Jill M. Cainey

The CLAW hypothesis: a review of the major developments

Environmental context. Understanding the role of clouds in the warming and the cooling of the planet and how that role alters in a warming world is one of the biggest uncertainties climate change researchers face. Important in this regard is the influence on cloud properties of cloud condensation nuclei, the tiny atmospheric particles necessary for the nucleation of every single cloud droplet. The anthropogenic contribution to cloud condensation nuclei is known to be large in some regions through knowledge of pollutant emissions; however, the natural processes that regulate cloud condensation nuclei over large parts of the globe are less well understood. The CLAW hypothesis provides a mechanism by which plankton may modify climate through the atmospheric sulfur cycle via the provision of sulfate cloud condensation nuclei. The CLAW hypothesis was published over 20 years ago and has stimulated a great deal of research. Abstract. The CLAW hypothesis has for 20 years provided the intriguing prospect of oceanic and atmospheric systems exhibiting in an intimately coupled way a capacity to react to changing climate in a manner that opposes the change. A great number of quality scientific papers has resulted, many confirming details of specific links between oceanic phytoplankton and dimethylsulfide (DMS) emission to the atmosphere, the importance of DMS oxidation products in regulation of marine atmospheric cloud condensation nucleus (CCN) populations, and a concomitant influence on marine stratocumulus cloud properties. However, despite various links in the proposed phytoplankton–DMS–CCN–cloud albedo climate feedback loop being affirmed, there has been no overall scientific synthesis capable of adequately testing the hypothesis at a global scale. Moreover, significant gaps and contradictions remain, such as a lack of quantitative understanding of new particle formation processes in the marine atmospheric boundary layer, and of the extent to which dynamical, rather than microphysical, cloud feedbacks exist. Nevertheless, considerable progress has been made in understanding ‘Earth System Science’ involving the integration of ocean and atmospheric systems inherent in the CLAW hypothesis. We present here a short review of this progress since the publication of the CLAW hypothesis.

Chloride and Bromide Loss from Sea-Salt Particles in Southern Ocean Air

Datasets on aerosol composition in Southern Ocean air at Cape Grim and Macquarie Island, and rainwater composition at Cape Grim, have been analysed for sea-salt components in order to test the validity of the multiphase autocatalytic halogen activation process proposed initially by Sander and Crutzen (1996) and developed fully for clean marine air by Vogt et al. (1996). Four distinct datasets from the two locations were analysed. All four datasets provided consistent evidence in support of three predictions of the autocatalytic model: (1) overall Cl- deficits in sea-salt aerosol were small, difficult to quantify against analytical uncertainty and at most a few percent; (2) Br- deficits were large, averaging −30% to −50% on an annual basis, with strong seasonality ranging from about −10% in some winter samples to −80% or more in some summer samples; and (3) the Br- and Cl- deficits were clearly linked to the availability of strong, S-acidity in the aerosol, confirming the importance of acid catalysis to the dehalogenation process.

Shipboard measurements of dimethyl sulfide and SO2southwest of Tasmania during the First Aerosol Characterization Experiment (ACE 1)

Measurements of seawater dimethylsulfide (DMS), atmospheric dimethylsulfide, and sulfur dioxide (SO 2 ) were made on board the R/VDiscoverer in the Southern Ocean, southeast of Australia, as part of the First Aerosol Characterization Experiment (ACE 1). The measurements covered a latitude range of 40°S–55°S during November-December 1995. Seawater DMS concentrations ranged from 0.4 to 6.8 nM, with a mean of 1.7±1.1 nM (1σ). The highest DMS concentrations were found in subtropical convergence zone waters north of 44°S, and the lowest were found in polar waters south of 49°S. In general, seawater DMS concentrations increased during the course of the study, presumably due to the onset of austral spring warming. Atmospheric DMS concentrations ranged from 24 to 350 parts per trillion by volume (pptv), with a mean of 112±61 pptv (1σ). Atmospheric SO2 was predominantly of marine origin with occasional anthropogenic input, as evidenced by correlation with elevated 222 Rn and air mass trajectories. Concentrations ranged from 3 to 1000 pptv with a mean of 48.8± 49 pptv (1σ) and a median 15.8 pptv. The mean SO2 concentration observed in undisturbed marine air was 11.9±7.6 pptv (1σ), and the mean DMS to SO2 ratio in these conditions was 13±9 (1σ). Diurnal variations in

Active chlorine in the remote marine boundary layer: Modeling anomalous measurements of δ13C in methane

Measurements of δ13C in methane in the marine boundary layer (MBL) of the extratropical Southern Hemisphere imply a kinetic isotope fractionation much larger than would be expected if the hydroxyl radical were the only tropospheric methane sink. We use a simple chemical box model to show that the assumption of a MBL active chlorine (Cl•) sink can explain these anomalous observations provided there is a seasonal cycle in the Cl• sink with a summer-winter concentration difference ∼ 6 · 10³ cm−3. The required summer maximum and yearly mean Cl• concentrations are plausible, and imply a global Cl• sink strength for methane of < 5 Tg yr−1. Choice of a Cl• sink seasonal cycle linked to the nonsinusoidal dimethyl sulfide seasonal cycle gives the observed fractionation with a smaller yearly mean Cl• concentration than equivalent sinusoidal Cl• cycles.

DMS and SO2 at Baring Head, New Zealand: Implications for the Yield of SO2 from DMS

Atmospheric dimethyl sulfide (DMS) and sulfur dioxide (SO2) concentrations were measured at Baring Head, New Zealandduring February and March 2000. Anti-correlated DMS and SO2 diurnalcycles, consistent with the photochemical production of SO2 from DMS, were observed in clean southerly air off the ocean. The data is used to infer a yield of SO2 from DMS oxidation. The estimated yields are highly dependent on assumptions about the DMS oxidation rate. Fitting the measured data in a photochemical box model using model-generated OH levels and the Hynes et al. (1986) DMS + OH rate constant suggests that theSO2 yield is 50–100%, similar to current estimates for the tropical Pacific.However, the observed amplitude of the DMS diurnal cycle suggests that the oxidation rate is higher than that used by the model, and therefore, that theSO2 yield is lower in the range of 20–40%.

Dimethylsulfide, a limited contributor to new particle formation in the clean marine boundary layer

[1] Condensation sinks for sulfuric acid are predicted using measured aerosol number distributions for five sites, Baring Head and Cape Grim (clean marine), Tarawa (equatorial clean marine), the East Antarctic Plateau and the free troposphere (ultra clean). The condensation sink determined at each site allowed an estimate of the minimum level of sulfur dioxide required for new sulfate particle formation at that site. The sulfur dioxide concentration required for new particle production is nearly two orders of magnitude larger than the observed average at the sites assessed, except the free troposphere. In the marine boundary layer any available sulfur dioxide is converted heterogeneously to sulfate via the condensation sink. This implies that a significant proportion of the sub-micron sulfate aerosol in the marine boundary layer is likely to have been entrained from the free troposphere, where a limited condensation sink and sufficiently high levels of sulfur species can support homogeneous nucleation.

Precursors to Particles (P2P) at Cape Grim 2006: Campaign Overview

Environmental context. Understanding the role of clouds in assessing the impact of climate change is a challenging issue. It is thought that plankton and seaweed contribute to the formation of clouds by emitting gases that lead to the particle production necessary for cloud formation. Macroalgae (kelp) at Mace Head, Ireland, produce large quantities of iodine when exposed to sunlight at low tide and this iodine results in the rapid production of particles. Cape Grim, Tasmania, also has large colonies of kelp and the role of Bull Kelp (Durvillaea potatorum) in particle production was assessed. Abstract. Iodine emissions from coastal macroalgae have been found to be important initiators for nucleation events at Mace Head, Ireland. The source of this iodine is the large beds of the brown kelp Laminaria digitata, which are significantly exposed at low tide. On the coast around Cape Grim, Tasmania, there are beds of the brown kelp Durvillaea potatrum. The Precursors to Particles 2006 (P2P 2006) campaign at the Cape Grim Baseline Air Pollution Station in late summer (February) 2006 focused on the role of this local kelp in providing precursor gases to particle formation. Durvillaea potatorum does not produce iodated precursor gases at the levels observed at Mace Head. IO was measured at 0.5 ± 0.3 ppt, while OIO was below detection limits (9 ppt). The dominant atmospheric iodated species was methyl iodide and the average concentration measured at the Cape Grim Station was 1.5 ± 0.3 pptv in baseline conditions, but showed significant variation in discrete samples collected immediately above the ocean surface. Nucleation events were not detected at the Cape Grim Station, except for one period where the plume of a local bushfire interacted with air of marine origin. The passage of four fronts did not result in nucleation bursts and measurements on the beach 94 m below the Cape Grim Station suggested that Durvillaea potatorum was only a weak source of new particles.

Flux chamber study of particle formation from Durvillaea potatorum

Environmental context. Kelp at Mace Head, Ireland, produces large quantities of iodine when exposed to sunlight at low tide and this iodine results in the rapid production of particles. Cape Grim, Tasmania, also has large colonies of kelp (Durvillaea potatorum) but its role in particle formation appears limited. A flux chamber was used to better understand the response of Durvillaea potatorum to light stress and ozone. Abstract. Brown kelp, in particular Laminara digitata at Mace Head, Ireland, has been shown to emit iodine when under stress, resulting in new particle formation. The Cape Grim Baseline Air Pollution Station, Tasmania, is surrounded by rocky reefs that support large colonies of the brown kelp Durvillaea potatorum. During an intensive campaign in February 2006 at Cape Grim, levels of IO, OIO and methyl iodide remained at background levels and no particle formation events could be associated with locally generated precursor iodine species. In order to better understand the limitations of the local kelp to provide a source of precursor species, samples of Durvillaea potatorum were collected from the beach below the Cape Grim Station and tested for their capacity to initiate particle formation using a flux chamber technique. Particles were observed only when the kelp was exposed to both very high levels (>100 ppb) of ozone and natural solar radiation. There was a high correlation between ozone level and particles produced. The particles resulting from exposure to high levels of ozone were aromatic and volatile. Durvillaea potatorum appears to plays a very limited role in contributing to particle formation at Cape Grim, but it does represent a source of atmospheric iodine under photo-oxidative stress, of 18 pmol g–1 (fresh weight) min–1 and is likely to have a significant role in atmospheric chemistry at this site.

Understanding the origin of clouds

Clouds are an important part of our atmosphere and they have a critical role in controlling the amount of the sun’s energy that reaches the earth’s surface. Clouds can have a cooling effect on the atmosphere, which counteracts increases in temperature caused by climate change.[1] Understanding exactly how clouds impact on our climate and ensuring that we can accurately model the current role and extent of clouds is critical to determine how any changes in climate will affect clouds and how clouds will affect climate in the future. Clouds consist of many drops of liquid water and these droplets form when water vapour condenses on the surface of a tiny particle. The sources of these particles in the atmosphere are many and varied. Some are generated from the surface of the land or ocean by wind action, and some of these primary particles can act as cloud condensation nuclei (CCN).[2] There are also several natural processes that emit gases, which then react in the atmosphere to form secondary particles. This includes the emission of gases from plankton and seaweed in the ocean and plants on land. Man’s activities, such as the burning of fossil fuels can also result in the production of particles either directly or through precursor gases. There is a complicated interaction between existing particles and gases in the atmosphere. Larger particles can mop up the precursor gases before they can form a particle, which limits the role for secondary particle formation.[3] Even clouds themselves have a very important role in reactions between gases and between particles and gases.[4] The CLAW Hypothesis[5] was published in 1987 and stated that plankton in the ocean emitted dimethyl sulfide (DMS), which once in the atmosphere eventually forms tiny sulfate particles that could act as CCN. It was suggested that plankton emitted more DMS when under stress from higher sea surface temperatures and so more CCN and clouds would result, which created a feedback loop to limit warming.[5] Many studies have shown that the seasonal cycles of DMS, sulfate particles and CCN numbers are strongly correlated.[6] However, other studies have shown that DMS-derived sulfur dioxide is removed by larger particles before it can form tiny sulfate CCN.[3] DMS isn’t a major source of CCN in the marine boundary layer,[7] but has an important role in modifying the chemistry of

Where to now? A synthesis of current views of the CLAW hypothesis

Abstract. The CLAW hypothesis was published 20 years ago, building on suggestions that the sulfur cycle provided a natural feedback mechanism whereby plankton in the ocean had a role in modifying climate by providing the precursors for cloud condensation nuclei, which leads to the formation of high albedo clouds. In this issue, the 10 preceding articles represent the opinions of several leading scientists working on various aspects of the CLAW hypothesis and here we synthesise these varied opinions to answer the questions: Does the CLAW hypothesis operate as described in the original 1987 publication? and What steps and advances are needed to better understand CLAW and resolve any outstanding areas of difficulty?