozone. Ozone destruction is well known in the stratosphere where the ‰ÛÏozone hole‰Û appears above Antarctica every spring. Photolysis products of iodine species generated by sunlight can also deplete ozone near the Earth’s surface ‰ÛÒ where the effect is more benign than in the stratosphere ‰ÛÒ and influences the capacity of the lower atmosphere to oxidize other atmospheric constituents. In addition, new particles are formed when relatively high concentrations of iodine react with ozone during the day.

The first indications of a strong coastal source of iodine came from observations of iodine monoxide (IO) at the Mace Head site on the west coast of Ireland.⁴ The anti-correlation between tidal height and the concentration of IO, a key intermediate molecule in iodine reactions, allowed scientists to trace the source of iodine to exposed seaweeds. Kelps of the Laminaria genus were identified as a major iodine source. Although the precursors of (atomic) iodine were initially thought to be volatile iodocarbons, small, iodine-containing organic molecules such as methyl iodide and diiodomethane, this suggestion did not stand up to closer scrutiny because iodocarbons do not photolyse rapidly enough to explain the observed IO concentrations. The current consensus is that most iodine is emitted as molecular iodine (I₂), a molecule that has a lifetime during the day of a few seconds, as opposed to iodocarbons, where the lifetime ranges from several minutes to hours.⁵

Despite research over the last decade pinpointing seaweeds as strong emitters of iodine, quantifying the amount of iodine released by seaweeds poses an unusual and tricky challenge for atmospheric scientists (Figure 1). First, seaweeds are exposed and covered twice daily by the tide, which itself varies in a two-week cycle. The level of exposure of lower littoral species such as Laminaria species is therefore highly variable, which presents a challenge for taking expensive and fragile instrumentation right to the seaweed. Second, light-driven iodine reactions are extremely rapid during the day. Consequently, instrumentation should be both fast and sensitive. Finally, the density and distribution of seaweeds is highly variable, with seaweeds growing thickly in some areas and sparsely or not at all in others. Because measurements over large areas only give average concentrations, techniques with sufficient spatial resolution are needed to monitor local concentrations, which could be much higher or lower than average concentrations. Here, we look at ways to address these challenges.

Measurement Strategies

Absorption spectroscopy is an attractive analytical approach because many iodine species have strong and structured absorption at visible wavelengths. Early observations of atmospheric iodine were made by long-path differential optical absorption spectroscopy (LP-DOAS) measurements.⁴ As its name implies, LP-DOAS monitors the absorption of light across a very long path through the atmosphere. The length of the light path is typically several kilometers in order to attain the sensitivity needed to detect trace concentrations of absorbing molecules. The LP-DOAS systems are capable of monitoring IO down to the low parts-per-trillion level, and IO has also been observed by satellite, predominantly over Antarctica.⁶ After I₂ was identified as the main precursor for IO, absorption measurements of I₂ became the main focus of research. In the context of studying iodine chemistry, however, the drawback of the long-path approach is that it only provides information about the average concentration across the path. The concentration may vary enormously between hot spots of iodine and other areas where iodine levels are low. It is therefore necessary to find approaches to carry out sensitive measurements across a much smaller length.

It is possible to produce long path lengths of light across short separations if two highly reflective mirrors are used to reflect light backward and forward through the sample. Such an optical cavity can generate effective path lengths of kilometers in a setup with typical dimensions of about a meter. Instruments exploiting this approach can achieve sensitivities comparable to LP-DOAS systems but with high spatial resolution. In Cork, a method known as incoherent broadband cavity enhanced absorption spectroscopy (IBBCEAS) has been pioneered and applied to iodine detection.⁷ In an IBBCEAS system, light from a bright source, such as a short-arc Xe-lamp or a light emitting diode, is transmitted through an optical cavity and the spectrum is recorded. By measuring a broad spectrum, target species can be identified and quantified based on their known absorption.

Iodine Emissions by Seaweeds

The compact size and fast response of IBBCEAS allows some of the difficulties of field observations of seaweed emissions to be avoided. By studying seaweed samples in a controlled environment, such as in a flow tube or an atmospheric simulation chamber, emissions can be studied in isolation from the confounding effects of tidal height, population density, and adverse weather conditions. We have used this approach to examine the emissions of individual Laminaria digitata seaweeds, and have found some surprising results. In one set of experiments, seaweed specimens were removed from water and placed in an enclosed container. Exposure to air triggered the release of iodine, which was monitored over time by an IBBCEAS system. What was expected was a relatively constant rate of emission of iodine, and there was indeed a strong rise in iodine emission over the first few minutes of exposure, followed by a decline in emission over tens of minutes. What was surprising, however, was that short, irregular bursts of iodine were observed in addition to the broad, continuous emission. Unlike the main emission profile, which was typically 90 percent complete in less than an hour, these pulses continued over many hours (Figure 2).⁸ These periodic pulses of iodine could potentially be the first naturally-occurring example of a ‰ÛÏclock reaction,‰Û or an unusual type of reaction in which the concentrations of reaction intermediates fluctuate in a semi-periodic fashion. The physiological reason and mechanism for the time dependence of the iodine emission has not yet been adequately explained. One theory is that it is a biological defense mechanism to protect the exposed seaweed from harsh chemicals such as ozone, or from microorganisms. To probe this issue, the amount of iodine emitted from different parts of the seaweed was examined. Here again the results were unexpected: there was a surprisingly large variation in the amount of iodine emitted from different parts of the seaweed, with the stipe, the long stem-like part of the seaweed, emitting 10 times more iodine than the uppermost blades of the plant (shown in Figure 3).⁸

To understand the impact of iodine emission from seaweed on atmospheric chemistry, quantitative information is needed on the amounts, rates of emission, and emission changes over time. Although various groups have studied seaweed emissions, estimates of emissions from single seaweed specimens have varied by two or three orders of magnitude. To better estimate emission rates from Laminaria digitata seaweeds, we carried out a series of experiments in an atmospheric simulation chamber.¹⁰ Individual seaweeds were initially submerged in a basin of seawater, which was gradually pumped out of the container to mimic the retreating tide (Figure 4). The emission time profiles were similar to those seen in previous studies, but unlike these studies, a large number of individual Laminaria seaweeds were studied under identical conditions. What was striking was the very large variability in emissions of seaweeds. In all cases, the photosynthetic response of individual seaweeds appeared to be normal, and all seaweeds emitted some iodine. However, the emission rate varied by two orders of magnitude in going from the weakest to the strongest emitter. From these studies, it is apparent that, even among the same species, high sample-to-sample variability is an intrinsic feature of a given seaweed population. Light levels had no obvious effect on the emissions of the seaweeds. Because a relatively large number of samples were studied, we were able to place an improved estimate on iodine emissions from Laminaria seaweeds despite the observed variability.

Coastal measurements

In recent field work, we have used so-called denuders in which the iodine reacts on a specially coated surface of a tube.¹¹ After sampling, the denuder tubes were collected and their iodine content was quantified in the laboratory. Denuder samplers are cheap and robust, making them an attractive option for sampling right above the seaweed beds. Instead of just looking at Laminaria species, however, recent work has reexamined the emissions by other brown seaweed species, notably Fucus species. Earlier studies had found only very small emissions from Fucus species. On closer inspection, however, it appears these species release iodine progressively over a much longer period.¹¹ Although cumulative emissions of iodine are lower than those by Laminaria, their position around the mid-to upper shore means they are exposed more frequently and for longer periods, making their contributions to coastal iodine emissions potentially significant.

Outlook

Quantifying the emissions of iodine from seaweeds poses an unusual analytical challenge that requires ad hoc approaches and adaptation of existing techniques. Although significant progress has been made in understanding and quantifying these emissions, questions remain. The importance of non-kelp species has yet to be studied in detail, although early findings indicate that Fucus and other species are potentially significant sources of iodine. As iodine concentrations within seaweed are known to vary over the year, the seasonal dependence of iodine emissions from different seaweeds should be investigated. In addition, most studies have taken place on north Atlantic coastlines; these observations need to be extended to other parts of the world, particularly to the tropics. Finally, the impact of coastal iodine on the local atmosphere is important, as iodine in air has strong interactions with city pollution. The unusual reactions are an active area of research.

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