Surfactants in cloud droplet activation: mixed organic-inorganic particles

Transcription

1 Atmos. Chem. Phys., 0, , doi:94/ap Author(s) 200. CC Attribution 3.0 Liense. Atmospheri Chemistry and Physis Surfatants in loud droplet ativation: mixed organi-inorgani partiles N. L. Prisle,4, T. Raatikainen 2, A. Laaksonen 2,3, and M. Bilde 4 University of Helsinki, Department of Physis, P.O. Box 48, 0004, University of Helsinki, Finland 2 Finnish Meteorologial Institute, Erik Palmenin Aukio, 000, Helsinki, Finland 3 University of Kuopio, Department of Physis, P.O. Box 627, 702, Kuopio, Finland 4 University of Copenhagen, Department of Chemistry, Universitetsparken 5, 200, Copenhagen, Denmark Reeived: 3 Otober 2009 Published in Atmos. Chem. Phys. Disuss.: 8 November 2009 Revised: 27 May 200 Aepted: 4 June 200 Published: 29 June 200 Abstrat. Organi ompounds with surfatant properties are ommonly found in atmospheri aerosol partiles. Surfae ativity an signifiantly influene the loud droplet forming ability of these partiles. We have studied the loud droplet formation by two-omponent partiles omprising one of the organi surfatants sodium otanoate, sodium deanoate, sodium dodeanoate, and sodium dodeyl sulfate, mixed with sodium hloride. Critial supersaturations were measured with a stati diffusion loud ondensation nuleus ounter (Wyoming CCNC-00B). Results were modeled from Köhler theory applying three different representations of surfatant properties in terms of surfatant surfae partitioning and redued droplet surfae tension. We here onfirm previous results for single-omponent organi surfatant partiles, that experimental ritial supersaturations are greatly underpredited, if redued surfae tension is used while ignoring the effets of surfae partitioning in droplets. Furthermore, disregarding surfatant properties by ignoring surfae partitioning and assuming the onstant surfae tension of pure water an also lead to signifiant underpreditions of experimental ritial supersaturations. For the mixed partiles omprising less than 50% by mass of surfatant, this approah however still provides a good desription of the observed droplet ativation. A omprehensive aount for surfatant properties, inluding both surfae tension redution and effets of surfae partitioning in ativating droplets, generally predits experimental ritial supersaturations well. Correspondene to: N. L. Prisle (nonne.prisle@helsinki.fi) Introdution The influene of atmospheri aerosol partiles on loud formation and properties onstitutes the single largest unertainty in assessing anthropogeni limate foring (IPCC, 2007). Cloud droplets form when water vapor ondenses onto partile surfaes. In the proess, partile onstituents may dissolve into the aqueous phase and form solution droplets. The ability of partiles to at as loud ondensation nulei (CCN) therefore depends on hemial omposition, as well as size. Surfae ative moleules (surfatants) onentrate in the surfae and an redue the surfae tension of an aqueous solution. Redued surfae tension, ompared to that of pure water, has been demonstrated in bulk samples of atmospheri loud and fog water (Fahini et al., 999, 2000) and in aqueous extrats of olleted atmospheri aerosol samples from a wide variety of soures and environments, inluding biomass (Asa-Awuku and Sullivan, 2008) and oal burning (Oros and Simoneit, 2000), and marine (Mohida et al., 2002), rural (Kiss et al., 2005), and polluted environments (Dinar and Taraniuk, 2006). The goal of this work is to advane the fundamental understanding of the role of surfatants in loud mirophysis, whih is essential for onsistent representations of aerosol effets in atmospheri models. Fatty aids and their salts onstitute an important lass of atmospheri surfatants and have been identified in aerosol samples from both marine (Mohida et al., 2003), urban (Yassaa et al., 200), and ontinental (Cheng et al., 2004) environments. We have previously addressed the loud droplet formation of single-omponent partiles omprising a series of saturated fatty aid sodium salts and demonstrated Published by Copernius Publiations on behalf of the European Geosienes Union.

2 5664 N. L. Prisle et al.: Mixed surfatant-salt CCN the importane of a omprehensive aount for surfatant properties in thermodynami model preditions (Prisle et al., 2008). Atmospheri partiles are however generally mixtures of both organi and inorgani speies (Murphy et al., 2006). In this work, we therefore proeed by investigating two-omponent partiles omprising organi surfatants mixed with sodium hloride (NaCl). These partiles an be regarded as simple model systems for marine aerosols (O Dowd et al., 2004). We ompare experimental observations of loud droplet formation by mixed surfatant-nacl partiles to thermodynami model preditions using different representations of surfatant properties in ativating droplets. Speifially, we have studied the sodium salts of n-otanoi (apryli), n- deanoi (apri) and n-dodeanoi (lauri) aid. In addition, we inluded sodium dodeyl sulfate (SDS), whih is a strong industrial surfatant with well-quantified properties. SDS is not found in the atmosphere but has previously been targeted as a model ompound for water-soluble atmospheri surfatants in loud droplet formation studies of both singleomponent partiles and mixed partiles with NaCl (Li et al., 998; Rood and Williams, 200; Sorjamaa et al., 2004). The fatty aid salts (FAS) and SDS are amphiphiles. Eah moleular struture is haraterized by a polar head-group (the COO or OSO 3 funtional groups) and a non-polar hydroarbon tail (the unbranhed (CH 2 ) n CH 3, n= 6,8,0, hain). To redue destabilizing interations with the polar water moleules in the aqueous phase, the surfatant moleules preferentially aumulate at the air-water interfae, with the hydrophili ends dissolved in the aqueous solution and the hydrophobi tails pointing outwards to the air. This is the origin of their surfae ativity. 2 Experimental We have measured the ritial supersaturation as a funtion of dry partile diameter for two-omponent laboratorygenerated partiles omprising organi surfatant (SFT) and sodium hloride (NaCl) in different relative amounts. Partile ompositions inluded one of the surfatants sodium otanoate (CH 3 (CH 2 ) 6 COONa; C8Na), sodium deanoate (CH 3 (CH 2 ) 8 COONa; C0Na), sodium dodeanoate (CH 3 (CH 2 ) 0 COONa; C2Na), and sodium dodeyl sulfate (CH 3 (CH 2 ) 0 OSO 3 Na; SDS), mixed with NaCl. Chemials were obtained at the highest purities available from ommerial soures: sodium otanoate (Sigma, apillary GC, minimum 99%), sodium deanoate (Fluka, purum. 98%), sodium dodeanoate (Sigma, Sigma Grade 99 00%), sodium dodeyl sulfate (Sigma, >99%), and sodium hloride (RiedeldeHaën, >99.8%). Before use, all hemials were baked overnight at moderate temperatures (<80 C) to evaporate any volatile impurities. Table. Weighed surfatant solute mass-frations (W p,sft ) in the atomizer solutions. SFT 20% 50% 80% 95% C C C SDS CCN measurements Critial supersaturations were measured with a stati thermal-gradient diffusion-type loud ondensation nuleus ounter (Wyoming CCNC-00B) (Snider et al., 2006). The experimental set-up and proedures employed are desribed in detail in previous work (Prisle et al., 2008, and referenes therein). Partiles were generated by atomization of aqueous solutions using a onstant output atomizer (TSI-3076), operated in reirulation mode. The wet aerosol produed was subsequently dried by passing through a set of two siliagel ontaining diffusion-driers in series, followed by dilution with dry air. The relative humidity (RH) in the dry aerosol was typially 5 8%, exept during one measurement (for about.5 h) where the RH inreased to nearly 20%. Atomizer solutions were prepared with de-ionized and purified water (8.2 M m resistivity, produed in a Milli-Q Plus Ultra Pure Water System). Total solute onentrations were gL. Compositions of the dried partiles are assumed to reflet the weighed relative mass frations of organi and inorgani solutes in the atomizer solutions. Studied partile ompositions thus omprise eah FAS in approximate mass frations of 20, 50, 80, and 95%, and SDS in approximate mass frations of 20, 50, and 80%, relative to NaCl. Exat weighed surfatant mass-frations (W p,sft ) are given in Table. From the dried polydisperse aerosol, a narrow eletrialmobility partile size-fration was seleted with an eletrostati lassifier (TSI-3080). The number onentration of ativated partiles (CCN [m ]) at seleted water vapor supersaturations (SS ) was then measured with the CCNC. The total partile number onentration (CN [m ]) was measured in parallel, using a ondensation partile ounter (TSI-300). Partile eletrial-mobility diameters (D p ) were 25 40nm and CCNC supersaturations were %. Purified and dried high-pressure air was used in all experiments. Laboratory temperatures (T ) were kept onstant at K during the ourse of eah experiment by a thermostat air-onditioning system. Atmos. Chem. Phys., 0, , 200

3 N. L. Prisle et al.: Mixed surfatant-salt CCN CCNC alibration and data fitting The CCN experiments give the fration of ativated partiles (CCN/CN) with a seleted mobility diameter as a funtion of the supersaturation that the partiles were exposed to. These data were fitted with a four-parameter sigmoidal funtion, y=y 0 +a/[+exp( (x x 0 )/b)] (Prisle, 2006; Prisle et al., 2008). Inluding orretion for multiply-harged partiles simultaneously seleted in the lassifier, the experimental ritial supersaturation (SS exp ) was then determined from the midpoint of the singly-harged partile ativation step (x 0 ): the CCNC supersaturation was alibrated with a linear relation, obtained from measurements using monodisperse ammonium sulfate ((NH 4 ) 2 SO 4 ) and NaCl partiles (Bilde and Svenningsson, 2004), suh that SS exp = x Error bars given for SS exp depit ±2τ (approximate 95%-onfidene intervals), where τ is the ombined standard deviation from the sigmoidal fit to ativated frations and the linear supersaturation alibration (Prisle, 2006). 3 Theory 3. Köhler theory Cloud droplet formation is desribed in Köhler theory (Köhler, 936) by the equilibrium growth and ativation of an aqueous solution droplet. The Köhler equation relates the equilibrium water vapor saturation ratio (S) over a spherial solution droplet to its diameter (d): S p ( ) w 4νw σ pw 0 = a w exp RT d where p w is the equilibrium partial pressure of water over the solution droplet, pw 0 is the saturation vapor pressure over a flat surfae of pure water, a w is the droplet solution water ativity, ν w is the partial molar volume of water in the solution, σ is the droplet surfae tension, R is the universal gas onstant, and T is the Kelvin temperature. The water ativity (a w ) in Eq. () is equivalently alled the Raoult term and desribes how the equilibrium partial pressure of water over an aqueous solution is suppressed from the pure water saturation vapor pressure by dissolved solutes aording to (the extended) Raoult s Law, p w = a w pw 0. The exponential, or Kelvin, term of Eq. () aounts for the vapor pressure enhanement over a urved droplet surfae by the Kelvin effet. A plot of water saturation ratio versus droplet diameter (S d) gives the Köhler urve for a growing droplet and its maximum defines the ritial saturation ratio (S ) and orresponding ritial droplet diameter (d ). Droplets that have been exposed to ambient water saturation ratios larger than their respetive threshold values (S S ), and thus surpassed their ritial diameters (d d ), are assumed to be () ativated loud droplets that will ontinue to grow into fullsized loud drops, only limited by the transport of water vapor to the droplet surfae. Below, we equivalently refer to the water vapor supersaturation, SS/[00%] S, and analogously to the droplet ritial supersaturations (SS ). 3.2 Surfae partitioning Surfatant moleules preferentially aumulate at the airwater interfae of aqueous solutions. This generates a onentration gradient between the solution bulk (supersript B in the following) and surfae (supersript S) phases. The relative distribution of surfatant moleules between these distint solution phases is here referred to as the surfatant (bulk-)surfae partitioning: it an be thought of as governed by some equilibrium onstant, K Ɣ/ B, where Ɣ (the surfae exess) is the number onentration of surfatant moleules per unit area in the surfae, and B is the onentration per unit volume in the solution bulkphase. Surfae partitioning thus depends on the intrinsi propensity of surfatant moleules for the surfae (from the value of K), as well as the relative dimensions of the solution bulk and surfae phases, due to the impliit dependeny of Ɣ/ B on surfae area (A) and bulk volume (V ). For spherial liquid droplets with diameters of d=0.,, and 0 µm, the surfae-area-to-bulk-volume ratio (A/V = 6/d) is 60, 6, and 0.6 µm, respetively, and A/V 0 as d for bulk solutions with flat surfaes. The small solution droplets involved in loud droplet formation typially have sub-mirometer diameters, whereas the dimensions of bulk solutions haraterized by onventional laboratory methods or olleted in field samples are muh larger. We here denote bulk aqueous solutions as marosopi, and ativating droplets as mirosopi. The term bulk is generally used to designate the isotropi bulk phase within an aqueous solution and distinguish it from the anisotropi surfae phase. A similar pratie was used in our previous work (Prisle et al., 2008) and is also employed by Ruehl et al. (200). Due to surfae partitioning, A/V beomes a key fator in determining surfatant solution properties. Equilibrium water ativity (a w ) and surfae tension (σ ) are expressed as funtions of solution bulk-phase omposition ( B ). However, surfae partitioning depletes the bulk phase of surfatant moleules and thereby dereases B. Given the same surfatant onentration per unit of total (supersript T ) solution volume ( T ), the larger A/V of mirosopi droplets auses a larger fration of the total number (n T ) of surfatant moleules in solution to partition to the surfae (n S ) and a smaller fration to remain in the bulk phase (n B ), ompared to marosopi solutions. The hange in bulk-phase omposition from the total surfatant onentration due to surfae partitioning is negligible for marosopi solutions ( B T ). In mirosopi droplets, bulk-phase depletion of surfatant moleules an on the other hand be signifiant Atmos. Chem. Phys., 0, , 200

4 5666 N. L. Prisle et al.: Mixed surfatant-salt CCN ( B T ). The importane of surfae partitioning for hanging mirosopi solution properties, ompared to marosopi solutions of the same overall omposition, has previously been noted by Seidl and Hanel (983), Biano and Marmur (992), and Laaksonen (993). Surfatant surfae partitioning in mirosopi droplets potentially affets loud droplet ativation through both Raoult and Kelvin effets. A smaller surfatant bulk-phase onentration will be refleted in a larger water ativity (a diminished water partial pressure suppression by the Raoult effet) and a larger surfae tension (a diminished surfae tension redution). At a given droplet size, the latter thus leads to an amplified urvature enhanement of water vapor pressure by the Kelvin effet. A simple illustration of the effets of surfae partitioning on surfatant bulk-phase onentration and surfae tension in droplets of different sizes is presented in Appendix A. 4 Thermodynami model The details of our thermodynami model have been desribed in previous work (Sorjamaa et al., 2004; Prisle et al., 2008, and referenes therein). Some modifiation have been made, ompared to the original version; for ompleteness, the model struture, equations and assumptions are therefore reviewed here. 4. Köhler alulations Model alulations determine ritial supersaturations (SS ) for growing droplets formed on dry partiles of known omposition and diameter, by iterating the respetive Köhler urve maxima from Eq. (). Dry partiles are assumed to be spherial with (volumeequivalent) diameters (Dp e = D p) orresponding to the eletrial-mobility diameter mode seleted by the DMA during experiments. Partile volume is thus obtained as V p = π 6 Dp 3, without any mobility shape-fator orretion. Partile ompositions are given by the relative mass frations (W p ) of moleular surfatant (SFT) and NaCl, where W p,sft + W p,nacl =. As mentioned in Set. 2., these are assumed to reflet the weighed solute mass-frations in the atomizer solutions (see Table ). The total molar amounts (n T i ) of surfatant and NaCl solute (i = SFT,NaCl) available in the ativating droplets are then alulated, assuming volume additivity of the dry partile omponents, from n T i = W p,i M i ( π Wp,SFT 6 D3 p + W ) p,nacl (2) ρ SFT ρ NaCl Unit mass density is assumed for eah of the fatty aid salts (ρ FAS = gm 3 ), owing to lak of bulk-density information for the pure solid FAS. For growing droplets, the total molar amount of water (n T w ) is alulated as a funtion of diameter (d), assuming volume additivity of water and dry partile omponents within the aqueous droplets, as n T w = ρ w M w π 6 (d3 D 3 p ) (3) All partile omponents are assumed to be ompletely dissolved in the droplet solution, equivalent to assuming full water solubility of both surfatant and NaCl solutes. Chemial reations within the droplet solutions and physial reations, suh as ondensation or evaporation of droplet and gas-phase omponents, are assumed to be absent. For eah droplet size and orresponding total omposition (n T i, nt w ), the droplet bulk-phase molar omposition (n B i, nb w ) is subsequently determined, either without onsideration of surfae partitioning, as simply equated to the total droplet omposition (n B i = n T i, nb w = nt w ), or from iterating the partitioning equilibrium, as desribed in the following Set Water ativity and surfae tension are evaluated as funtions of the droplet solution bulk-phase omposition aording to relations for a marosopi solution, suh that a w = aw B(nB i,nb w ) and σ = σ B (n B i,nb w ). Droplet solutions are assumed to be ideal and bulk-phase onentrations are therefore used instead of the appropriate ativities for all droplet omponents. In partiular, the droplet bulk-phase water ativity is set equal to the orresponding water mole-fration onentration (aw B = xb w ). Both surfatant and NaCl solutes are assumed to be fully dissoiated in the droplet solution, with dissoiation fators δ SFT = δ NaCl = 2. Furthermore, the partial molar volume of water (ν w ) is approximated with the molar volume of pure water, given by the water molar mass and mass density as νw 0 = M w/ρ w, aording to the assumption of volume additivity for all droplet omponents. Droplet surfae tension is either evaluated from a onentration-dependent ternary SFT-NaCl aqueous solution surfae tension parametrization, σ = σ (SFT B,B NaCl ), as desribed in Set. 4.3 below, or set equal to the onstant value for pure water (σ = ). Then, the equilibrium water saturation ratio (S) for the given droplet size (d) is finally alulated from Eq. (). Köhler model preditions are made with three different representations of surfatant properties:. (denoted σ,p ) expliitly aounting for bulk-surfae partitioning in the droplets upon evaluating both Raoult and Kelvin terms in Eq. () and using a onentrationdependent droplet surfae tension, 2. (denoted σ,b ) negleting the effets of surfae partitioning on droplet bulk-phase omposition, but still using a onentration-dependent droplet surfae tension, that is, evaluating both Raoult and Kelvin terms diretly orresponding to a marosopi (bulk aqueous) solution with the same total omposition, and 3. (denoted ) ompletely disregarding surfatant properties of the organi and treating it as a ommon Atmos. Chem. Phys., 0, , 200

5 N. L. Prisle et al.: Mixed surfatant-salt CCN 5667 Table 2. Physial properties of the studied ompounds used in model alulations: molar mass M [gmol ], dissoiation fator δ, and bulk mass-density ρ [gm 3 ]. In all ases we have assumed δ = 2, and for the fatty aid salts we assumed ρ FAS = gm 3. moleular formula M [gmol ] δ ρ [gm 3 ] CH 3 (CH 2 ) 6 COONa CH 3 (CH 2 ) 8 COONa CH 3 (CH 2 ) 0 COONa CH 3 (CH 2 ) 0 OSO 3 Na NaCl solute, by negleting surfae partitioning and using the onstant surfae tension of pure water ( ) throughout droplet growth and ativation. Calulations involved in the partitioning representation are desribed in more detail in the following Set. 4.2, and the onentration-dependent surfae tension parameterizations are given in Set Model assumptions are summarized in Set Physial properties of the surfatants and NaCl used in the alulations are given in Table Partitioning model To determine the bulk-surfae partitioning equilibrium, the surfae phase is defined as an infinitely thin region separating the droplet bulk-phase from the gas phase (Gibbs et al., 928). The position of the surfae is seleted to yield the bulk-phase volume (V B ) equal to the total (equimolar) droplet volume (V T ) of all droplet omponents j V B = j n B j v j = j n T j v j = V T = V (4) where v j is the partial molar volume of j. This ensures onsisteny with the assumption that the droplet surfae tension is independent of droplet size (Sorjamaa et al., 2004). Bulk-phase onentrations (j B ) of all droplet omponents are evaluated by solving the the Gibbs adsorption equation (Gibbs et al., 928) n T j RT d ln(aj B )+Adσ = 0 (5) where A[m 2 ] is the spherial droplet surfae area, σ = σ B [Nm ] is droplet surfae tension, given as funtion of bulk-phase omposition, and n T j [mol] is the total molar amount and aj B( γ j BB j ) is the bulk-phase ativity (with orresponding ativity oeffiient γj B ) of j. Mole-fration based ativities are used for all droplet omponents and, equivalent to the Köhler alulations desribed in Set. 4. above, bulk-phase ativities are approximated with the orresponding onentrations by assuming γj B = for all speies. Thus, aj B = xb j, where xb j is the bulk-phase mole fration of j. As both surfatant and NaCl solutes are assumed to be fully dissoiated, droplet solutions thus omprise water (w), organi surfatant anion (SFT ), and inorgani sodium (Na + ) and hloride (Cl ) ions. Conentrations are still reported for the respetive formula units of both organi and inorgani salts, sine the droplet bulk and surfae phases must eah be eletrially neutral. The number of independent variables in Eq. (5) is redued by noting that droplet omponent total molar amounts, as evaluated from Eqs. (2) and (3) with the assumed dissoiation fators, are onserved upon partitioning n T j = nb j +ns j (6) Assuming the partial molar volume of eah droplet omponent is the same in both bulk and surfae phases, the partitioning equilibrium also onserves droplet omponent total volumes V T j = V j B +V j S (7) where Vj T = nt j v j and analogously for Vj B and Vj S. Eq. (4) implies V S = j ns j v j = 0; thus, if some speies has enhaned surfae molar number (n S j > 0), the orresponding surfae volume (Vj S ) must be balaned by simultaneous depletion of an equivalent volume of the remaining speies from the surfae. By furthermore assuming that water and NaCl have fixed molar ratios (Sorjamaa et al., 2004) n B w n B NaCl = nt w n T NaCl only a single independent variable, the surfatant anion bulkphase molar amount (n B SFT ), remains. This quantity is found by numerially solving n T j RT d ln(ab j ) dn B SFT (8) +A dσ B dn B SFT = 0 (9) for eah dry partile size (D p ) and omposition (W p,sft ), and for one droplet size (d) at a time, orresponding to known total molar amounts (n T j ) of all droplet omponents. For a given value of n B SFT (= nb SFT ), bulk-phase molar amounts of all other droplet speies (i = SFT) are related to this quantity by V n B SFT v SFT = n B k v k (0) whene n B i = n T i k =SFT = nb i n T i = nb i n T i V n B SFT v SFT V n T SFT v SFT k =SFT n T k v k () (V n T SFT v SFT) (2) (3) Atmos. Chem. Phys., 0, , 200

6 5668 N. L. Prisle et al.: Mixed surfatant-salt CCN Droplet surfae tension (σ B ) and bulk-phase ativities of dissoiated speies (aj B ) are given from the omponent molar amounts, under the assumptions mentioned here and in Set. 4. above. With these quantities, n B SFT is iterated from Eq. (9), until a solution is found. 4.3 Surfae tension parameterizations Model representations σ,p and σ,b employ onentrationdependent equilibrium surfae tensions for the growing droplets (Prisle et al., 2008). Experimental ternary surfatant-nacl bulk aqueous solution surfae tensions were obtained from Prisle et al. (200b) for the fatty aid salts (FAS), and from Rehfeld (967) and Matijevi and Pethia (958) for SDS. Surfae tensions were parameterized as funtions of both surfatant and NaCl bulk-phase onentrations by fitting data with the Szyskowski equation (Szyskowski, 908). Experimental surfae tensions derease with inreasing respetive onentrations of both surfatant and NaCl ( SFT, NaCl ), over the reported ranges. For a given NaCl, the surfatant strength in the ternary aqueous solutions, in terms of the surfae tension redution from the pure water value ( σ ) attained for a given SFT, inreases in the same order as for the orresponding binary surfatant bulk aqueous solutions, as C8Na < C0Na < C2Na < SDS. For the FAS-NaCl solutions, the Szyskowski equation was fitted to experimental surfae tensions in the form: ( ) dσnacl σ = + m NaCl aln(+m FAS /b) (4) dm NaCl where m NaCl and m FAS are the NaCl and FAS molal onentrations (moles of solute speies per kilogram of water) and = 72.2mNm is the pure water surfae tension at K. The surfae tension gradient for aqueous NaCl, ( dσnacl dm NaCl ) =.6[mN m /mol kg ], is obtained by linear fitting to data from Vanhanen et al. (2008). Szyskowski equation fitting-parameters a and b depend on the relative FAS and NaCl solute mass-frations, where w FAS +w NaCl =, as a = a +a 2 w FAS +a 3 w FAS 2 b = b +b 2 w FAS +b 3 w FAS 2 (5) (6) Fitted parameters are given in Table 3. Note that, due to surfae partitioning, w FAS and w NaCl are generally not equal to the orresponding total dry partile mass-frations (W p,fas and W p,nacl ). The SDS-NaCl solution surfae tension parametrization is obtained from the Szyskowski equation with SDS and NaCl molar onentrations (moles of solute speies per liter of solution) SDS and NaCl : ( ) dσnacl σ = + NaCl aln(+ SDS /b) (7) d NaCl As solutions are dilute, the same numerial value is used for the binary NaCl surfae tension gradient with Table 3. Szyskowski (Eq. 4) fitting parameters for ternary FAS-NaCl aqueous surfae tension parameterizations. Units are mn m and mol kg for parameters a and b, respetively. a a 2 a 3 b b 2 b 3 C8Na C0Na C2Na respet ( ) to molar and molal onentrations, so that dσnacl d NaCl =.6[mN m /mol L ]. Fitted parameters are a = 3.9mN m and b = (9.27E 6 mol 2 L 2 )/( NaCl E 3 mol L ). A ternary SDS-NaCl aqueous solution surfae tension parametrization, based on the same data from Rehfeld (967) and Matijevi and Pethia (958), is also provided by Li et al. (998). This parametrization is however not ontinuous, whih would ause problems in our alulations, and therefore the new parametrization was made. Surfatants an form mielles in onentrated aqueous solutions above the so-alled ritial mielle onentration (m). Mielles are aggregate strutures in whih the amphiphili surfatant moleules are oriented with the polar head-groups faing the aqueous medium, thus shielding a ore onsisting of the non-polar hydroarbon tails (Corrin and Harkins, 947). As mielles begin to form, the surfae tension gradient with respet to inreasing onentrations of dissolved surfatant sharply levels off and the surfae tension beomes approximately onstant. Surfae tension parameterizations were here obtained by fitting data orresponding to surfae tension values above those at the respetive ternary solution ms, estimated by visual inspetion of the data. In the Köhler model alulations, these onstant values are furthermore imposed as lower limits for the droplet surfae tensions evaluated from the resulting ontinuous parameterizations in eah ase. The minimum ternary solution surfae tensions estimated from the data were 24, 27, 22, and 22 mnm for C8Na, C0Na, C2Na, and SDS, respetively. It must be noted, that ternary solution ms and minimum surfae tensions will in general depend on both surfatant and salt solutes. 4.4 Summary of model assumptions Equilibrium is established between different phases within the droplet solution systems and with respet to the surroundings, and no kineti effets influene solution properties or droplet growth and ativation. Dry partiles are spherial, suh that the volumeequivalent diameter equals the seleted eletrialmobility diameter and no mobility shape-fator is applied. Atmos. Chem. Phys., 0, , 200

7 N. L. Prisle et al.: Mixed surfatant-salt CCN 5669 Dry partile mass-fration ompositions reflet those of the solutes within the atomizer solutions from whih partiles are generated. All omponents in the dry partile mixtures and aqueous droplet solutions have zero exess mixing volumes. Fatty aid sodium salt bulk mass-densities are all unity. No hemial or physial reations hange the overall ompositions of the droplet systems. All dry partile omponents are infinitely soluble in the aqueous droplet solutions. All speies in solution are fully dissoiated. All omponents behave ideally in the aqueous droplet solution, by having unit ativity oeffiients on the onentration sales of their respetive ativity referene states. Droplet solution surfae tensions are limited to the estimated minimum values at the approximate ternary ms. The droplet surfae is an infinitely thin spherial shell with zero volume, suh that the droplet bulk-phase volume equals the total equimolar droplet volume. The partitioning equilibrium onserves total molar amounts and volumes of the individual droplet omponents. The molar ratios of droplet speies other than surfatant are fixed. 5 Results and disussion 5. Experimental observations Figure shows measured ritial supersaturations (SS exp ) as a funtion of seleted dry partile mobility-diameter (D p ) for the mixed surfatant-nacl partiles. Individual panels (a) (d) display results for inreasing surfatant (SFT) massfrations in the dry partiles (W p,sft ). In panels (a) and (b), it is seen that with W p,sft 50%, differenes in SS exp for partiles of a given D p omprising different surfatants are omparable to experimental unertainties. Any potential differenes in droplet ativation behavior due to individual moleular properties of the surfatants are thus dominated by the presene of the inorgani salt. Panels () and (d), on the other hand, show that, when W p,sft > 50%, partiles with a given surfatant mass-fration for eah D p ativate with inreasing in the order of inreasing surfatant moleular mass (M SFT ), as C8Na < C0Na<C2Na<SDS. It SS exp should be noted that, in panel (), W p,c0na = 73% is somewhat lower than for the other surfatants ( 80%), whih affets the apparent trend in SS exp with M SFT. We have previously observed single-omponent partiles of these same surfatants to similarly ativate with SS exp inreasing in the order of inreasing M SFT (Sorjamaa et al., 2004; Prisle et al., 2008). As noted in Set. 4.3, surfatant strength also inreases with inreasing M SFT in both binary SFT (Campbell and Lakshminarayanan, 965) and ternary SFT-NaCl (Matijevi and Pethia, 958; Rehfeld, 967; Prisle et al., 200b) bulk aqueous solutions. For ativating solution droplets formed on pure surfatant partiles of a given D p, any effet of enhaned surfae tension redution with inreasing surfatant strength was insuffiient to overome the derease in Raoult effet arising diretly from the inrease in M SFT and possibly also from enhaned droplet bulk-phase depletion by surfae partitioning of the stronger surfatants. Inorgani salts an influene individual organi surfatant properties, as seen for the ternary SFT-NaCl bulk aqueous solution surfae tensions in Set Inreased organi ativity in an aqueous salt solution an derease the orresponding solubility and lead to preipitation or enhaned surfae partitioning. This is often alled the salting-out effet (Lin et al., 2005; Tukermann, 2007; Vanhanen et al., 2008). In marosopi solutions, enhaned surfae partitioning may inrease surfatant strength. In mirosopi droplets with large surfae-area-to-bulk-volume ratios, it will however also inrease bulk-phase depletion of dissolved surfatant moleules. The resulting effet of organi-inorgani interations on droplet surfae tension and surfatant ativation properties is therefore not immediately antiipated. Properties of inorgani salts also diretly influene loud droplet ativation of the mixed partiles. The moleular mass of NaCl is lower than for any of the studied surfatants (see Table 2). Therefore, dissolved NaCl by dry partile mass-fration ontributes a larger Raoult effet in the solution droplets formed than the organis, even in the absene of surfatant partitioning. The present results for mixed SFT-NaCl partiles show that, when NaCl omprises half, or more, of the dry partile mass, the larger Raoult effet of the inorgani salt appears to dominate any mutual differenes in surfatant moleular properties for determining droplet ativation. On the other hand, for partiles with eah surfatant omprising more than half of the mass, the effets of individual surfatant properties beome evident. Still, at a given W p,sft, just as for the pure surfatant partiles, any inreased droplet surfae tension redution with inreasing surfatant strength evidently annot overome the simultaneous derease in Raoult effet from inreased M SFT and possibly enhaned surfae partitioning. Comparing results for the individual surfatants between panels (a) (d) in Fig. shows that SS exp for a given D p inreases with inreasing W p,sft in the dry partiles. This Atmos. Chem. Phys., 0, , 200

8 5670 N. L. Prisle et al.: Mixed surfatant-salt CCN (a) 20% surfatant + 80% NaCl (b) 50% surfatant + 50% NaCl exp % C8 20% C0 23% C2 20% SDS exp % C8 50% C0 53% C2 5% SDS () 80% surfatant + 20% NaCl (d) 95% surfatant + 5% NaCl exp % C8 73% C0 8% C2 8% SDS exp % C8 94% C0 95% C Fig.. Experimental ritial supersaturations (SS exp ) as a funtion of dry partile diameter (D p ) for all dry partile ompositions. Individual panels show results for approximate surfatant mass frations (W p,sft ) of (a) 20%, (b) 50%, () 80% and (d) 95%. was also observed by Rood and Williams (200) for mixed SDS-NaCl partiles. Thus, as NaCl is gradually replaed by surfatant in the dry partiles, any redution in droplet surfae tension and Kelvin effet is dominated by the derease in Raoult effet, from inreasing partile omponent mean moleular mass (as M SFT > M NaCl ) and potentially also from an inreased degree of bulk-phase depletion due to surfae partitioning, as the surfatant omprises a larger fration of the total amount of solute within the droplets. 5.2 Comparison with Köhler model preditions Experimental ritial supersaturations (SS exp ) are ompared to Köhler model preditions with eah of the three different surfatant representations (SS σ,p, SS σ,b Figs. 2 (C8Na), 3 (C0Na), 4 (C2Na), and 5 (SDS)., and SS ) in It is immediately lear that the bulk solution representation (σ,b) greatly underpredits SS exp for all dry partile sizes and ompositions studied here. Predited ritial supersaturations (SS σ,b ) are well outside the range of experimental unertainty in all ases. The simple solute representation ( ) in many ases predit SS exp well, in partiular for the Atmos. Chem. Phys., 0, , 200 less strong surfatants. However, an also lead to signifiant underpreditions of experimental values, albeit to a muh lesser degree than σ,b. The partitioning representation (σ,p) generally desribes experimental ativation behavior well. Speifially, for the mixed partiles omprising SDS, σ,p is onsistently superior to the bulk property representations (σ,b and ) over the full ranges of dry partile sizes and ompositions studied here. For partiles omprising one of the fatty aid salts (FAS), σ,p in some ases overpredits : generally, this ours for the smaller partiles omprising the larger mass frations of the stronger surfatants. For the largest partiles omprising FAS, all surfatant representations underpredit SS exp. For partiles with FAS, the logss logd p slope is generally steeper for model preditions than observed experimentally. This suggests that there may be some size-dependent effet not aounted for with the equilibrium Köhler model used here. Possible sizedependent effets on dry partile properties and droplet ativation are disussed in more detail in Set. 5.4 below. SS exp Figure 6 shows measured (SS exp ) and model predited (SS σ,p, SS σ,b, and SS ) ritial supersaturations for partiles of seleted dry sizes as funtions of dry partile

9 N. L. Prisle et al.: Mixed surfatant-salt CCN % C8Na + 80% NaCl 5% C8Na + 49% NaCl % C8Na experimental % C8Na experimental % C8Na + 2% NaCl 95% C8Na + 5% NaCl % C8Na experimental % C8Na experimental Fig. 2. Critial supersaturations (SS ) as funtions of dry partile diameter (D p ) for partiles with sodium otanoate (C8Na) in dry mass frations (W p,c8na ) of (a) 20%, (b) 5%, () 79%, and (d) 95%, measured in experiments and modeled with the three different surfatant representations (σ,p, σ,b, and ). surfatant mass-fration (W p,sft ). Eah panel shows results for one of the surfatants studied. Upper urves and data points are results for D p = 40nm, representing the smaller partile sizes, and lower urves and data points are results for D p = 00nm, representing the larger partiles studied. Experimental ritial supersaturations reported for pure surfatant partiles (Sorjamaa et al., 2004; Prisle et al., 2008) and mixed SDS-NaCl partiles (Rood and Williams, 200) in previous studies, and ritial supersaturations measured for pure NaCl partiles during alibration of the CCNC in the present work, are inluded for omparison. The perspetive presented in Fig. 6 elaborates on the findings of Figs. 2, 3, 4, and 5. In all ases, the underpreditions of SS exp by σ,b inrease with inreasing surfatant mass-fration in the dry partiles. The partitioning representation (σ,p) generally desribes experimental ativation well, but overpredits for the smaller partiles (D p = 40nm) with the largest mass-frations ( 95%) of FAS. The simple solute representation ( ) also desribes experimental ativation well, in SS exp partiular for the smaller W p,sft, but underpredits SS exp for the stronger surfatants (C2Na and SDS) when partiles ontain signifiant surfatant mass-frations (e.g. W p,sft > 50%). 5.3 Evaluation of surfatant representations Comparing model preditions using the different surfatant representations reveal some of the dynamis behind the effets of surfatant properties on loud droplet ativation for the mixed partiles. The partitioning representation (σ,p) aounts for the influene of surfae partitioning on droplet bulk-phase omposition and uses redued droplet surfae tension, as would be antiipated from the presene of watersoluble surfatants in the dry partiles. It is therefore expeted to provide the most omprehensive and thermodynamially onsistent desription of surfatant properties applied in the present Köhler alulations. In our previous work, σ,p and the simple solute representation ( ) nevertheless gave very similar preditions of ritial supersaturations for single-omponent partiles of the C8 C2 fatty aid salts (Prisle et al., 2008) and of SDS (Sorjamaa et al., 2004). This was due to mutual differenes in the Kelvin and Raoult terms nearly aneling at the respetive points of Atmos. Chem. Phys., 0, , 200

10 5672 N. L. Prisle et al.: Mixed surfatant-salt CCN 20% C0Na + 80% NaCl 50% C0Na + 50% NaCl % C0Na experimental % C0Na experimental % C0Na + 27% NaCl 94% C0Na + 6% NaCl % C0Na experimental % C0Na experimental Fig. 3. Critial supersaturations (SS ) as funtions of dry partile diameter (D p ) for partiles with sodium deanoate (C0Na) in dry mass frations (W p,c0na ) of (a) 20%, (b) 50%, () 73%, and (d) 94%, measured in experiments and modeled with the three different surfatant representations (σ,p, σ,b, and ). droplet ativation, for the speifi partile ompositions studied and omponent properties applied. Speifially, for pure C2Na partiles, predited SS σ,p SS, whereas for pure C8Na and pure C0Na, SS σ,p >SS, and for pure SDS, SS σ,p <SS. Predited loud droplet ativation for the mixed partiles in the present study depends on the properties of both surfatant and NaCl. The Raoult effet of the inorgani salt, as well as the influene of NaCl on surfatant properties of the organi, will affet the relative importane of the predited Kelvin and Raoult effets within the ativating droplets. Figures 2 5 and 6 show that, for the mixed SFT-NaCl partiles, σ,p and also produe similar results at the smaller surfatant mass-frations (e.g. W p,sft < 50%). This supports the suggestion in relation to our experimental observations that, for these partile ompositions, the large Raoult effet of NaCl dominates any differenes in ativation behavior arising from the different moleular properties of the surfatants (see Fig. ). As the surfatants omprise larger massfrations within the dry partiles (e.g. W p,sft > 50%), predited SS σ,p >SS. Thus, even if NaCl inreases surfatant strength in the marosopi ternary solutions, the predited effet of enhaned surfae partitioning is in these ases even greater on inreasing the water ativity in the ativating droplets. and SS inrease with inreasing W p,sft, but the preditions eventually re-merge, as partiles beome almost pure surfatant (for W p,sft 00%). Based on our previous model results for pure surfatant partiles, predited SS W p,sft urves with σ,p and would thus be expeted to ross at some point, for SDS and C2Na. This ross-over is indeed observed for SDS (see Fig. 5) at very high mass frations (W p,sds 00%), but not for mixed partiles with C2Na. Model alulations in the present work were made with new ternary aqueous solution surfae tension parameterizations, as opposed to the binary surfae tensions previously employed in alulations for pure FAS partiles (Prisle et al., 2008) and the parameterization used for SDS-NaCl solutions by Sorjamaa et al. (2004). As a onsequene, mixed partile preditions in the limit of W p,sft = 00% are here made for somewhat different onditions than was previously applied for the pure surfatant partiles. Differenes between SS σ,p Atmos. Chem. Phys., 0, , 200

11 N. L. Prisle et al.: Mixed surfatant-salt CCN % C2Na + 77% NaCl 53% C2Na + 47% NaCl % C2Na experimental % C2Na experimental % C2Na + 9% NaCl 95% C2Na + 5% NaCl % C2Na experimental % C2Na experimental Fig. 4. Critial supersaturations (SS ) as funtions of dry partile diameter (D p ) for partiles with sodium dodeanoate (C2Na) in dry mass frations (W p,c2na ) of (a) 23%, (b) 53%, () 8%, and (d) 95%, measured in experiments and modeled with the three different surfatant representations (σ,p, σ,b, and ). Preditions of SS σ,p > SS may appear opposed to the derease in ritial supersaturations that ould immediately be expeted from the expliit dependeny of the Kelvin term on redued droplet surfae tension (Shulman et al., 996). However, predited ativation with the different surfatant representations generally ours for different ritial droplet diameters (d ). Even if the partitioning representation aounts for the redued droplet surfae tension, predited ritial supersaturations an still be higher than with the simple solute representation that ompletely disregards speifi surfatant properties: this ours when surfae ativity via surfatant partitioning auses an even greater suppression of the droplet Raoult effet than the Kelvin effet suppression attained from redued droplet surfae tension, at the respetive d s predited with σ,p and. Notably, when experimental values SS exp >SS for the mixed surfatant-nacl partiles, it thus suggests a real influene of surfatant partitioning effets within the ativating loud droplets. Other studies have onsidered surfatant properties in ativating droplets by using redued surfae tensions orresponding to marosopi (bulk) aqueous solutions with the same overall omposition (Shulman et al., 996; Fahini et al., 999; Dinar and Taraniuk, 2006; Svenningsson et al., 2006). Calulations have also been simplified by assuming droplets suffiiently dilute at the point of ativation that the onstant surfae tension of pure water ould be applied (Bilde and Svenningsson, 2004; Hartz et al., 2005). Li et al. (998) and Rood and Williams (200) partially aounted for surfae partitioning in loud droplet formation of mixed SDS-NaCl partiles, by inluding the effet of bulk-phase depletion on predited droplet surfae tension. Li et al. (998) showed that replaing NaCl with SDS in the dry partiles inreases predited ritial supersaturations. This result is analogous to those of our Köhler model preditions using the partitioning representation, although the detailed underlying mehanism is inherently somewhat different. Rood and Williams (200) thus argue that surfatants similar to SDS will inhibit the ability of NaCl partiles to ativate in louds. Here, we onfirm this suggestion experimentally and show Atmos. Chem. Phys., 0, , 200

12 5674 N. L. Prisle et al.: Mixed surfatant-salt CCN 20% SDS + 80% NaCl 5% SDS + 49% NaCl % SDS experimental % SDS experimental % SDS + 9% NaCl 00% SDS % SDS experimental.5 0. Sorjamaa, Fig. 5. Critial supersaturations (SS ) as funtions of dry partile diameter (D p ) for partiles with sodium dodeyl sulfate (SDS) in dry mass frations (W p,sds ) of (a) 20%, (b) 5%, and () 8%, measured in experiments and modeled with the three different surfatant representations (σ,p, σ,b, and ). Experimental results from Sorjamaa et al. (2004) for pure SDS are shown in panel (d) and ompared to preditions with the model of the urrent study, using the ompound parameters given in Table 2. for the first time, for a series of fatty aid sodium salts that our in the real atmosphere, that displaing NaCl with surfatant signifiantly inreases partile ritial supersaturations. Rood and Williams (200) present experimental data for the CCN ativity of mixed SDS-NaCl partiles with similar ompositions as was studied in this work. Their measured ritial supersaturations follow the same trends with dry partile size and surfatant mass-fration as presented here. Experimental SS values given by Rood and Williams (200) for partiles omprising SDS are however onsistently lower than ours and appear to be best desribed by our Köhler model alulations using the simple solute representation ( ). As no experimental unertainty estimates are given by Rood and Williams (200), we however annot say if the differenes between the two experimental data sets are signifiant. Rood and Williams (200) also do not provide details of the experimental onditions and the differenes in observed ativation ould potentially be aused by temperature differenes during measurements. For all mixed partiles omprising SDS studied here, the partitioning representation (σ,p) gives the best predition of experimental ritial supersaturations (SS exp ). For partiles omprising FAS, σ,p and the simple solute representation ( ) are both greatly superior to the bulk solution representation (σ,b), but our urrent results do not provide a lear disrimination between σ,p and as the better representation for the studied FAS. Model alulations using σ,p somewhat overpredit experimental ritial supersaturations (SS σ,p > SS exp ), espeially for the smaller partiles omprising the highest mass-frations ( 95%) of FAS. For these partiles, on the other hand tends to underpredit experimental values (SS < SS exp ). Smaller dry partiles ativate with ritial droplets orresponding to smaller diameter growth fators (d /D p ), and thus with larger total solute onentrations ( T (d /D p ) 3 ). The larger surfae-area-to-bulk-volume ratios (A/V d ) of the smaller ritial droplets however also failitate greater relative surfatant bulk-phase depletions ( S / B ). The smaller partiles with the higher surfatant Atmos. Chem. Phys., 0, , 200

13 N. L. Prisle et al.: Mixed surfatant-salt CCN 5675 (a) C8Na + NaCl partiles (b) C0Na + NaCl partiles.5 0. D p =40 nm D p =00 nm experimental Prisle et al., 2008 NaCl alibration data.5 0. D p =40 nm D p =00 nm experimental Prisle et al., 2008 NaCl alibration data C8Na dry mass fration, W p,c8na C0Na dry mass fration, W p,c0na () C2Na + NaCl partiles (d) SDS + NaCl partiles.5 0. D p =40 nm D p =00 nm experimental Prisle et al., 2008 NaCl alibration data.5 0. D p =40 nm D p =00 nm experimental Sorjamaa, 2004 NaCl alibration data Rood, C2Na dry mass fration, W p,c2na SDS dry mass fration, W p,sds Fig. 6. Critial supersaturations (SS ) for partiles with inreasing dry mass frations (W p,sft ) of (a) sodium otanoate (C8Na), (b) sodium deanoate (C0Na), () sodium dodeanoate (C2Na), and (d) sodium dodeyl sulfate (SDS), measured in experiments and modeled with the three different surfatant representations (σ,p, σ,b, and ). Experimental values from Sorjamaa et al. (2004) and Prisle et al. (2008) for pure surfatant partiles, and from Rood and Williams (200) for mixed SDS-NaCl partiles, as well as alibration data for pure NaCl partiles, are inluded for omparison. Results are shown for seleted dry partile sizes, D p = 40nm (upper urves and data points) and D p = 00nm (lower urves and data points). mass-frations thus aentuates the importane of a orret desription of surfatant onentration and onentrationdependent surfae tension and solution effets in the ativating droplets. The ternary FAS-NaCl solution surfae tension parameterizations applied in our model alulations (see Set. 4.3) are not onstrained by measurements for bulk-phase massfrations (w FAS ) relative to NaCl below about 20%. In small droplets, surfatant surfae partitioning an however yield very low predited w FAS (< 0.%). The parameterizations used may therefore not reprodue the orret surfae tension properties for suh droplets. Test alulations indiate a signifiant effet of the surfae tension parameterizations applied on predited ritial supersaturations, in partiular with σ,p. Unfortunately, we do not at present have the experimental data to obtain better onstraints on the Szyskowski fitting parameters. Future work should emphasize employing surfae tension parameterizations that are well-onstrained by measurements for bulk aqueous solution ompositions orresponding to those predited in the bulk-phase of ativating solution droplets. The sensitivity of our Köhler model preditions was tested to seleted properties of the experimental systems, in partiular to unertainties in FAS mass-density, dry partile omposition, size, and shape, and droplet solution effets in terms of non-ideality and solute dissoiation and limited bulk-phase onentration. The sensitivity alulations are desribed in detail and seleted results are presented in Appendix B. None of the tested effets proved apable of resolving the very large underpreditions of experimental ritial supersaturations by the bulk solution representation (SS σ,b << SS exp ). Thus, σ,b does not appear to provide a satisfatory representation of the effets of surfatant properties on loud droplet ativation for the partiles studied here. In priniple, the differenes between SS exp and preditions using either σ,p or might be explained by model Atmos. Chem. Phys., 0, , 200