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Atmospheric Environment 45 (2011) 4123e4132

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Atmospheric Environment

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The impact of organic coatings on light scattering by sodium chloride particles

Yan Li a, Michael J. Ezell b, Barbara J. Finlayson-Pitts b,*

a School of Chemical Engineering, Nanjing University of Science and Technology, Nanjing, Jiangsu, ChinabDepartment of Chemistry, University of California, Irvine, CA 92697-2025, USA

a r t i c l e i n f o

Article history:Received 15 February 2011Received in revised form10 May 2011Accepted 10 May 2011

Keywords:Light scatteringSea saltSecondary organic aerosolParticle measurements

* Corresponding author. Tel.: þ1 949 824 7670; faxE-mail address: bjfinlay@uci.edu (B.J. Finlayson-Pi

1352-2310/$ e see front matter � 2011 Elsevier Ltd.doi:10.1016/j.atmosenv.2011.05.031

a b s t r a c t

Light scattering by airborne particles plays a major role in visibility degradation and climate change. Thecomposition and structure of particles in air can be complex, so that predictions of light scattering a priorihave significant uncertainties. We report here studies of light scattering by NaCl, a model for airborne saltparticles from the ocean and alkaline lakes, with and without an organic coating formed from the lowvolatility products of the reaction of a-pinene with ozone at room temperature at 1 atm in air. Lightscattering at 450, 550 and 700 nm was measured using an integrating nephelometer on particles whosesize distribution was independently determined using a scanning mobility particle sizer (SMPS). Forcomparison, polystyrene latex spheres (PSL) of a known size and dioctylphthalate (DOP) particlesgenerated with a narrow size distribution were also studied. The measured values were compared tothose calculated using Mie theory. Although excellent agreement between experiment and theory wasfound for the PSL and DOP particles, there were large discrepancies for a polydisperse NaCl sample. Thesewere traced to errors in the size distribution measurements. Despite the use of 85Kr neutralizers, theBoltzmann charge equilibrium distribution assumption used to derive particle size distributions fromSMPS data was shown not to be valid, leading to an overestimate of the concentration of larger particlesand their contribution to light scattering. Correcting for this, the combination of experiments and theoryshow that as salt takes up low volatility organics in the atmosphere and the geometric mean diameterincreases, the effect on light scattering may be reasonably approximated from the change in sizedistribution under conditions where the organic coating is small relative to the core size. However, fora given particle diameter, light scattering decreases as the relative contribution of the organic componentincreases. Thus, light scattering by salt particles with a specific size distribution will be reduced whenorganics comprise a significant portion of the particles. This will lessen their impact on the visual rangecompared to pure salt particles, but also lead to less counterbalancing of the tropospheric warming dueto greenhouse gases.

� 2011 Elsevier Ltd. All rights reserved.

1. Introduction

Aerosol particles are important for a number of reasons,including detrimental effects on human health (Pope III andDockery, 2006). Particles scatter light, which leads to degradationof visibility and is responsible for the direct effect of particles onclimate which partially counterbalances the warming effects ofgreenhouse gases; they also have indirect effects on climatethrough their impact on cloud properties and lifetimes (Finlayson-Pitts and Pitts, 2000; IPCC, 2007). There is a great deal of interest inquantifying the effects of aerosols on climate because this currentlyrepresents the greatest uncertainty in predicting changes in radi-ative forcing (IPCC, 2007).

: þ1 949 824 2420.tts).

All rights reserved.

Sea salt is a major contributor to the burden of atmosphericparticles over oceans and in coastal areas (Woodcock, 1953, 1972;Murphy et al., 1998; Lewis and Schwartz, 2005; Andreae andRosenfeld, 2008). In addition, salt particles are emitted from alka-line dry lakes and salt lakes such as the Dead Sea in Israel and theGreat Salt Lake in Utah (Stutz et al., 2002; Levin et al., 2005). Thesesea salt particles, of which NaCl is a major component, scatter lightand are major contributors to the aerosol optical depth over oceans(Murphy et al., 1998; Shinozuka et al., 2004). Salt particles are wellknown to take up water and deliquesce (Twomey, 1953; Orr et al.,1958; Hänel and Zankl, 1979; Tang et al., 1997) which changesboth the particle size and refractive index, and thus changes thelight scattering as well (Covert et al., 1980; Tang, 1996; Hinds, 1999;Fierz-Schmidhauser et al., 2010).

During their formation by bubble-bursting at the ocean surface,sea salt particles acquire organic coatings due to biological materialin the oceans (Blanchard, 1964; Modini et al., 2010; Russell et al.,

Y. Li et al. / Atmospheric Environment 45 (2011) 4123e41324124

2010). Organic coatings can also be acquired by uptake, duringtransport, of low volatility organics formed by chemical reactions ofgaseous precursors (Gill et al., 1983; Middlebrook et al., 1998;Ellison et al., 1999; Russell et al., 2010). For example, McInneset al. (McInnes et al., 1998) reported a significant contribution ofcarbonaceousmaterial tomarine particles that had been influencedby anthropogenic emissions. However, modeling and field studiessuggest that the coexistence of organic compounds with sea saltcan also affect the light scattering properties due to effects onparticle size as well as on chemical composition/refractive index(McInnes et al., 1998; Randles et al., 2004). In addition, organicsurfactants can impact water uptakewhich also affects particle size,composition and light scattering (Hansson et al., 1990, 1998;Andrews and Larson, 1993; Wagner et al., 1996; Kaku et al., 2006;Alshawa et al., 2009; Harmon et al., 2010).

The goal of the present study was to probe the effects of organiccoatings on light scattering by NaCl particles as a model for emis-sions from the oceans and from dry lakes. The coatings consisted oflow volatility organics formed in the oxidation of a-pinene,a common atmospheric biogenically derived constituent which ispresent in many coastal regions along with sea salt. In addition, theoxidation of a-pinene to form low-volatility products is well-studied (Zhang et al., 1992; Kamens et al., 1999; Koch et al., 2000;Jaoui and Kamens, 2001; Jenkin, 2004; Chen and Griffin, 2005;Lee and Kamens, 2005; Pathak et al., 2007; Yu et al., 2008). Totest the experimental system and the calculations, the study alsoincluded experiments on monodisperse polystyrene latex spheres

a

b

c

d

Fig. 1. Flow system configurations used to generate four aerosols in this study: (a) monodispdistribution; (c) polydisperse NaCl; and (d) monodisperse NaCl with and without an organ

and a polydisperse (but with a narrow distribution) sample ofdioctyl phthalate particles. We show that the organic coatingaffects the optical properties of NaCl particles and discuss theimplications for light scattering in the atmosphere.

2. Experimental

2.1. Generation and characterization of particles

The aerosols described below were all generated by atomizingprepared solutions with a constant output atomizer (TSI, Model3076) with either synthetic air (Oxygen Services Company, blend ofO2 and N2, THC (total hydrocarbon) < 0.01 ppm, H2O < 2.0 ppm,CO < 0.5 ppm, CO2 < 0.5 ppm) or UHP (ultra-high purity) nitrogen(Oxygen Services, 99.999%) at a pressure of 20e36 psig.Compressed air, purified by passing through an FTIR (FourierTransform Infrared spectrometer) purge gas generator (ParkerBalston Model 75-62), filtered through activated carbon/aluminamedia (Perma Pure LLC) and finally an inline 0.1 micron filter (DIF-N70, Headline Filters), provided diluent gas for all aerosols.

Experimental requirements such as high particle concentrationsneeded for nephlometry measurements, well-defined and stableparticle size distributions for calculations and, in the case of organic-coated salt particles, the need to avoid homogeneous nucleation oforganics, added constraints on the experimental systems. Fourdifferent flow system configurations shown in Fig. 1 were used togenerate particles in this study, as described in more detail below.

erse PSL spheres; (b) polydisperse dioctyl phthalate (DOP) particles with a narrow sizeic coating. Flow controllers (FC) and krypton-85 neutralizers (85Kr) are shown.

Table 1Refractive indices used in Mie calculationsa.

l (nm) PSLb DOPc NaCld SOAe

450 1.613 1.500 1.558 1.48550 1.596 1.489 1.547 1.45700 1.582 1.479 1.539 1.41

a All organics taken to be non-absorbing at 450, 550 and 700 nm.b From Kasarova (Kasarova et al., 2007).c From Calvalier and Larche (Cavalier and Larche, 2006).d From the Handbook of Chemistry and Physics (Lide, 1994).e From best match to data for pure SOA in separate experiments. These values are

in excellent agreement with direct measurements of the refractive index of SOAfrom pinene ozonolysis which were in the range of 1.4e1.5 (Kim et al., 2010).

Y. Li et al. / Atmospheric Environment 45 (2011) 4123e4132 4125

2.1.1. Polystyrene latex spheres (PSL)Diluted suspensions of a PSL microsphere size-standard

(Duke Scientific, 3300A, 300 � 5 nm) were prepared by adding15 drops of a concentrated aqueous suspension to 200 mL of18.0 MU-cm water. As seen in Fig. 1a, the solution was atomizedand flowed at w2 lpm through two diffusion dryers (TSI Model3062) into a 3-L glass mixing bulb where the aerosol was dilutedwith a 8 lpm flow of purified air. The diluted mixture total flowwas passed through a 85Kr neutralizer (TSI Model 3012) beforebeing simultaneously sampled by the nephelometer (9.7 lpmflow) for light scattering measurements (TSI Model 3563) anda scanning mobility particle sizer (sample flow 0.3 lpm, sheathflow 3.0 lpm) (SMPS) consisting of an electrostatic classifier(TSI, Model 3080) with a differential mobility analyzer (DMA;TSI Model 3081) and a condensation particle counter (CPC; TSIModel 3022a). Prior to entering the DMA, aerosol passedthrough an impactor (0.071 cm orifice) and a smaller, internal85Kr neutralizer (TSI Model 3077).

2.1.2. Dioctylphthalate (DOP)Particles of DOP (Aldrich, 99%) were generated by atomizing

a 0.25% (w/w) solution of DOP in ethanol that also contained tracelevels (3.5 ppm) of anthracene that acted as seed particles. Subse-quently, the aerosol passed through a vertical glass tube (45 cm inlength and 1.9 cm inside diameter), the top half of which washeated with electrical heating tape to 250e300 �C so as to vaporizeDOP. The flow of DOP vapor continued through the lower unheatedhalf of the tube where the vapor cooled to room temperature andcondensed to form particles with a relatively narrow size distri-bution (Liu and Lee, 1975). A portion of the aerosol flow (0.3 lpm)was dried and diluted with purified air in a 5-L mixing bulb (flow18 lpm), passed through the 85Kr neutralizer and subsequentlydivided for simultaneous measurements by the nephelometer(flow 17.7 lpm) and SMPS (sample flow 0.3 lpm, sheath flow3.0 lpm) (Fig. 1b).

2.1.3. Polydisperse NaClSalt particles, generated from an aqueous solution (1.0 g L�1) of

NaCl (Fischer, Certified A.C.S.) were passed through two diffusiondryers (Fig. 1c). The aerosol (2.4 lpm), along with diluent air(20 lpm), entered a high-volume slow-flow reactor described indetail elsewhere (Ezell et al., 2010) and then through a 85Krneutralizer. Briefly, the main body of the flow reactor is cylindrical,7.3 m in length and 0.46m in diameter. Aerosol and diluent air werewell mixed in the first 1.2 m before entering the 6.1 m reactionsection which is equipped with five equally spaced sampling ports.Uncoated NaCl aerosol was sampled simultaneously from thefurthest downstream port by a nephelometer (flow 9.9 lpm) anda SMPS (sample flow 0.3 lpm; sheath flow 3.0 lpm).

2.1.4. Monodisperse NaClSodium chloride aerosol was obtained using the same atomized

NaCl solution, diffusion dryers and neutralizer along with the SMPSdescribed above but with the CPC removed (Fig. 1d). Instead ofscanning, the upstream DMA was set at a voltage selected toprovide a narrow distribution of particles around a selected elec-trical mobility diameter of 131 nm, and the monodisperse output(2.0 lpm) was directed to a 3-L continuous flow glass reactor alongwith diluent air and oxygen (flow 6.3 lpm), and at times 300 ppb a-pinene, and an O3/O2 mixture (1.5 ppm O3) to produce an organiccoating. The output of the reactors was apportioned between thenephelometer (8.5 lpm) and a second SMPS system for particlesizing (sample flow 0.3 lpm, sheath flow 3.0 lpm). This secondSMPS system was identical to the one described above except theCPC was a TSI Model 3075.

2.2. Measurement of light scattering

Light scattering at 450, 550 and 700 nmwas measured using anintegrating nephelometer (TSI Model 3563) with angular integra-tion from 7� to 170�. In the normal configuration, heating from thehalogen lamp can cause a temperature rise in the sample of up to4.5 �C (Heintzenberg and Erfurt, 2000; Carrico et al., 2003; Fierz-Schmidhauser et al., 2010). In order to minimize the temperatureincrease, lamp power was reduced to 40% of the maximum. Inaddition, the instrument top cover was removed and an externalfan directed towards the lamp and cell. These steps kept themeasured temperature increase to less than 1 �C. Particle numberand size distributions were measured at the inlet and outlet to testfor particle losses, which were observed to be less than 3%. Thenephelometer had been calibrated against particle free air andcarbon dioxide and daily or hourly zero baseline measurementswere made to correct for any drift in background scattering.

2.3. Mie scattering calculations

Light scattering coefficients, ssp, were calculated using Mietheory based on the algorithmof BohrenandHuffmann (Bohren andHuffman, 1983). The refractive indices used for Mie calculations at450, 550 and 700 nm for PSL, DOP, NaCl, and organic coating, aresummarized in Table 1. Particles were assumed to be sphericalwhich will certainly be the case for all but the uncoated NaClparticles, which are cubic. However, the potential correction to sspfor this non-sphericity has been shown to be less than 5% for sub-micron particles (Chamaillard et al., 2006), significantly smallerthan calculational errors estimated to be 15% arising mainly frommanufacturers stated uncertainty in CPC number counts (10%).

After charge correction, which assumes a Boltzmann equilib-rium charge distribution (Hinds, 1999), the number of particlesfrom 14.2 to 736.5 nm was taken as measured by the SMPS andused to compute the scattering for each bin of the size distributionfor scattering from 7� to 170�. The final value of ssp was obtained asa sum over themeasured distribution. This gave smaller errors thanusing log normal distributions for the polydisperse samples. In thecase of the monodisperse NaCl, there was a single exception to thispractice: the distribution of the “monodisperse” NaCl also includedsmaller peaks corresponding to particles with multiple charges;a small distinct peak at 269 nm, corresponding to triply chargedparticles, was fit with a Gaussian function because of the weaksignal and potential for overlap with very small signals fromparticles with four or more charges. Distributions of particles withone or two charges were well-defined and used as measured.

The scattering coefficients were then calculated usinga composite distribution consisting of the measured distributionfor the smaller diameter particles combined with the Gaussian fitfor the 269 nm peak. In the case where monodisperse NaCl aerosolwas coated with SOA, the coated particles were treated as a shell of

a

b

Fig. 2. Measured and Mie calculated light scattering coefficients of (a) polystyrenelatex spheres and (b) monodisperse DOP particles in three experiments. Also shown(inset) is a typical particle number distribution for DOP particles with a geometricmean diameter of 237 nm and geometric standard deviation of 1.29.

Y. Li et al. / Atmospheric Environment 45 (2011) 4123e41324126

organic around a core of NaCl rather than an internal mixture(Vaden et al., 2010). This should be a good assumption since theNaCl particles were dry, but as an additional check, Mie calculationsfor hypothetically internally mixed NaCl and SOA were performedand compared with calculated results of external mixing. In theinternally mixed case, the volume weighted refractive index wascalculated (Lang-Yona et al., 2010) as NaCl particles with a set initialdiameter of 202 nm grew by accretion of organic material to300 nm. The resulting scattering coefficients for the internallymixed particles were found to be within 6% of the analogouslycalculated externally mixed particles at all three wavelengths.

1.6x10-3

1.2

0.8

0.4

0.0

sp (

m

-1

)

450 550

Wave

Fig. 3. Measured and Mie calculated light scattering coefficients of polydisperse NaCl partdiameter of 114 nm and a geometric standard deviation of 1.66.

3. Results and discussion

3.1. Test of Mie algorithm and particle size measurements usingPSL and DOP particles

Fig. 2 compares the measured scattering coefficients at 450, 550and 700 nmwith those calculated using Mie theory for the 300 nmPSL spheres (Fig. 2a). The calculated values of ssp are in excellentagreement with the measurements, within 8% or better at allwavelengths. Fig. 2b shows the measured and calculated scatteringcoefficients for three different experiments with DOP particles. Asseen for the typical size distribution shown in the inset, the numberdistribution is relatively narrow, with a geometric standard devi-ation of 1.29. Again, the Mie scattering calculations agree to within10% of the measured values of the scattering coefficients. Thisagreement for both a monodisperse sample and for a widerdistribution of particles of known composition and refractive indexvalidates both the Mie scattering routine and the measurement ofthe particle numbers and size distributions.

3.2. Sodium chloride particles

3.2.1. Polydisperse NaClFig. 3 shows the measured and calculated scattering coefficients

for a polydisperse distribution of NaCl particles (averaged over fiveconsecutive measurements of the size distribution) as shown in theinset. Dry NaCl was used in order to isolate the effects of organiccoatings on particle sizes and refractive indices, independent of thepotential impact of organics on water uptake (Fierz-Schmidhauseret al., 2010; Kaku et al., 2006; Tang, 1996). These studies are alsorelevant to aerosols generated by dry salt lakes (Stutz et al., 2002;Levin et al., 2005), as well as to cases where deliquesced particlesare dried during sampling (Chamaillard et al., 2003, 2006). There isclearly a large discrepancy between the measurements and calcula-tions, between 81 and 121% over the three wavelengths. This ispuzzling, given the excellent agreement for PSL and DOP, since onlya small (<5%) effect is expected due to the non-sphericity of NaClparticles of this size (Chamaillard et al., 2003). In order to probe this,further experimentswere carried out onmonodisperseNaCl samples.

3.2.2. Monodisperse NaClFig. 4a shows the size distribution of particles measured by the

second SMPS after isolation of particles with 131 nm electrical

2.0x103

1.5

1.0

0.5

0.0

Con

cent

ratio

n (#

cm

-3)

600400200Particle diameter (nm)

700

length (nm)

icles. The inset shows typical number distribution of particles with a geometric mean

Y. Li et al. / Atmospheric Environment 45 (2011) 4123e4132 4127

mobility diameter obtained by holding the upstreamDMA at a fixedvoltage. As expected, when one size is selected in the first DMA,larger particles with multiple charges but the same electricalmobility are also co-selected. These multiply charged particles withthe same electrical mobility exit the first DMA and pass througha 85Kr neutralizer ahead of the second DMA, which converts thedistribution of charges to an equilibrium Boltzmann distribution(Hinds, 1999). In this case, the peaks at 202 and 269 nm areparticles that had, on exiting the first DMA, þ2 and þ3 charges,respectively. While there were no distinct peaks with largernumbers of charges, very small contributions from such highlycharged particles cannot be ruled out. The small peaks observed at69 and 88 nm correspond to 131 nm particles that now bear,respectively, þ3 and þ2 charges. Data reduction software correctsfor multiple-charged particles based on the assumption thatparticles acquire an equilibrium Boltzmann charge distribution onpassage through the 85Kr neutralizers. However, the presence of thesmall peaks at 69 and 88 nm represent insufficient accounting ofthese multiply charged particles.

All positively charged particles that pass through the 85Krneutralizer ahead of the second DMA, should receive an equilib-rium Boltzmann distribution (Hinds, 1999), which should then bereflected in the relative intensities of the 131 (þ1), 202 (þ2) and269 (þ3) peaks. Using the measured particle size distribution andthe assumption of an equilibrium Boltzmann charge distribution,then the percentage of charged particles represented by þ1, þ2and þ3 should be 63, 28 and 9.1%, respectively (Hinds, 1999). The

2.5x103

2.0

1.5

1.

a

b

0

0.5

0.0

Co

nc

en

tra

tio

n (#

cm

-3

)

200100

Particle dia

69 88

131

202(202,

2500

2000

1500

1000

500

0

C

on

ce

ntra

tio

n (#

cm

-3

)

200100

Particle diam

15%

40Concentration700550450

Fig. 4. (a) Measured number concentration (solid line) for monodisperse NaCl aerosol size s269 nm peak. Numbers above peaks indicate measured mode diameters; values in parenthessingly charged 131 nm particles. (b) Number concentration and calculated scattering coefpercentage contribution of each peak to the total ssp for 450 nm. The contributions at 550

measured values (after charge correction) in averaged runs such asthat shown in Fig. 4a were actually 73 � 6, 22 � 3 and 5.6 � 2.8%(2 s).

Clearly, these values, particularly for the 269 nm (þ3) particles,are not in good agreement with the Boltzmann distribution. Thelikely reason for this is that the particles do not actually come toa complete Boltzmann equilibrium distribution on passing throughthe 85Kr neutralizers. This has been reported in a few other cases aswell, particularly at higher flow rates through the neutralizer(Covert et al., 1997; Ji et al., 2004; Kim et al., 2005). For example,results from Kim et al. (Kim et al., 2005) indicate that for 200 nmNaCl particles, the measured fraction of þ2 particles was about 20%smaller than the fraction for a Boltzmann equilibrium distributionof charges. Since the SMPS charge correcting algorithm assumes anequilibrium charge distribution, this would result in overestimateof multiply charged particle number concentration and hence, anoverestimate of the calculated scattering coefficients.

In order to take into account this unexpectedly large contribu-tion to light scattering from the 269 nm particles and to separateout their contribution from the 202 and 131 nm particles (as well asfrom any larger particles that were not accurately measured butcould still contribute to light scattering), the 269 nm peak in Fig. 4awas fit with a Gaussian. The individual contributions to the lightscattering at the three wavelengths were then calculated usinga composite distribution consisting of the measured population forthe 131 and 202 nm peaks and the Gaussian fit for the 269 nm peak.As seen in Fig. 4b, the 269 nm particles contributed 46% of the total

400300

meter (nm)

MeasuredGaussian fit to 269 nm peak

+2)

269(265,+3)

400300

eter (nm)

20x10-6

15

10

5

0

sp (

m

-1

)

% 46%

elected by DMA at 131 nm mobility diameter. The dashed line is the Gaussian fit to thees are calculated diameters and charge that would have the same electrical mobility asficients using a composite distribution (see text). Numbers above each peak are theand 700 nm were similar.

2.5x103

2.0

1.5

1.0

0.5

0.0

Co

nc

en

tra

tio

n (#

c

m-3

)

35030025020015010050

Particle diameter (nm)

NaCl seed

Organic coated seed

Fig. 6. Size distributions of sodium chloride particles with and without an organiccoating from a-pinene ozonolysis.

Y. Li et al. / Atmospheric Environment 45 (2011) 4123e41324128

light scattering at 450 nm, while the 202 and 131 nm particlescontributed 40% and 15% respectively. Thus, despite the very smallnumber of 269 nm particles, they dominate the light scattering. Asimilar observation has been reported for ammonium sulfate andsodium chloride particles at relative humidity between 40 and 90%(Fierz-Schmidhauser et al., 2010).

Fig. 5a, using the composite distribution, shows that the Miescattering calculations for the three wavelengths are now in goodagreement with the measurements. The discrepancy varies from�3% at 450 nm to �26% at 700 nm, much better agreement thanwas the case for polydisperse NaCl. That the calculated scattering isless than the measured values may be due to a small contributionfrom larger particles with þ4 and greater charges that are co-selected with the 131 nm particles. Given that incomplete chargeneutralization occurs for the monodisperse case, the same is likelyto be true for the polydisperse sample. Thus, the significant over-prediction of the light scattering coefficients for the polydispersecase (Fig. 3) likely arises from overprediction of the particle numberconcentration from the assumption of acquiring an equilibriumBolztmann charge distributions during passage through the 85Krneutralizers.

3.2.3. Organic-coated NaCl particlesFig. 6 shows that as expected, the particle size distribution shifts

to larger diameters when the NaCl particles are coated withorganics. Within the resolution of the instrument, the shift in thepeak of the distribution appeared to be 5.0 nm for both the 131 and202 nm peaks (the small signal and its breadth for the peak at 269

4x10-4

a

b

2

3

1

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sp (m

-1

)

450 550

Wav

Wav

Measured Calculated

5x10-4

4

3

2

1

0

sp (m

-1

)

450 550

Fig. 5. Measured and Mie calculated light scattering coefficients of monodisperse sodiumonodisperse NaCl aerosol, and (b) NaCl coated with SOA from a-pinene ozonolysis.

nm made the shift difficult to assess.), giving a coating thickness of2.5 nm. Experiments by Zelenyuk et al. (Zelenyuk et al., 2008) showthat smaller particles of NaCl which become coated with DOP fromthe vapor phase acquire thicker organic coatings compared to thelarger particles. However, since we are not able to resolve differ-ences in coating thickness experimentally in the present experi-ments, we treat all particles as if they acquire the 2.5 nm thickorganic coating. Using the approach of Zelenyuk et al. (Zelenyuket al., 2008) to estimate the different coating thicknesses basedon the shift in the peak for 131 nm particles would actually givea coating thickness of 1.9 nm for the 202 nm particles and 1.5 nm

700

elength (nm)

elength (nm)

700

m chloride particles after Gaussian fitting the þ3 charged particle distribution. (a)

1. 0

0. 9

0. 8

0. 7

0. 6

sp

(c

oa

te

d) /

sp

(p

ure

N

aC

l )

1. 0 0. 8 0. 6 0. 4 0. 2 0. 0

D(co re )/D(total)

100

80

60

40

20

0

Org

an

i c co

at in

g vo

l u

me

%

Fig. 7. Calculated effect of organic coating thickness on light scattering coefficients for particles with constant diameter, D(total) ¼ 202 nm and varying diameters of the NaCl core.D(core)/D(total ¼ 1) corresponds to pure 202 nm NaCl particles with 0% volume due to organic coating.

Y. Li et al. / Atmospheric Environment 45 (2011) 4123e4132 4129

for the 269 nm particles; however, such relatively small differencesin organic coating thickness on the much larger NaCl core affect theMie scattering calculation results by less than w5%. Note thatZelenyuk and coworkers (Vaden et al., 2010) have shown thataspherical NaCl particles exposed to the products of ozonolysis of a-pinene become spherical, suggesting that the low volatility prod-ucts form a uniform coating around the NaCl core.

Using the refractive indices in Table 1 for NaCl and SOA, the lightscattering coefficients at 450, 550 and 700 nm were calculated.Fig. 5b compares these to the measured scattering coefficients atthe three wavelengths. The agreement between the measured andcalculated values varies between 7 and 38%, with the calculatedvalues being consistently smaller than themeasurements. This maybe due to a small contribution from larger particles with þ4 ormore charges. However, given the complexity of the system, thisagreement between theory and experiment is quite satisfactory.

The thickness of the coatings acquired in these experiments waslimited by several factors. First, the reaction time in the continuousflow reactor was limited to w0.3 min. Second, higher gas phaseconcentrations that would increase the rate of formation of the lowvolatility products and the coating thickness could not be usedbecause they lead to significant concentrations of pure secondaryorganic aerosol from homogeneous nucleation. Pure SOA havedifferent light scattering coefficients than NaCl alone or with a thinorganic coating, which could not be taken into account in thecalculations since the pure SOA could not be distinguished exper-imentally from coated NaCl particles if they were the same size.

Although the coating thicknesses could not be varied overawide range in the experiments, the agreement betweenmeasuredand calculated scattering coefficients for the systems describedabove allows one to extend the effects of coating thickness on lightscattering to thicker coatings using the Mie calculations. Fig. 7shows the ratio of the calculated scattering coefficients for thecoated particles to that for pure NaCl for varying core sizes andcoating thicknesses but assuming a total, fixed particle diameter of202 nm. This removes the effect of particle size so that changes aresolely due to the different contributions of NaCl and organicmaterial. The ratio of scattering coefficients decreases as thecoating becomes thicker due to the smaller refractive index for theorganic shell compared to pure NaCl. For example, when the corediameter is 162 nm and the coating thickness is 20 nm, the lightscattering coefficients are w75e90% of those for pure NaCl,depending on the wavelength. In addition to this direct effect oforganics on light scattering, they may also reduce the uptake ofwater and hence hygroscopic growth relative to uncoated saltparticles (Randles et al., 2004), which will also reduce the light

scattering from that expected in the absence of organics. This isconsistent with the field studies of McInnes et al. (McInnes et al.,1998) which showed that light scattering by marine particles atSable Island at a given RHwas smaller for those particles associatedwith anthropogenic influences that contained a higher organiccontent.

As discussed by Hinds (Hinds, 1999) the visual range Lv forviewing a dark object against the horizon during the day is given byLv ¼ 3.9/se, where se is the total extinction coefficient that includesboth light absorption and scattering by gases and particles. In thecommon situation where light scattering by particles dominates,the visual range will be inversely proportional to ssp. A reduction inssp due to substitution of some of the particle volume by organicmaterial will therefore increase the visual range over that for pureNaCl. For the case in which sea salt particles of certain diametersacquire organic coatings in air, not only their refractive index butalso their sizes will change. Fig. 8 (solid lines) shows the scatteringcoefficients for the situation where 202 nm particles acquireorganic coatings of increasing thickness up to 50 nm (corre-sponding to 70% by volume of organic material). The scatteringcoefficients increase because the effect of increased particle sizeoutweighs that due to the smaller refractive index of the organiccoating. Taking the data at 550 nm, the scattering coefficientincreases by nearly a factor of five when the 202 nm NaCl particleacquires an organic coating of thickness 50 nm, and the visual rangewill decrease by the same factor.

For comparison, Fig. 9 (solid lines) also shows the scatteringcoefficients if the particles grew in size by adding NaCl instead ofthe organic coating (dashed lines). As expected from the previousdiscussion, the scattering coefficients for pure NaCl particles of thesame size as the coated particles are larger compared to the casewhere they have the organic coating. For an organic coatingthickness of 50 nm on a 202 nm diameter NaCl core, the scatteringcoefficient at 550 nm is 76% of that of a pure NaCl particles of thesame size, which will increase the visual range by 32%. In addition,the decreased light scattering will lead to less counterbalancing ofthe warming effect in the troposphere due to greenhouse gases.

Aerosol mass scattering efficiencies are often used in assessingthe contribution of different components of particles to light scat-tering. The mass scattering efficiency for a selected aerosolcomponent (ai) is defined as ai ¼ ssp,i/Mi, where Mi is the mass ofcomponent i (Hinds, 1999). Based on a review of the literature,Hand and Malm (Hand and Malm, 2007) recommended values ofthe aerosol mass scattering efficiency at 0% RH for fine mode seasalt of 4.5� 0.9m2 g�1 and for fine organics of 3.9�1.5m2 g�1, withmany but not all of the measured values being made at 550 nm and

1.0x10

-4

0.8

0.6

0.4

0.2

sp (

m

-1

)

300280260240220200

Particle diameter (nm)

80

60

40

20

0

O

rg

an

ic

c

oa

tin

g v

olu

me

%

Fig. 8. Calculated scattering coefficients of 202 nm NaCl particles acquiring increasing amounts of organic coating. The dashed line is the percent by volume of the organic on theparticles.

Y. Li et al. / Atmospheric Environment 45 (2011) 4123e41324130

the fine mode being typically defined as particles with diametersless than either 1 or 2.5 mm. These averages represent a relativelybroad range of reported values and are particularly sensitive to theparticle size distribution. For example, Quinn et al. (Quinn et al.,

1.0x10

-4

0.8

0.6

0.4

0.2

sp (m

-1

)

240220200

= 450 nm

7x10

-5

6

5

4

3

2

1

sp (m

-1

)

240220200

= 550 nm

3.0x10

-5

2.5

2.0

1.5

1.0

0.5

sp (m

-1

)

240220200

Particle

= 700 nm

Fig. 9. Calculated scattering coefficients of 202 nm core particles acquiring increasing amoscattered light, 450, 550 and 700 nm.

2002) reported values ranging from 1.8 to 5.1 m2 g�1 for sub-micron sea salt particles at Barrow, Alaska. Our averaged data fora polydisperse NaCl sample with a geometric number meandiameter of 115 nm and a geometric standard deviation of 1.7 gave

300280260

300280260

300280260

diameter (nm)

unts of NaCl (solid lines) or organic coating (dashed lines) at the three wavelengths of

Y. Li et al. / Atmospheric Environment 45 (2011) 4123e4132 4131

a value of aNaCl ¼ 1.1 m2 g�1 at 550 nm. Given that the particle sizedistribution in our studies was smaller than typically found in air,the agreement with the published values is reasonable.

4. Conclusions

These studies demonstrate the importance of directly andexperimentally determining the number of particles with differentsizes and charges, but the same electrical mobility diameter.Despite the use of 85Kr neutralizers, the assumption that theBoltzmann charge equilibrium distribution applies may not, in fact,be valid. While this error may not make a difference in themeasurement of many particle properties, it is highly significant forsome such as light scattering measured here especially at largerparticle sizes where there are more multiply charged particles.

Our combination of experiments and theory show that as thegeometric mean diameter of salt particles increases due to uptakeof thin organic coatings, the effect on light scattering may bereasonably approximated from the change in size distributionunder conditions where the organic coating is small relative to thecore size. However, it is well known that particle composition alsoaffects hygroscopic growth, which determines particle diameter asa function of atmospheric aging and relative humidity. For example,particles containing hygroscopic salts and/or highly oxidizedorganics can grow significantly as a function of relative humidity. Inaddition, atmospheric aging of particles can lead to formation ofabsorbing materials, which can be important in urban areas(Finlayson-Pitts and Pitts, 2000). Thus, while changes in sizedistribution can be used to estimate effects on light scattering,some understanding of particle composition is needed to accu-rately predict the actual growth of the particles.

In addition, for a fixed particle diameter, light scatteringdecreases as the relative contribution of the organic componentincreases. As a result, light scattering by salt particles with a specificsize distribution will be reduced when organics comprise a signifi-cant portion of the particles. This will lessen their impact on visi-bility reduction compared to pure sea salt particles, but also lead toless counterbalancing of the tropospheric warming due to green-house gases.

Acknowledgments

We are grateful to the U.S. Department of Energy (Grant # DE-FG02-05ER64000) for support of this work and toWilliam.C. Hinds,Alla Zelenyuk, Maynard Havlicek, and Yong Yu for helpful discus-sions. We thank David Covert and Ted Anderson for discussions onthe integrating nephelometer. We are also very grateful to SergeyNizkorodov for the use of a second SMPS system.

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