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Cooling Enhancement of Aerosol Particles Due to Surfactant Precipitation Melinda R. Beaver,†,‡,| Miriam A. Freedman,†,‡ Christa A. Hasenkopf,†,§ and Margaret A. Tolbert*,†,‡ Department of Chemistry and Biochemistry, CooperatiVe Institute for Research in EnVironmental Sciences, CIRES, and Department of Atmospheric and Oceanic Sciences, UniVersity of Colorado, Boulder, Colorado 80309 ReceiVed: March 17, 2010; ReVised Manuscript ReceiVed: May 24, 2010
Light extinction by particles in Earth’s atmosphere is strongly dependent on the particle size, chemical composition, and ability to take up water. In this work, we have measured the optical growth factors, fRHext(RH, dry), for complex particles composed of an inorganic salt, sodium nitrate, and an anionic surfactant, sodium dodecyl sulfate. In contrast with previous studies using soluble and slightly soluble organic compounds, optical growth in excess to that expected based on the volume weighted water uptake of the individual components is observed. We explored the relationship between optical growth and concentration of surfactant by investigating the role of particle density, the effect of a surfactant monolayer, and increased light extinction by surfactant aggregates and precipitates. For our experimental conditions, it is likely that surfactant precipitates are responsible for the observed increase in light scattering. The contribution of surfactant precipitates to light scattering of aerosol particles has not been previously explored and has significant implications for characterizing the aerosol direct effect. Introduction The direct interaction of atmospheric particulate matter with solar radiation affects Earth’s climate and local visibility. Light extinction, the sum of scattering and absorption, is dependent on the particle size, composition, and morphology. Ambient relative humidity (RH) affects extinction by causing changes in particle size and refractive index due to hygroscopic growth. Currently, the largest uncertainties in predicting anthropogenic effects on climate are associated with the aerosol direct and indirect effects.1 Laboratory quantification of the optical growth of particles of varying composition is necessary for inclusion into radiative transfer calculations. Atmospheric particulate matter has a large contribution from organic compounds.2-4 Often, both inorganic salts and organic compounds are found internally mixed within the same aerosol particle. Surface-active organic compounds have been observed to be present on both marine and continental aerosol particles.5-8 Soluble surfactants are a particularly interesting class of organic compounds to study because they can be located either on the surface9-11 or as aggregates within the aerosol interior.12 Aerosol surface coatings may affect heterogeneous reaction rates, aerosol optical properties, and water uptake.11 Donaldson and Vaida9 suggest that insoluble surfactants could form ideally packed coatings that act to slow transport. They also suggest that soluble surfactants will likely form porous coatings with less rigid structures, which will not slow the rate of transport of molecules through them. Various groups have performed aerosol particle studies to probe and understand the effects of both soluble and insoluble organic coatings on water uptake * To whom correspondence should be addressed. E-mail: tolbert@ colorado.edu. † Department of Chemistry and Biochemistry. ‡ Cooperative Institute for Research in Environmental Sciences, CIRES. § Department of Atmospheric and Oceanic Sciences. | Current address: Environmental Science and Engineering Program, California Institute of Technology, Pasadena, California 91125.
and phase transitions by hygroscopic salts.13-16 Water evaporation rates have been seen to decrease when surface-active compounds are present in particles.13 Studies on phase transitions in particles containing surfactants have found that these transitions are not prevented by the coating.14-16 Overall, the results suggest that the structure and packing nature of the organic coating determines the impact of the coating on water uptake.15,16 Particle coatings can also increase extinction efficiency by aerosol particles.17,18 The effect of the surface activity and partitioning of organic compounds at high relative humidities and in the supersaturated regime has also been previously explored.19-21 In particular, partitioning of the organic compound to the surface layer needs to be considered to obtain the correct hygroscopicity and critical supersaturation for organic aerosols.19-21 Tabazadeh12 suggests that micelles or aggregates of soluble organic matter might exist within the aerosol particle at concentrations lower than that required to form a monolayer, producing a colloidal aerosol particle. It is also suggested that when micelles form, several physical properties of aerosol particles would be affected. For example, water uptake is suggested to decrease because of the change in distribution of charges within the aerosol particle.12,22 However, light scattering is suggested to increase because of the increase in particle turbidity.12,22 Organic aggregates have been found to constitute a large fraction of the submicrometer organic matter in particles generated from natural seawater.8 Surface-active compounds, or surfactants, are amphiphiles consisting of a hydrophobic tail and a hydrophilic headgroup.22 Here we have studied sodium dodecyl sulfate (SDS); the chemical structure is shown in Figure 1. SDS, although not an atmospherically relevant compound, has been used in a number of laboratory investigations as a model surface-active organic compound. SDS is a soluble, anionic surfactant with a sulfate hydrophilic group and a 12-carbon hydrophobic tail. Even though we have used an anionic surfactant as a model surface active molecule here, most surface active organics in atmo-
10.1021/jp102437q 2010 American Chemical Society Published on Web 06/10/2010
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Figure 1. Chemical structure of sodium dodecyl sulfate (SDS), the soluble anionic surfactant used in this work.
Figure 3. Schematic of the experimental apparatus for generating aerosol particles, size selecting, humidifying, and measuring aerosol extinction.
Figure 2. Generalized aerosol morphologies studied in this work. In the top panel, at lower concentrations of SDS, only a monolayer of surfactant is expected to form on the aerosol particle. In the bottom panel, at higher SDS concentrations, there is sufficient surfactant to form a monolayer as well as aggregates within the particle.
spheric particulate matter will not be ionized because of the acidic pH of most ambient aerosol particles. The hygroscopic growth of SDS/sodium nitrate and SDS/ sodium chloride aerosol system has been studied by Zelenyuk et al.23 using a hygroscopic tandem differential mobility analyzer, HTDMA, to measure diameter changes upon humidification. Various groups have experimentally investigated the structure and morphology of particles containing SDS across a range of relative humidities and surfactant concentrations.24,25 These groups found that the particle morphology is dependent on the concentration of surfactant in the particles as well as on the water content of the particle. The role of SDS in affecting reactions at aqueous surfaces has also been investigated, and, in general, it has been found that the presence of SDS slows or inhibits the surface reaction.26,27 When studying surfactant aerosol systems, it has generally been assumed that a monolayer of organic forms before organic aggregates or micelles form because of the large surface-area-to-volume ratios present in particles.23,24,26 The present work investigates the optical growth upon humidification of the complex aerosol particle system consisting of SDS mixed with the inorganic salt, sodium nitrate (SN) in the subsaturated regime. The light extinction technique is a particularly interesting method with which to probe this aerosol system because the effect of micelles, if any, on optical growth can be probed. A diagram of the theoretical particle morphologies studied here is shown in Figure 2. In the top panel, a monolayer coverage of SDS is shown to form on the wet particle. In the bottom panel, at higher mass percents of SDS, both a monolayer and aggregates of surfactant might form.
Previous work has investigated the optical growth factors of organic compounds such as dicarboxylic acids, polyfunctional aromatic acids, sugars, amino acids, as well as complex internal mixtures of the organics with ammonium sulfate.28-30 The organic compounds studied to date span a range of water solubilities from very water-soluble to only slightly watersoluble. These studies found a linear relationship between optical growth factors and the organic content of the particles. The growth thus appears to follow a Zdanovskii-Stokes-Robinson (ZSR) relationship,31 which assumes that each component of the mixture acts independently and that no interaction between components occurs. The majority of the particle systems studied thus far have exhibited additive optical growth when volume weighted using appropriate particle densities. Therefore, an aerosol particle system where complex particle morphologies may exist is an interesting extension of previous work. Experimental Section A tandem cavity ring-down aerosol extinction spectrometer, CRD-AES, was used to determine the dependence of aerosol extinction on RH at 532 nm and is shown schematically in Figure 3. The instrument has been previously described in Baynard et al.32 and in Beaver et al.,30 so only a brief description will be provided here. Mixed SDS/SN aerosol particles were prepared via atomization of aqueous solutions of mixtures of the pure compounds. Solutions were prepared using HPLC grade water, SN (SigmaAldrich, ACS reagent grade), and SDS (Sigma-Aldrich, 99+%, ACS reagent grade) to a total solution concentration of ∼0.05 wt %. The ratios of SN to SDS were varied to produce aerosol particles with a range of concentrations of inorganic salt to surfactant from pure SN to pure SDS. In all cases, it was assumed that the ratio of SN to SDS in the starting solution remained the same in the aerosol particle. The total concentration of the solutions was kept low to minimize the peak diameter of the number size distribution (typically 85 nm) to reduce the interference of doubly charged particles from the differential mobility analyzer. In this work, no correction has been applied to account for doubly charged particles. After atomization, the particles were dried through two diffusion driers to a RH < 10%. The first drier contained molecular sieves, and the second
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Figure 5. Optical growth factors, fRHext(86.5%RH, dry), versus mass percent SDS for internally mixed SN/SDS aerosol particles. Three particle sizes are included: 200 (9), 300 (b), and 400 nm (2). Lines connecting the data points are included for clarity. Figure 4. Comparison of the measured optical growth factors for pure SN versus particle diameter, fRHext(86.5%RH, dry) (9), with fRHext values calculated from the thermodynamic model of Clegg et al.33 (-), as described in the text. The dotted lines show the uncertainty bands that result from the accuracy ((3%RH) of the humidity probes.
contained silica gel, with a total residence time in the driers of ∼75 s. After drying, the polydisperse particles entered the electrostatic classifier (TSI 3080) and differential mobility analyzer (DMA) (TSI 3081) for size selecting over the diameter range of 200 to 550 nm. Size selection was performed at a sheath flow of 5 LPM and with an atomized sample aerosol flow of 1.5 LPM. The large aerosol to sheath flow causes a wider distribution of particles to be size-selected than the more standard 1:10 flow ratio. The size-selected aerosol particles enter the first cell of the CRD-AES where the extinction due to dry aerosol, σext(dry), is measured at 21 °C. The particles then enter a temperaturecontrolled humidification cell where they are exposed to an RH of 90%. The humidification cell consists of a water vapor permeable membrane surrounded by liquid water that is heated to ∼30 °C. Then, the extinction of the humidified particles at 86.5%RH, σext(86.5%RH), is measured in the second cell of the CRD-AES. The humidity and temperature in both cells are monitored with Vaisala Humitter 50Y probes (accuracy (3%). The ratio of the humidified extinction to the dry reference extinction, from the tandem cells, defines the optical growth factor fRHext(86.5%RH, dry)
fRHext(86.5%RH, dry) )
σext(86.5%RH) σext(dry)
The particle concentration is then measured with a condensation particle counter, CPC (TSI 3022A). Results and Discussion Before investigating the optical growth factors of the SDS/ SN mixed particle system, the size dependence of the fRHext(86.5%RH, dry) of pure SN particles was studied. Results from this study are shown in Figure 4, along with a comparison to calculated fRH values. Error bars indicate the standard deviation of multiple experiments. The calculated values were determined by using growth factors, Gf ) Dwet/Ddry, to convert each dry particle size to a humidified size. Zelenyuk et al.23 measured a growth factor for SN particles of 1.73 at 86%RH ((1%RH). For the calculation shown in Figure 4, a growth factor of Gf ) 1.82 was used as determined from the aerosol
inorganics model (AIM).33 This growth factor agrees well with that calculated from parametrizations of hygroscopic growth factors determined by an electrodynamic balance, 1.81.34,35 To convert Gf to fRHext, we need the refractive indices of the dry and humidified particles. We assumed refractive indices of 1.33 and 1.56 for water and SN, respectively. The literature value for the refractive index of SN is 1.587.35 However, on the basis of the measured dry extinction cross sections determined for SN in this work, we found 1.56 to be the best fit to the dry extinction cross sections. Whereas residual water is expected to be absent at relative humidities 50% by mass, the best agreement with the measurements is the prediction based on a volume weighting of the densities reported in the literature for the pure compounds SDS and SN. However, using the densities reported by Zelenyuk at al.23 at these concentrations give only a slightly poorer fit. It can be seen in Figure 6 that none of the ZSR fits describe the entire data set particularly well. At lower concentrations of SDS, we observe more light extinction than predicted by each weighting method, most notably for particles containing ∼20% by mass SDS. This trend is shown in more detail in Figure 7, where the ratio of the measured to predicted fRHext is shown for a variety of mixtures of organic compounds with ammonium sulfate. In this Figure, the predicted fRHext(RH, dry) values were determined from volume weighting the literature densities of each pure compound. As can be seen, we have commonly observed agreement between measured and predicted fRHext values for a range of soluble and slightly soluble organics (Figure 7b,c). In some cases, we have also observed negative deviations from the prediction, which have in general been attributed to dry particle densities being less than predicted (b and c); therefore, fRHext values were lower than predicted.29,30 The SDS/SN system is the first mixture where we have observed positive deviations from the ZSR prediction (Figure 7a). Possible explanations for this positive deviation are the effect on water uptake by a monolayer coating of the surfactant as well as increased light extinction by aggregate formation within the particle. When studying surfactant aerosol systems, it has generally been assumed that a monolayer of organic forms before organic aggregates or micelles form because of the large surface-areato-volume ratios present in particles.23,24,26 To predict what concentration of SDS is necessary to form a monolayer on humidified particles, the growth factors from Zelenyuk et al.23 at 86%RH were used to determine the approximate humidified particle diameter. Then, the available surface area per molecule of surfactant was calculated for each mass percent SDS. A range of molecular footprints for surfactants is found in the literature. It can be assumed that a monolayer of surfactant forms when each SDS molecule has a surface area of 20 Å2, a value known for carboxylic acids of 12 carbons and longer.10,22,23,26 In contrast, Persson et al.,38 found the molecular footprint of SDS in dilute ionic solutions to be closer to 50 Å2. On the basis of these molecular footprints, the dry particle mass percent SDS at which a monolayer forms can be calculated. The results from this calculation are shown in Figure 8. As can be seen, the mass percent SDS at which a monolayer should be present varies slightly with particle diameter (based on the surface area to volume ratio). Assuming a 20 Å2 footprint, for a 200 nm particle exposed to 86%RH, a monolayer should exist on particles of
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Figure 8. Surface area per molecule of surfactant versus mass percent SDS in the dry particle calculated for the humidified particles based on reported hygroscopic growth factors.23 200 (9), 300 (b), and 400 nm (2). The region bound by the dotted lines is the approximate mass percent SDS required to form a monolayer, depending on the assumption of the area of the molecular footprint.
Figure 7. Ratio of measured to predicted fRHext for a range of organic compounds studied (a) here and in previous work refs (b) 29 and (c) 30.
∼10% by mass SDS, whereas for 400 nm diameter particles, a monolayer should exist on particles of ∼8% by mass SDS. When a larger footprint of 50 Å2 per molecule of SDS is used, the monolayer forms at much lower concentrations of SDS, 35% remained transparent. However, at an SDS mass percent of ∼33%, which corresponds to 0.41 M SN, the solutions became cloudy (with a bright white color) and visibly scattered light. The high ionic strength and the temperature of the solution are the likely causes for the turbidity. For example, for solutions of 0.4 M NaCl and 0.069 M SDS, the critical micelle temperature is ∼24 °C.41 The results from these simple experiments are shown with the fRHext particle data for 200 nm particles in Figure 9, with the concentration range where turbid solutions were observed shaded in gray. Although these experiments were performed with bulk solutions and are overall less concentrated than the humidified particles studied, the transition from clear to cloudy occurred at similar SDS mass percents to where we measured more extinction by humidified particles than predicted by ZSR. An increase in ionic strength or surfactant concentration, closer to that found in an aerosol particle, would enhance the degree of aggregation and precipitation of the surfactant. Therefore, it is possible that the positive deviations observed in the system at low SDS mass percents could be due to increased light extinction by a precipitate in the particles. Although there is evidence of an increase in light extinction due to the presence of phase separation in these laboratory-generated particles, the largest difference from the volume weighted approximation is ∼40% and occurs at ∼25% by mass surfactant. Therefore, the ZSR prediction only slightly underestimates the aerosol extinction in this complex aerosol system. Conclusions and Atmospheric Implications Internally mixed SN/SDS aerosol particles were produced over a range of concentrations. The size-resolved, optical growth of the mixed particle system was investigated as a function of the concentration of surfactant present in the aerosol particle. Optical growth factors of the individual components agree well with thermodynamic predictions as well as with literature values. For the mixed particles, nearly constant optical growth was observed at concentrations of SDS up to 20% by mass, and then optical growth was found to decrease with increasing mass of SDS. The trend of optical growth versus mass percent SDS can be explained by a ZSR additive uptake parametrization using literature density values for the pure components at 50% by mass SDS and higher. It is likely that organic aggregates form
J. Phys. Chem. A, Vol. 114, No. 26, 2010 7075 at lower mass percent SDS, and increased light extinction due to precipitate formation was observed. The results obtained in this study have significant implications for understanding the aerosol direct effect. While SDS is not an atmospherically relevant molecule, soluble surfactants are commonly found in tropospheric aerosols. In addition, most particles found in the atmosphere are internal mixtures of multiple components including organic compounds, salts, and water. Depending on the temperature, pH, and ionic strength of the aerosol particles, surfactant precipitates could form in aerosol particles. As shown through the experiments in this study, precipitate formation could cause an increase in light scattering, which would result in a larger negative radiative forcing than previously expected. While the possible importance of monolayer formation and micelle formation has been addressed in the atmospheric literature,11,12 the potential for aggregation and precipitation of organic compounds to enhance significantly light extinction has not been explored. Our study indicates that aggregate and precipitate formation in aerosol particles warrants further investigation. Acknowledgment. We acknowledge financial support from NASA under grant no. NNX09AE12G. M.R.B. was supported by an EPA STAR graduate research fellowship (FP-91654601). C.A.H. was supported with an NSF Graduate Research Fellowship. M.A.F. acknowledges support from the NOAA Climate and Global Change Postdoctoral Fellowship Program administered by the University of Corporation for Atmospheric Research. The research described in this article has been partially funded by the United States Environmental Protection Agency (EPA) under the Science to Achieve Results (STAR) Graduate Fellowship Program. EPA has not officially endorsed this publication, and the views expressed herein may not reflect the views of the EPA. References and Notes (1) Forster, P.; Ramaswamy, V.; Artaxo, P.; Berntsen, T.; Betts, R.; Fahey, D. W.; Haywood, J.; Lean, J.; Lowe, D. C.; Myhre, G.; Nganga, J.; Prinn, R.; Raga, G.; Schulz, M.; Van Dorland, R. Changes in Atmospheric Constituents and in Radiative Forcing. In Climate Change 2007: The Physical Science Basis. Contribution of Working Group I to the Fourth Assessment Report of the IntergoVernmental Panel on Climate Change; Solomon, S., Qin, D., Manning, M., Chen, Z., Marquis, M., Averyt, K. B., Tignor, M., Miller, H. L., Eds.; Cambridge University Press: New York, 2007. (2) Saxena, P.; Hildemann, L. M. J. Atmos. Chem. 1996, 24, 57. (3) Murphy, D. M.; Cziczo, D. J.; Froyd, K. D.; Hudson, P. K.; Matthew, B. M.; Middlebrook, A. M.; Peltier, R. E.; Sullivan, A.; Thomson, D. S.; Weber, R. J. J. Geophys. Res. 2006, 111, D23S32. (4) Zhang, Q.; Jimenez, J. L.; Canagaratna, M. R.; Allan, J. D.; Coe, H.; Ulbrich, I.; Alfarra, M. R.; Takami, A.; Middlebrook, A. M.; Sun, Y. L.; Dzepina, K.; Dunlea, E.; Docherty, K.; DeCarlo, P. F.; Salcedo, D.; Onasch, T.; Jayne, J. T.; Miyoshi, T.; Shimono, A.; Hatakeyama, S.; Takegawa, N.; Kondo, Y.; Schneider, J.; Drewnick, F.; Borrmann, S.; Weimer, S.; Demerjian, K.; Williams, P.; Bower, K.; Bahreini, R.; Cottrell, L.; Griffin, R. J.; Rautiainen, J.; Sun, J. Y.; Zhang, Y. M.; Worsnop, D. R. Geophys. Res. Lett. 2007, 34, 2007GL029979. (5) Tervahattu, H.; Juhanoja, J.; Kupiainen, K. J. Geophys. Res. 2002, 107, 7. (6) Tervahattu, H.; Juhanoja, J.; Vaida, V.; Tuck, A. F.; Niemi, J. V.; Kupiainen, K.; Kulmala, M.; Vehkamaki, H. J. Geophys. Res. 2005, 110, 9. (7) Peterson, R. E.; Tyler, B. J. Appl. Surf. Sci. 2003, 203-204, 751. (8) Facchini, M. C.; Rinaldi, M.; Decesari, S.; Carbone, C.; Finessi, E.; Mircea, M.; Fuzzi, S.; Ceburnis, D.; Flanagan, R.; Nilsson, E. D.; de Leeuw, G.; Martino, M.; Woeltjen, J.; O’Dowd, C. D. Geophys. Res. Lett. 2008, 35, 5. (9) Donaldson, D. J.; Vaida, V. Chem. ReV. 2006, 106, 1445. (10) Gill, P. S.; Graedel, T. E.; Weschler, C. J. ReV. Geophys. 1983, 21, 903. (11) Ellison, G. B.; Tuck, A. F.; Vaida, V. J. Geophys. Res. 1999, 104, 11633. (12) Tabazadeh, A. Atmos. EnViron. 2005, 39, 5472.
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