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Using laser light to trap and explore the cloud-forming properties

of  single aerosol particlesJim Walker, Jon Wills and Jonathan ReidDepartment of Chemistry, University of Bristol

IntroductionThe atmospheric radiative forcing of aero-sols is one of the greatest sources of uncer-tainty in our current understanding of global climate change (Solomon et al., 2007). Aerosols perturb the radiation balance by directly scattering and absorbing radiation (a direct effect), and by influencing the albedo and lifetime of clouds (an indirect effect). In order to improve our ability to quantify the impact of aerosols on the cli-mate system, the thermodynamic proper-ties and kinetics of change of aerosol particles need to be fully resolved.

The equilibrium size adopted by an aero-sol particle is a property that must be under-stood. Not only does size influence the particle’s optical cross-section but also whether or not the particle is likely to act as a cloud condensation nuclei (CCN) and form a cloud droplet. Equilibrium size is determined by the partitioning of water between the particle and the surrounding gas phase. As the ambient relative humidity (RH) changes, a particle takes up or loses water so as to remain in thermodynamic equilibrium with its surroundings. This behaviour is governed by the aerosol hygro-scopicity and is dependent on the aerosol chemical composition and structure. Atmospheric aerosol particles usually con-tain a variety of components, including organic and inorganic compounds and black carbon, and often exist as aqueous droplets through the take-up of water.

Köhler theory is used to predict the hygroscopic behaviour of simple aerosol particles (Köhler, 1936). This describes the interplay between the surface curvature (Kelvin effect) and solute (solute effect) in determining the vapour pressure of water for an aqueous droplet. A droplet has a curved surface which raises the vapour pressure above that of an equivalent flat-surfaced solution. The solute contained within the droplet opposes the Kelvin effect

and acts to decrease the vapour pressure to below that of pure water. When the vapour pressure is balanced by the ambient RH, a droplet is considered to be in thermody-namic equilibrium with its surroundings and the droplet radius will then remain steady. Figure 1 illustrates the impact of the Kelvin and solute effects on the droplet size. A simple way of considering the figure is that it shows the RH required to stabilize a droplet, in this case a sodium chloride droplet with a dry particle diameter of 10 nanometres (nm = 10−9m), at a certain droplet size. For larger droplet diameters (>2μm: μm = 10−6m) the vapour pressure and droplet size are dominated by the sol-ute effect. However, for smaller or more dilute droplets the Kelvin effect becomes increasingly important, leading to a decrease in droplet size.

Hygroscopic changes in droplet size are often expressed in terms of a growth factor (GF) which is simply the ratio of dry (rdry) to wet (r) particle radii at a given RH:

GF(RH) = r(RH) ____ rdry

(1)

This removes the dependence on dry parti-cle size and allows measurements from a range of droplets to be compared to a single prediction curve.

Figure 2 shows hygroscopic growth curves for three single component aerosols predicted by the Aerosol Diameter Dependent Equilibrium Model (ADDEM) (Topping et al., 2005). The predictions clearly show that as the local RH is increased the droplets grow in size (increasing droplet radii leads to increasing droplet growth fac-tors). Above 90% RH, the size of each drop-let becomes increasingly sensitive to changes in RH: a small RH change can lead to very large changes in size.

Because of the Kelvin effect, the sur-rounding RH must surpass a critical super-saturation (RH > 100%) before an aerosol droplet can become a cloud droplet (Figure 1). Below the critical supersatura-tion a droplet adopts a unique size at each RH. Once the critical supersaturation is reached the droplet will continuously take up water and grow unhindered provided the RH does not drop below 100%. Understanding the relationship between

critical supersaturation, droplet size and composition remains one of the main chal-lenges in fully resolving the hygroscopic behaviour of atmospheric aerosols.

Of the examples in Figure 2, and assum-ing equivalent dry particle sizes, a sodium chloride particle takes up the most water at a given RH and adopts the largest equilib-rium droplet size. Glutaric acid is the least hygroscopic and ammonium sulphate falls between these two. Typically, aerosols con-taining inorganic salts are more hygroscopic

Figure 1. A Köhler curve showing the variation in wet droplet size with RH for a sodium chloride droplet with a dry particle diameter of 10nm (black line). The blue dashed line shows the behaviour if only the Kelvin effect is considered and the red dashed line indicates just the solute effect. Inset is a magnified section of the Köhler curve.

Figure 2. Modelled aerosol hygroscopic growth curves from the Aerosol Diameter Dependent Equilibrium Model (ADDEM). Black is sodium chloride, blue is ammonium sulphate and red is glutaric acid.

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excitation of the OH-stretching vibration of the water molecules present within the droplet. Superimposed above the spontane-ous band are peaks of stimulated Raman intensity at discrete wavelengths. The wave-lengths of these peaks are used to accurately infer the size of the droplet. The peaks occur at wavelengths corresponding to Whispering Gallery Modes (WGMs), named in reference to the famous whispering galleries found at such places as St Paul’s Cathedral. In these cases the wavelength of the spontaneous Raman emission is such that a standing wave forms around the circumference of the droplet. This stimulates further Raman emis-sions, giving rise to cavity-enhanced peaks in the Raman spectrum. The pattern of stim-ulated Raman scattering at WGM wave-lengths provides a unique fingerprint of the droplet size, allowing the size to be deter-mined with nanometre accuracy. A more detailed explanation of the sizing process can be found in Mitchem and Reid (2008).

The following account shows how these techniques have been used to confirm the validity of commonly used thermodynamic models such as the Aerosol Diameter Dependent Equilibrium Model (ADDEM) at RHs approaching saturation. In particular, we examine the accuracy of the model for predicting the hygroscopic behaviour of ammonium sulphate aerosol, one of the most important cloud-forming aerosols (McFiggans et al., 2006).

High precision hygroscopicity measurements Figure 4 shows a sequence of images recorded from two droplets tweezed in neighbouring optical traps. In this case the droplet at the bottom is a sodium chloride control droplet and the one at the top is an ammonium sulphate droplet, the drop-let of interest. The RH within the trapping cell is changed by introducing a humidified nitrogen (N2) gas flow. This provides a pure, inert atmosphere allowing aerosol hygroscopicity to be isolated and examined

as opposed to traditional capacitance probes to measure RH. Capacitance probes typically have an uncertainty >> ±1% RH, far larger than the uncertainty associated with the control droplet. The control droplet also probes the RH in the immediate vicinity of the droplet of interest.

To trap a droplet, an aerosol plume, which is generated by nebulising a bulk solution, is passed through the trapping cell. When a droplet passes close to the focal point of the laser it can become trapped. Comparative experiments require two chemically dissimi-lar droplets to be tweezed in traps situated only a few tens of micrometres apart. This presents a problem because the respective bulk solutions must be nebulised consecu-tively into the trapping cell. It is important to ensure that the initially trapped droplet is not contaminated by collisions with aero-sols from the second nebulised species. This is solved by incorporating a reflective spatial light modulator (SLM) in the optical set-up prior to the trapping cell. Holographic phase patterns (holograms) are displayed on the SLM, modifying the incoming laser beam and creating a unique optical landscape inside the trapping cell. In this experiment the SLM is used to create a protective ring of light intensity around a trapped droplet, deflecting incoming droplets and prevent-ing contamination. This method allows a control droplet to be trapped and isolated before the droplet of interest is trapped (Wills et al., 2009). Other uses of the SLM include creating arrays of traps to tweeze multiple droplets at once and animating trap positions to move droplets around within the trapping plane. Animations are created by applying a sequence of holo-grams onto the SLM (Butler et al., 2009).

Trapped droplets are sized with nanome-tre accuracy using Raman spectroscopy (Mitchem and Reid, 2008). Figure 3 shows a Raman spectrum that is typical of an opti-cally trapped aqueous aerosol droplet. The broad underlying band of spontaneous Raman intensity arises from laser-induced

than those containing other species and will activate into cloud droplets given suitable ambient conditions. It is important to note that atmospheric aerosols are usually far more complex than these single component examples. However, before thermodynamic aerosol models can be trusted to accurately represent the equilibrium behaviour of typi-cal atmospheric aerosols, their robustness at a basic level must be rigorously tested. To trust predictions of critical supersatura-tions for aerosols of interest, models must be tested for their accuracy at RHs approach-ing saturation. Validation of models can be achieved through comparison of predicted trends with experimental results.

The optical tweezing techniqueCollecting the experimental data with which to test thermodynamic aerosol mod-els requires controlled laboratory tech-niques. In recent years optical tweezing has emerged as a valuable tool for probing indi-vidual aerosol droplets (Mitchem et al., 2008). In such experiments a droplet (typically between 3μm and 6μm in radius) is trapped indefinitely by a tightly focussed laser beam within a trapping cell. The droplet is then interrogated as conditions inside the trap-ping cell are changed. For example, the hygroscopicity of the droplet can be mea-sured from the change in radius as the RH within the cell is changed (Hargreaves et al., 2010). Optical tweezing is a powerful tech-nique and has already been used to inves-tigate aerosol properties including optical properties (Reid, 2009), equilibrium state (Hanford et al., 2008) and composition (Laurain and Reid, 2009).

A significant strength of the optical tweez-ing technique is the ability to trap and isolate different droplets in neighbouring optical traps, separated by just tens of micrometres (Wills et al., 2009; Butler et al., 2009). This allows for comparative measurements to be made on droplets of distinct chemical com-position. Indeed, if one of the traps is occu-pied by an aqueous sodium chloride droplet, for which the hygroscopic behaviour is very well understood, we have a highly sensitive and responsive probe of the gas phase con-ditions. RH changes can be determined from the size change of the sodium chloride con-trol droplet with an accuracy of better than ±0.09% RH and one second time response (Butler et al., 2008), an accuracy and time-resolution that are impossible with conven-tional RH probes. Consequently, the hygroscopic behaviour of a nearby droplet of interest can be measured with an accuracy previously unobtainable, which is particularly crucial for investigating aerosol properties at high RH as CCN activation is approached.

The precision with which changes in RH can be determined is one of the key benefits in using a sodium chloride control droplet

Figure 3. A typical Raman spectrum collected from, and used to accurately size, an optically tweezed aerosol droplet. The peaks of stimulated Raman emission are clearly visible above the broad spontaneous Raman band.

Figure 4. A series of photographs showing the change in size of two droplets tweezed in neighbouring optical traps as the RH within the cell is increased (from left to right). The black scale bar represents a distance of approxi-mately 10μm. The ammonium sulphate droplet is at the top and the sodium chloride control droplet is at the bottom.

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ReferencesButler JR, Laura Mitchem, Hanford KL, Lennart Treuel, Reid JP. 2008. In situ comparative measurements of the proper-ties of aerosol droplets of different chemi-cal composition. Faraday Discuss. 137: 351–366.Butler JR, Wills JB, Laura M, Burnham DR, David McGloin, Reid JP. 2009. Spectroscopic characterisation and manipulation of arrays of sub-picolitre aerosol droplets. Lab Chip 9: 521–528. doi:10.1039/b814545hHanford KL, Laura M, Reid JP, Clegg SL, Topping DO, McFiggans GB. 2008. Comparative thermodynamic studies of aqueous glutaric acid, ammonium sulfate and sodium chloride aerosol at high humidity. J. Phys. Chem. Lett. A 112: 9413–9422. doi:10.1021/jp802520dHargreaves G, Kwamena N-OA, Zhang YH, Butler JR, Rushworth S, Clegg SL, Reid JP. 2010. Measurements of the equilibrium size of supersaturated aque-ous sodium chloride droplets at low relative humidity using aerosol optical tweezers and an electrodynamic balance. J. Phys. Chem. Lett. A 114(2): 1806–1815, doi:10.1021/jp9095985Köhler H. 1936. The nucleus in and the growth of hygroscopic droplets. Trans. Faraday. Soc. 32: 1152–1161.Laurain AMC, Reid JP. 2009. Characterizing internally mixed insoluble organic inclusions in aqueous aerosol droplets and their influence on light absorption. J. Phys. Chem. Lett. A 113(25): 7039–7047, doi:10.1021/jp902248pMcFiggans G, Artaxo P, Baltensperger U, Coe H, Facchini MC, Feingold G, Fuzzi S. 2006. The effect of physical and chemi-cal aerosol properties on warm cloud droplet activation. Atmos. Chem. Phys. 6: 2593–2649.Miles REH, Knox KJ, Reid JP, Laurain AMC, Laura M. 2010. Measurements of mass and heat transfer at a liquid water surface during condensation or evapora-tion of a subnanometer thickness layer of water. Phys. Rev. Lett. 105: 116101, doi:10.1103/PhysRevLett.105.116101Mitchem Laura, Reid JP. 2008. Optical manipulation and characterisation of aerosol particles using a single-beam gradient force optical trap. Chem. Soc. Rev. 37(4): 756–769.Reid JP. 2009. Particle levitation and laboratory scattering. J. Quant. Spectrosc. Radiat. Transf. 110(14): 1293–1306, doi:10.1016/j.jqsrt.2009.02.019Solomon S, Qin D, Manning M, Chen Z, Marquis M, Averyt KB, Tignor M, Miller HL. 2007. Climate Change 2007: The Physical Science Basis – Contribution of Working Group to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change (IPCC). Cambridge University Press: Cambridge, UK and New York, NY.Tong H-J, Reid JP, Bones DL, Luo BP, Krieger UK. 2011. Measurements of the timescales for the mass transfer of water in glassy aerosol at low relative humidity and ambient temperature. Atmos.

physical properties of atmospherically rele-vant aerosol droplets. Thermodynamic mea-surements of ammonium sulphate have shown very close agreement with thermo-dynamic predictions for the 84–96% RH range. These results suggest that estimates of the critical supersaturation, the RH required to activate an aerosol particle into a cloud droplet, from thermodynamic mod-els such as ADDEM, are accurate to better than ±0.002% RH for ammonium sulphate aerosol, an important cloud-forming aerosol (Walker et al., 2010). This work has pro-gressed previous hygroscopicity studies examining mixed organic/inorganic parti-cles at high RH (Hanford et  al., 2008) and single component inorganic particles over the low RH range (Hargreaves et al., 2010).

Laboratory work of this kind is essential to help reduce the uncertainty associated with the effects of atmospheric aerosols on climate. Further optical tweezing work has examined the phase structure and optical properties of mixed phase inorganic/organic aerosol particles (Laurain and Reid, 2009), increasing the complexity of studied aerosols composition towards that found in  atmospheric aerosols. Research is now examining the oxidative ageing of particles containing benchmark organic species such as glutaric acid. The controlled manip-ulation of arrays of droplets will also enable precise measurements of the short-range interactions between droplets, and improve studies of the dynamics of droplet coales-cence in the future.

Currently, experiments are underway to not only examine the equilibrium state of the aerosol, but also to investigate the kinetics of mass- and heat-transfer during water condensation to more fully under-stand the growth rate of cloud droplets (Miles et al., 2010). Kinetic studies of this kind are potentially very important if water uptake rates, and not thermodynamics, are shown to play the key role in atmospheric cloud droplet formation.

Kinetics studies have also examined the change in water-uptake behaviour of sucrose aerosol as it changes from a liquid to a glassy state. When liquid, the sucrose droplet equil-ibrates on a timescale comparable to the change in environmental conditions, whilst in the glassy state the droplet can remain in disequilibrium for timescales of more than 10 000 seconds (Tong et al., 2011).

AcknowledgementsThe authors gratefully acknowledge sup-port from the Engineering and Physical Sciences Research Council through the award of a Leadership Fellowship (JPR) and a research grant (JBW). JSW acknowledges the support of the Natural Environment Research Council for the award of a PhD studentship. This paper is based on a

independently from other properties. This is a benefit when one considers the impuri-ties, trace gases and oxidative nature of ambient air. As the RH changes, the drop-lets change size to maintain thermody-namic equilibrium with their surroundings. The sizes of both droplets are tracked as the RH changes. At all times during the experiment the RH is determined from the size of the control droplet using thermody-namic model predictions (Hanford et al., 2008). Combining the control droplet RH profile with the ammonium sulphate size profile gives high precision hygroscopicity data, directly mapping out the Köhler curve. Figure 5 shows the results of one such experiment compared with the ammonium sulphate GF predictions from ADDEM. Clearly, the results are in very good agreement with the modelled predictions over the entire experimental range in RH. The equilibrium size of ammonium sul-phate aerosol from several experimental runs is found to agree with predictions to within an uncertainty of ±0.2% over an RH range of 84–96% (Walker et al., 2010). This confirms the validity of the ADDEM model for predicting the hygroscopic behaviour of a highly relevant atmospheric aerosol in approaching cloud-forming conditions. This increases the confidence in the model to extrapolate accurately into the super-saturated regime. The experimental proce-dure is a delicate one, with the laser power set to trap droplets of a certain size range. As tweezed droplets change size they become less stable in the optical traps and eventually fall out. For this reason the RH range over which to collect data must be carefully selected, and in this case high RHs approaching cloud-forming conditions have been chosen.

ConclusionsThis work highlights just one way in which the optical tweezing technique can be used to improve our understanding of the

Figure 5. Experimental ammonium sulphate hygroscopic growth curve. Ammonium sulphate growth factors shown as a function of RH as measured from the NaCl control droplet. Data is shown against simulated growth factors from the ADDEM model (dashed black line).

presentation to the RMetS Student Conference in 2010.

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hygroscopic properties of ammonium sulfate and sodium chloride aerosol at relative humidities approaching satura-tion. J. Phys. Chem. A 114: 12682–12691, doi:10.1021/jp107802yWills JB, Butler JR, Palmer J, Reid JP. 2009. Using optical landscapes to con-trol, direct and isolate aerosol particles. Phys. Chem. Chem. Phys. 11: 8015–8020, doi:10.1039/b908270k

Chem. Phys. Discuss. 11(2): 4843–4879, doi:10.5194/acpd-11-4843-2011Topping DO, McFiggans GB, Coe H. 2005. A curved multi-component aerosol hygroscopicity model framework: part 2 – including organic compounds. Atmos. Chem. Phys. 5: 1223–1242.Walker JS, Wills JB, Reid JP, Liangyu W, Topping DO, Butler JR, Yun-Hong Z. 2010. Direct comparison of the

Correspondence to: Jim Walker, Department of Chemistry,University of Bristol

jw2324@bris.ac.uk

© Royal Meteorological Society, 2011

DOI: 10.1002/wea.815

Downslope winds at Greek coastlinesAlan LapworthSharnbrook, Beds

Greek coastal waters in summer with their long periods of sunshine, moderate winds, absence of tides, areas sheltered from swell, deep waters close to the shore (and hence few underwater rocks), and near-zero mag-netic variation, are an ideal area for sailing. Since the 1970s they have become home to an increasing number of flotillas and many people have learned to sail in these deserv-edly popular waters.

On the west coast (bounding the Ionian Sea) summer winds blow generally from the northwest and are known locally as the maestro. Although, in common with most parts of Greece, there is a northerly synoptic gradient component, the winds in the Ionian region are markedly diurnal and are mainly driven by the land-sea temperature difference. During the morning there is usu-ally very little wind. Around the middle of the day, the sea breeze arrives from the northwest and usually blows up to force 4 or 5 before dying away in the evening. During the evening, a land breeze with sig-nificant katabatic components from the inland mountains often blows from the southeast. Sometimes a strong, sandy, southerly blow (known as a Sirocco) from a small depression will be experienced, but this is rare in summer.

On the east coast, the sea breeze blows from the southeast. North of Cape Sounion, however, this is often replaced in summer by a northeasterly monsoon wind known as the Meltemi (or Etesian winds). This can blow up to near-gale force for several days at a time further out among the Aegean islands.

Downslope windsThe coast itself is generally mountainous and the hills can locally enhance the other-

wise moderate sea breeze to an extent that may come as a surprise to those first expe-riencing the effect, as it often happens sud-denly. These orographically-enhanced winds are the subject of this article. Apart from the effect they undoubtedly have on a sailing boat, they are extremely interesting both to observe and experience. A typical case might occur during a sail from the islands of Levkas or Meganisi southwest across the Ionian ‘inland’ Sea to Port Vathi on the island of Ithaca (traditionally associ-ated with Odysseus). The moderate after-noon sea breeze from the northwest gives a pleasant beam reach (i.e. at right-angles to the track) with a gentle rolling swell into the entrance of the Gulf of Molo (Kolpos Aetou) which runs southwestwards into the island, nearly dividing it in two. At the end of the Gulf lies the narrow channel into Port Vathi (Figures  1  and 2). On the right-hand (northwest) side of the Gulf, a ridge rises steeply towards Mount Neritos (2600 feet) and as you approach the land two unusual features may strike you. The first may be an apparently motionless line of cloud along the hillside (Figure 3); as you get closer, the cloud can be seen to be pouring down the hillside and dissolving at a fixed level (Figure  4). The other feature is that the sea surface under the cloud and across much of the Gulf appears to be boiling with white caps. Outside this disturbed area, the sea surface is eerily smooth, apart from little wavelets radiating outwards from the white water. What appears to be happening is that the northwesterly sea breeze is flowing over the top of the ridge. However, instead of leaving the lee side of the mountain in rela-tive shelter, it is blowing straight down the hillside onto the surface of the Gulf waters immediately adjacent to the land with greatly enhanced wind speed.

By this stage, anyone with a normal sense of preservation will have taken down all sail

ITHACAAEGEAN

EVIAIONIAN

Figure 1. Outline map of Greece showing the location of the islands of Ithaca and Evia.

Figure 2. An aerial photograph of the island of Ithaca showing the Gulf of Molo and Port Vathi.

and started the engine – if your engine is not reliable then at most you leave up a double-reefed main sail. You might also con-sider going somewhere other than Port Vathi: Kioni to the north is an alternative. Once into the Gulf, there is no avoiding the downdraught and as you reach the bound-ary of white-capped water, the whole boat will heel over somewhat, even under a bare mast. A rough ride ensues down the Gulf until you turn left (downwind) into the port entrance channel. Should you still be under sail at this stage then a gybe (a violent, potentially damaging, swing of boom and mainsail from one side to the other when