REPRESENTATIONS OF CLOUD DROPLET ACTIVATION PROPERTIES FOR SURFACE ACTIVE ORGANIC AEROSOL. Kuopio, Finland

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1 REPRESENTATIONS OF CLOUD DROPLET ACTIVATION PROPERTIES FOR SURFACE ACTIVE ORGANIC AEROSOL N. L. PRISLE 1,2 T. RAATIKAINEN 3 J. SMITH 2,4 M. DAL MASO 1 A. LAAKSONEN 3,4, and H. KOKKOLA 2 1 Department of Physics, Division of Atmospheric Sciences, University of Helsinki, P.O. Box 64, University of Helsinki, Finland 2 Finnish Meteorological Institute, Kuopio Unit, P.O. Box 1627, Kuopio, Finland 3 Finnish Meteorological Institute, P.O. Box 503, Helsinki, Finland 4 Department of Physics and Mathematics, University of Eastern Finland, P.O. Box 1627, Kuopio, Finland Keywords: organic aerosol, surfactants, cloud droplet activation, modelling INTRODUCTION Atmospheric aerosol particles commonly comprise surface active organic compounds (Gill et al., 1983). Surface active molecules (surfactants) accumulate in the surface of an aqueous solution and can reduce the solution surface tension from that of pure water. Reduced surface tension has been demonstrated in atmospheric cloud and fog water samples (Facchini et al., 1999, 2000) and in aqueous extracts of atmospheric aerosol samples from a variety of sources, including biomass and coal burning (Oros and Simoneit, 2000; Asa-Awuku and Sullivan, 2008), and from marine, rural and urban environments (Mochida et al., 2002; Kiss et al., 2005; Dinar and Taraniuk, 2006). Surface activity is expected to influence cloud droplet activation properties of organic aerosols, as the droplet surface tension enters directly into the Köhler equation (Köhler, 1936; Shulman et al., 1996). However, activating droplets in the atmosphere typically have sub-micrometer diameters (microscopic), whereas collected field samples and extracts are bulk solutions (macroscopic). Microscopic droplets have much greater surface-area-to-bulk-volume ratios than macroscopic solutions. For example, spherical droplets with diameters d = 0.1, 1, and 10 µm have A/V (= 6/d) = 60, 6, and 0.6 µm 1, respectively. As surfactant molecules accumulate in the surface of a microscopic aqueous droplet, the distribution (partitioning) between the bulk and surface phases that result can significantly change the droplet properties, compared those of a macroscopic solution with the same composition (Seidl and Hanel, 1983; Bianco and Marmur, 1992; Laaksonen, 1993). Surfactant partitioning may this way affect the activation of microscopic cloud droplets through both solute suppression of water activity (the Raoult effect) and surface tension reduction (the Kelvin effect). The detailed mechanism by which surface activity can overall influence droplet activation is therefore complex, as is the thermodynamic representation of surfactants in model predictions of cloud droplet formation. Li et al. (1998) used a model based on Köhler theory, in which the effect of surfactant partitioning on droplet surface tension was taken into account. Sorjamaa et al. (2004) furthermore included the effect on water activity. Laboratory measurements have shown that experimental critical supersaturations for particles containing the ionic surfactants sodium dodecyl sulfate (CH 3 (CH 2 ) 10 SO 4 Na; SDS), sodium octanoate (CH 3 (CH 2 ) 6 COONa; C8Na), sodium decanoate (CH 3 (CH 2 ) 8 COONa; C10Na), sodium dodecanoate (CH 3 (CH 2 ) 10 COONa; C12Na), and sodium tetradecanoate (CH 3 (CH 2 ) 12 COONa; C14Na) are greatly underpredicted when using reduced surface tension in Köhler theory while ignoring the effects of surface partitioning in droplets (Rood

2 and Williams, 2001; Sorjamaa et al., 2004; Prisle et al., 2008, 2009). In some cases, experimental observations are well described when surfactant properties are simply disregarded by ignoring surface partitioning and using the constant surface tension of pure water throughout droplet growth and activation. Other times, this approach can however lead to significant underpredictions of measured critical supersaturations. For such particles, organic component surface activity thus appears to significantly influence the cloud droplet forming ability and the overall effect of surfactant properties is to inhibit droplet activation by increasing critical supersaturations above those predicted from simple equilibrium Köhler theory (Seinfeld and Pandis, 2006). An increase in critical supersaturation resulting from surfactant properties can be predicted upon accounting for the effects of both surface partitioning and reduced surface tension in activating droplets (Prisle et al., 2009). These calculations however require specific knowledge of compositiondependent surface tension and molecular properties for all particle components, which are generally not available, even for the simple aerosol mixtures studied in the laboratory, let alone for complex ambient aerosols. Furthermore, evaluation of this detailed information within atmospheric models that include aerosol-cloud interaction will not be computationally feasible. The aim of this work is therefore to seek computationally simple model representations, capable of reproducing the main features of the detailed thermodynamic behaviour of surface active organics in cloud droplet activation without explicitly evaluating these properties. To be applicable to complex and/or ambient aerosol mixtures, such representations should furthermore only involve particle parameters that can be determined from direct aerosol measurements. METHODS Based on knowledge about detailed dynamics of the predicted effects of surface activity on droplet activation for organic aerosol systems of well-known properties, simplified representations of the behavior of organic surfactants in activating droplets are proposed. These simple surfactant representations are then used to predict critical supersaturations from Köhler theory for previously studied two-component particles comprising the above-mentioned organic surfactants C8Na, C10Na, C12Na, and SDS, mixed with sodium chloride (NaCl) in various relative amounts. Predictions with the simple representations are compared to those employing a comprehensive thermodynamic account for surface activity, as well as those resulting from completely disregarding surfactant properties and treating the organic as a regular solute within the activating droplets. All model predictions are furthermore compared to previous laboratory measurements of droplet activation for the particles in question. PRELIMINARY RESULTS AND DISCUSSION Critical supersaturations (SS c [%]) are calculated from Köhler theory with two simple representations of organic surfactant (SFT) properties. The first representation (rep1) simply assumes that all surfactant solute is partitioned to the droplet surface and as a consequence does not yield any suppression of droplet water activity according to the Raoult effect. Surface concentrations are furthermore assumed to be insufficient for any reduction of droplet surface tension, which is then set equal to the constant value for pure water (σ w = 72.2 mnm 1 ) throughout droplet growth and activaton. The second representation (rep2) accounts for the Raoult effect and surface tension reduction achieved from the organic upon incomplete surface partitioning at small dry particle diameters (D p [nm]) and large surfactant mass-fractions (w SFT ) within the particles. The surface tension and extent of organic surface partitioning are then parameterized as simple functions of D p and w SFT. Preliminary results for mixed SDS and sodium chloride (NaCl) particles are shown in Figures 1

3 Activation of 81% SDS + 19% NaCl particles Critical supersaturation, SS c [%] rep1 rep2 σ, p σ w Sorjamaa, 2004; Prisle, Dry particle diameter, D p [nm] Figure 1: Critical supersaturations (SS c [%]), measured in experiments (Prisle et al., 2009) and modeled from Köhler theory, as functions of dry particle diameter (D p [nm]) for mixed SDS-NaCl particles with w SDS = 81%. Surfactant are described using the detailed partitioning representation (σ, p), the regular solute representation (σ w ), and two simple representations (rep1 and rep2). See text for details. and 2. Both figures show critical supersaturations predicted with simple representations rep1 and rep2, together with values calculated using the comprehensive surfactant partitioning representation (σ, p), described by Sorjamaa et al. (2004); Prisle et al. (2008, 2009), and the regular solute representation (σ w ), corresponding to simple Köhler theory. Experimental critical supersaturations from Sorjamaa et al. (2004); Prisle et al. (2009) are also included. In Figure 1, critical supersaturations are displayed as functions of dry particle diameter for mixed SDS-NaCl particles with w SDS = 81%, and in Figure 2, values for particles with D p = 40 nm are given as functions SDS mass-fraction in the dry particles. Predictions of the partitioning representation are well-reproduced by calculations using rep1 over the majority of dry particle compositions and then for the full range of studied dry particle sizes. Only for large SDS mass-fractions (e.g. w SDS > 80%) do predictions deviate and it becomes necessary to account for the presence of the organic within the activating droplets by using rep2. Including activation properties of the organic with this representation is capable of reproducing the qualitative behavior of the comprehensive model, even for the largest SDS mass-fractions. Both rep1 and rep2 describe experimental activation more closely than the regular solute representation in the range 50% < w SDS < 80%. For larger SDS mass-fractions in the dry particles, rep1 increasingly deviate from both experimental observations and the partitioning model predictions, whereas rep2 succesfully predict the behavior of both even for the highest SDS mass-fractions. Considering our preliminary results for the other surfactants studied (not shown), it generally becomes necessary to include effects of the organic on droplet activaton at high surfactant massfractions in the dry particles. The mass-fractions at which predictions with rep1 and rep2 start to deviate are lower, and the deviations between predictions with rep1 and rep2, and between rep1 and the detailed partitioning model, increase, for the weaker the surfactants.

4 Critical supersaturation, SS c [%] Activation of 40 nm SDS + NaCl particles Sorjamaa, 2004; Prisle, 2009 σ, p σ w rep1 rep SDS mass fraction in dry particles, w SDS Figure 2: Critical supersaturations (SS c [%]) for mixed SDS-NaCl particles of D p = 40 nm, as functions SDS mass-fraction (w SDS ) in the particles. Specifications are as in Figure 1. See text for details. Atmospheric aerosol will rarely comprise almost pure surfactant, but are generally mixtures of both organic and inorganic components (O Dowd et al., 2004; Murphy et al., 2006). Very high water vapor supersaturations are also unlikely in warm clouds (Kokkola et al., 2006). The simplest rep1 therefore describes activation properties of SDS remarkably well, over ranges of both dry particle relative organic and inorganic composition and critical supersaturation that are realistic for the atmosphere. The predictive capability of the simple representations must be tested for other particle compositions, and ultimately for complex organic aerosol mixtures of unresolved speciation, such as laboratory-generated SOA particles of atmospheric relevance, and ambient aerosols. To our knowledge, these representations are the first to mimick detailed thermodynamic behaviour of surface active organics in activating droplets, while remaining as conceptually and computationally straightforward as using simple Köhler theory. For the studied surfactants other than SDS, it has been necessary to include a compound-specific correction in the surface partitioning and surface tension parameterizations, which is achieved by scaling the effect according to organic molecular mass or carbon number. Such a correction could potentially be related to the mean molecular mass of an organic aerosol mixture. CONCLUSIONS Our preliminary results show that, for the two-component mixed surfactant-nacl particles in question, Köhler model predictions with the comprehensive surfactant partitioning model can be closely reproduced using very simple representations of the cloud droplet activation properties for the organic aerosol component. In particular, simply omitting any effects on activation from the organic surfactant works remarkably well for describing activation behavior of mixed SDS-NaCl particles over ranges of relative organic-inorganic particle compositions and water vapor supersaturations

5 that are realistic for the atmosphere. Within these ranges, this simple surfactant representation notably describes experimentally observed activation behavior better than treating SDS as a regular solute in Köhler theory. The presented simple representations of organic cloud droplet activation properties could potentially be applied to complex organic aerosol mixtures of unknown composition, such as laboratory-generated SOA particles or ambient aerosols. ACKNOWLEDGEMENTS The authors are grateful for the financial support by the Academy of Finland Centre of Excellence program. N. L. Prisle furthermore gratefully acknowledge the support by the Carlsberg Foundation under grant REFERENCES Asa-Awuku, A. and Sullivan, A. (2008). Investigation of molar volume and surfactant characteristics of water-soluble organic compounds in biomass burning aerosol. Atmospheric Chemistry and Physics, 8: Bianco, H. and Marmur, A. (1992). The Dependence of the Surface Tension of Surfactant Solutions on Drop Size. Journal of Colloid and Interface Science, 151: Dinar, E. and Taraniuk, I. (2006). Cloud Condensation Nuclei properties of model and atmospheric HULIS. Atmospheric Chemistry and Physics, 6: Facchini, M., Decesari, S., Mircea, M., Fuzzi, S., and Loglio, G. (2000). Surface Tension of Atmospheric Wet Aerosol and Cloud/Fog Droplets in Relation to their Organic Carbon Content and Chemical Composition. Atmospheric Environment, 34: Facchini, M., Mircea, M., Fuzzi, S., and Charlson, R. (1999). Cloud Albedo Enhancement by Surface-Active Organic Solutes in Growing Droplets. Nature, 401: Gill, P. S., Graedel, T. E., and Weschler, C. J. (1983). Organic Films on Atmospheric Aerosol- Particles, Fog Droplets, Cloud Droplets, Raindrops, and Snowflakes. Reviews of Geophysics and Space Physics, 21: Kiss, G., Tombácz, E., and Hansson, H.-C. (2005). Surface Tension Effects of Humic-Like Substances in the Aqueous Extract of Tropospheric Fine Aerosol. Journal of Atmospheric Chemistry, 50: Köhler, H. (1936). The Nucleus in and the Growth of Hygroscopic Droplets. Transactions of the Faraday Society, 32: Kokkola, H., Sorjamaa, R., Peräniemi, A., Raatikainen, T., and Laaksonen, A. (2006). Cloud Formation of Particles Containing Humic-Like Substances. Geophysical Research Letters, 33:L10816,1 5. Laaksonen, A. (1993). The Composition Size Dependence of Aerosols Created by Dispersion of Surfactant Solutions. Journal of Colloid and Interface Science, 159: Li, Z., Williams, A., and Rood, M. (1998). Influence of Soluble Surfactant Properties on the Activation of Aerosol Particles Containing Inorganic Solute. Journal of the Atmospheric Sciences, 55:

6 Mochida, M., Kitamori, Y., Kawamura, K., Nojiri, Y., and Suzuki, K. (2002). Fatty acids in the marine atmosphere: Factors governing their concentrations and evaluation of organic films on sea-salt particles. Journal of Geophysical Research, 107:D17S4325. Murphy, D. M., Cziczo, D. J., Froyd, K. D., Hudson, P. K., Matthew, B. M., Middlebrook, M., Peltier, R. E., Sullivan, A., Thomson, D. S., and Weber, R. J. (2006). Single-particle mass spectrometry of tropospheric aerosol particles. Journal of Geophysical Research, 111:D23S32. O Dowd, C. D., Facchini, M. C., Cavalli, F., Ceburnis, D., Mircea, M., Decesari, S., Fuzzi, S., Yoon, Y. J., and Putaud, J.-P. (2004). Biogenically driven organic contribution to marine aerosol. Nature, 431: Oros, D. and Simoneit, B. (2000). Identification and emission rates of molecular tracers in coal smoke particulate matter. Fuel, 79: Prisle, N. L., Laaksonen, T. R. A., and Bilde, M. (2009). Surfactants in cloud droplet activation: mixed organic-inorganic particles. Atmos. Chem. Phys. Discuss., 9: Prisle, N. L., Raatikainen, T., Sorjamaa, R., Svenningsson, B., Laaksonen, A., and Bilde, M. (2008). Surfactant partitioning in cloud droplet activation: a study of C8, C10, C12 and C14 normal fatty acid sodium salts. Tellus, 60B: Rood, M. J. and Williams, A. L. (2001). Reply. Journal of the Atmospheric Sciences, 58: Seidl, W. and Hanel, G. (1983). Surface-Active Substances on Rainwater and Atmospheric Particles. Pure and Applied Geophysics, 121: Seinfeld, J. H. and Pandis, S. N. (2006). Atmospheric Chemistry and Physics -From Air Pollution to Climate Change. John Wiley and Sons, Inc., second edition. Shulman, M., Jacobson, M., Charlson, R., Synovec, R., and Young, T. (1996). Dissolution Behavior and Surface Tension Effects of Organic Compounds in Nucleating Cloud Droplets. Geophysical Research Letters, 23: Sorjamaa, R., Svenningsson, B., Raatikainen, T., Henning, S., Bilde, M., and Laaksonen, A. (2004). The Role of Surfactants in Köhler Theory Reconsidered. Atmospheric Chemistry and Physics, 4: