Characterizing the Morphology of Organic Aerosols at Ambient

Aug 20, 2010 - Part of the special issue “Atmospheric Analysis as Related to Climate Change”. , * To whom correspondence should be addressed. E-ma...
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Anal. Chem. 2010, 82, 7965–7972

Characterizing the Morphology of Organic Aerosols at Ambient Temperature and Pressure† Miriam A. Freedman,‡,⊥ Kelly J. Baustian,‡,§ Matthew E. Wise,‡ and Margaret A. Tolbert*,‡,| Cooperative Institute for Research in Environmental Sciences, Department of Atmospheric and Oceanic Sciences, and Department of Chemistry and Biochemistry, University of Colorado Boulder, Colorado 80309 The aerosol direct effect, which characterizes the interaction of radiation with aerosol particles, remains poorly understood. By determining aerosol composition, shape, and internal structure, we can predict aerosol optical properties. In this study, we performed a feasibility study to determine if tapping-mode atomic force microscopy (TM-AFM) and Raman microscopy can be effectively used to obtain information on aerosol composition, shape, and structure. These techniques are advantageous because they operate under ambient pressure and temperature. We worked with model aerosol particles composed of organic components of varying solubility mixed with ammonium sulfate. In particular, we explored whether aerosols could be differentiated on the basis of the solubility of the organic component. We also characterized the aerosol internal structure and investigated how this structure changes as the solubility of the organic compound is varied. To obtain indirect chemical information from AFM, we imaged particles supported on both polar, SiOx/ Si(100), and nonpolar, highly ordered pyrolytic graphite, surfaces. We have found that AFM can be used to differentiate the solubility of the organic component. In some cases, AFM can also be used to identify internal structure. With Raman microscopy, we can differentiate between core-shell structures and homogeneous structures. Surprisingly, we find that even for the most soluble compounds, core-shell structures are observed. To discuss consequences of our results for climate studies, we calculate the difference in radiative forcing caused by having a core-shell aerosol rather than a homogeneous particle. Overall, these techniques are promising for characterizing composition, shape, and internal structure of atmospheric particles. †

Part of the special issue “Atmospheric Analysis as Related to Climate Change”. * To whom correspondence should be addressed. E-mail: tolbert@ colorado.edu. ‡ Cooperative Institute for Research in Environmental Sciences. § Department of Atmospheric and Oceanic Sciences. | Department of Chemistry and Biochemistry. ⊥ Present address: Department of Chemistry, The Pennsylvania State University, University Park, PA 16802. 10.1021/ac101437w  2010 American Chemical Society Published on Web 08/20/2010

The aerosol direct effect represents a large uncertainty in calculations of anthropogenic contributions to radiative forcing.1 The direct effect quantifies the radiative forcing due to the interaction of radiation with aerosol particles. The optical properties of aerosol particles are in turn determined by their composition, shape, and structure. If the shape and internal structure of the particles could be generalized as a function of composition, a better and more comprehensive understanding of aerosol optical properties could be developed. In addition, the structure of a particle gives insight into its physical and chemical properties, such as water uptake and reactivity. Aerosols containing organic compounds are of particular interest because the observed ambient weight fraction of these compounds ranges from 0.20 to 0.70.2 To understand the interaction of radiation with complex ensembles of particles, we must first gain insight into how radiation interacts with single particles. By working with laboratory samples rather than field samples, we have more control of the systems and can evaluate the validity of approximations used to model the optical properties of aerosol particles. Using single particle studies, we can investigate the range of structures and shapes formed in the atmosphere or in laboratory studies. Knowledge of the internal structure and shape of individual particles and the range of structures formed allows us to predict ensemble optical properties. Several studies of aerosols have directly shown the presence of complex aerosol internal structures.3,4 Transmission electron microscopy (TEM) is the most commonly used technique to probe aerosol structure directly. In general, capabilities to perform electron microscopy are widespread, making it a convenient and powerful technique to characterize field samples. TEM instruments are often paired with electron dispersion spectroscopy (EDS) or electron energy loss spectrometry (EELS) instruments, which allow the user to obtain elemental information and/or valence state. Through these additional microscopies, information about particle composition can be obtained. TEM has significant disadvantages for the study of the aerosol particles, however, which limits its utility. A sample in a standard (1) Solomon, S.; Qin, D.; Manning, M.; Chen, Z.; Marquis, M.; Averyt, K. B.; Tignor, M.; Miller, H. L.; Eds.; Intergovernmental Panel on Climate Change, Climate Change 2007: The Physical Science Basis; Cambridge Univ. Press: New York, 2007. (2) Zhang, Q.; et al. Geophys. Res. Lett. 2007, 34, L13801. (3) Po´sfai, M.; Buseck, P. R. Annu. Rev. Earth Planet. Sci. 2010, 38, 17–43. (4) Moffet, R. C.; Henn, T. R.; Tivanski, A. V.; Hopkins, R. J.; Desyaterik, Y.; Kilcoyne, A. L. D.; Tyliszczak, T.; Fast, J.; Barnard, J.; Shutthanandan, V.; Cliff, S. S.; Perry, K. D.; Laskin, A.; Gilles, M. K. Atmos. Chem. Phys. 2010, 10, 961–976.

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TEM is held at pressures below 10-6 Torr, and in an environmental TEM, the maximum pressure is approximately 10 Torr.5 In both cases, semivolatile organic compounds that partition to the aerosol phase can evaporate under vacuum conditions, which is occasionally observed as a film surrounding a particle.6 In addition, resolution is determined by the focus of the electron beam. Higher resolution requires beams where more energy is directed into a given point, sometimes damaging the aerosol particles under study. Tapping-mode atomic force microscopy (TM-AFM) and Raman microscopy are performed under ambient conditions and, therefore, have the potential to be better for the study of organic aerosols. The absence of vacuum conditions limits the evaporation of the semivolatile organic components. Like TEM, these techniques also use standard instruments that are widely available and facile to use. AFM images particles in the nanometer size regime like electron microscopy techniques, but AFM is less likely to damage fragile samples. A few AFM studies to investigate field samples and laboratory-generated atmospheric aerosol particles have been performed.6–17 Like AFM, Raman microscopy is a single-particle technique that can be performed under ambient conditions, which has not been used widely for aerosol studies. Because it relies on optical microscopy to locate aerosols, particles must be in the micrometer size regime. The Raman spectrometer allows for more detailed chemical information about the particles to be obtained, in contrast to the elemental information acquired with EDS. A few studies have used Raman spectroscopy to investigate core-shell structures in laboratory-generated aerosols.18-20 In this study, we have investigated the internal structure of aerosol particles using AFM and Raman microscopies. We produced particles through the atomization of salts mixed with soluble and slightly soluble organics and by coating atomized salt particles with insoluble organics using a coating oven. AFM images were taken on polar and nonpolar substrates to obtain (5) Wise, M. E.; Biskos, G.; Martin, S. T.; Russell, L. M.; Buseck, P. R. Aerosol Sci. Technol. 2005, 39, 849–856. (6) Po´sfai, M.; Xu, H.; Anderson, J. R.; Buseck, P. R. Geophys. Res. Lett. 1998, 25 (11), 1907–1910. (7) Wittmaack, K.; Strigl, N. Environ. Sci. Technol. 2005, 39 (21), 8177–8184. (8) Friedbacher, G.; Grasserbauer, M.; Meslmani, Y.; Klaus, N.; Higatsberger, M. J. Anal. Chem. 1995, 67 (10), 1749–1754. (9) Gwaze, P.; Annegarn, H. J.; Huth, J.; Helas, G. Atmos. Res. 2007, 86 (2), 93–104. (10) Barkay, Z.; Teller, A.; Ganor, E.; Levin, Z.; Shapira, Y. Microsc. Res. Tech. 2005, 68, 107–114. (11) Abid, A. D.; Tolmachoff, E. D.; Phares, D. J.; Wang, H.; Liu, Y.; Laskin, A. Proc. Combust. Inst. 2009, 32, 681–688. (12) Mavrocordatos, D.; Kaegi, R.; Schmatloch, V. Atmos. Environ. 2002, 36 (36-37), 5653–5660. (13) Lanzuolo, G.; Sgro, L. A.; De Filippo, A.; Barone, A. C.; Borghese, A.; D’Alessio, A. Environ. Eng. Sci. 2008, 25 (10), 1365–1377. (14) Kollensperger, G.; Friedbacher, G.; Kotzick, R.; Niessner, R.; Grasserbauer, M. Fresenius’ J. Anal. Chem. 1999, 364 (4), 296–304. (15) Schmitz, I.; Schreiner, M.; Friedbacher, G.; Grasserbauer, M. Appl. Surf. Sci. 1997, 115 (2), 190–198. (16) Ramirez-Aguilar, K. A.; Lehmpuhl, D. W.; Michel, A. E.; Birks, J. W.; Rowlen, K. L. Ultramicroscopy 1999, 77 (3-4), 187–194. (17) Lehmpuhl, D. W.; Ramirez-Aguilar, K. A.; Michel, A. E.; Rowlen, K. L.; Birks, J. W. Anal. Chem. 1999, 71, 379–383. (18) Chan, M. N.; Lee, A. K. Y.; Chan, C. K. Environ. Sci. Technol. 2006, 40, 6983–6989. (19) Buajarern, J.; Mitchem, L.; Reid, J. P. J. Phys. Chem. A 2007, 111, 11852– 11859. (20) Wise, M. E.; Baustian, K. J.; Tolbert, M. A. Proc. Nat. Acad. Sci. 2010, 107, 6693–6698.

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indirect chemical information about the particles. Raman microscopy was used to explore chemical composition. EXPERIMENTAL SECTION A brief overview of our experimental procedure is given here with more details provided in the Supporting Information for this article. The aerosols studied in this paper are composed of palmitic acid (Sigma-Aldrich, 99%), succinic acid (Sigma-Aldrich, g99.0%, ACS reagent grade), sucrose (Mallinckrodt Baker, ACS reagent grade), glutaric acid (Sigma-Aldrich, 99%), and ammonium sulfate (Mallinckrodt Baker, g99.0%, ACS reagent grade). Chemicals were used without further purification. The properties of these compounds are given in the Supporting Information in Table S-1. Aqueous solutions were atomized (TSI 3076) and dried through a series of four driers to produce aerosol particles. Because palmitic acid is too insoluble to make an aqueous solution, mixed palmitic acid/ammonium sulfate particles were produced using a coating oven. This procedure produces ammonium sulfate particles that are coated in palmitic acid.20 Aerosol particles were collected on substrates via impaction using a pump running at the same speed as the aerosol flow. For the AFM studies, aerosol samples were deposited on SiOx/ Si(100) and highly ordered pyrolytic graphite (HOPG) substrates. AFM images were obtained in tapping mode using a Veeco Dimension 3100 AFM with a Nanoscope IVa controller. Silicon tips (Nanosensors PPP-FMR) with a spring constant of 2.5-4 N m-1 were used. The AFM images shown in the figures are in amplitude mode, which illustrates the boundaries of height differences on the sample. For Raman microscopy, aerosols were imaged on quartz substrates, which are treated with a commercial hydrophobic silanizing agent to produce a hydrophobic surface. An Olympus optical microscope with a 100× objective was used to detect the shape of the aerosol particles. An Almega Raman spectrometer was used to probe the chemical composition of individual aerosol particles using 532 nm light. Higher weight percent solutions (1-15 wt %) were used to increase the number of particles with diameters larger than 1 µm. RESULTS AND DISCUSSION Interpreting the AFM results depends upon understanding the process by which a particle is deposited on a surface. A growing literature exists on the fluid dynamics, spreading, and rebound of liquid droplets of sizes from hundreds of micrometers to millimeters on a variety of substrates.21 Depositional processes of nanometer to micrometer size particles have also been explored in terms of classical fluid dynamics.22 A microscopic understanding of how solid, nanoscale particles impact and deform on surfaces, to the best of our knowledge, is not available. We might expect that, when a particle is deposited on a surface, it impacts, deforms as it spreads along the surface, and restores due to intermolecular forces (Figure 1). Each of these processes depends on the physical properties of the aerosol particles and the substrate.

(21) Bergeron, V.; Bonn, D.; Martin, J. Y.; Vovelle, L. Nature 2000, 405 (6788), 772–775. (22) Friedlander, S. K. Smoke, Dust, and Haze: Fundamentals of Aerosol Dynamics, 2nd ed.; Oxford University Press: New York, 2000.

Figure 1. Illustration of the deposition of aerosol particles on surfaces. Particles deform when they impact on a surface. Due to intermolecular forces, some particles restore after they deform.

Figure 2. Schematic of a typical Raman microscopy experiment. Scans are taken 200 nm apart as the laser beam is scanned parallel to the substrate over the particle. The beam spot size is large compared to the particle size but can be used to differentiate the composition of the outer layer of the particle from the internal composition.

A schematic of how structural information is obtained from Raman microscopy is shown in Figure 2. For particles several micrometers in size, a large beam spot size of 1 to 2 µm allows the components of the surface of the particle to be differentiated from the components of the rest of the particle. As we scan the laser beam over the edge of the particle, we can differentiate core-shell from homogeneous structures if the surface species compose a sufficient thickness at the outermost layer of the particle. The particle can be scanned over in the xy plane (parallel to the substrate, across the particle) or along the z axis (perpendicular to the substrate, into the particle). Similar results are obtained in all cases, and below, we have only shown results obtained by scanning in the xy plane. Spectra were taken 200 nm apart. Because of the large beam spot size, internal structure cannot be obtained on our 2-4 µm diameter particles. Decreasing the beam spot size is possible but reduces the signal intensity. In addition,

Figure 3. AFM images of the palmitic acid/ammonium sulfate system on (a) HOPG and (b) SiOx/Si(100). The 10 µm × 10 µm images are shown in amplitude mode. Line scans through typical particles, which are marked in the images, are shown. The circled particle in (a) is enlarged in an inset. The width of this inset is 1 µm. Light lines running diagonally in the top right corner of (a) mark representative step edges between atomic layers of HOPG. Many of these ridges are visible on the HOPG substrate. In (b), a ridge of material originating from the aerosol particle is marked in the line scan.

focusing the beam increases the energy applied to a given area of the particle and can damage the samples, as is sometimes apparent by visual inspection. Below, we discuss our results for our AFM and Raman microscopy studies. Due to the different size ranges of particles explored, there may not be an exact correspondence between the two techniques. Because these are single-particle techniques, some degree of heterogeneity is visible in the AFM images and is observed in the Raman studies. Understanding the range of structures formed is of interest for better characterization of optical properties. Insoluble Organic Compound: Palmitic Acid. In Figure 3, AFM images of palmitic acid coated ammonium sulfate particles Analytical Chemistry, Vol. 82, No. 19, October 1, 2010

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are shown on nonpolar HOPG (Figure 3a) and polar SiOx/Si(100) (Figure 3b) substrates. The silicon sample varies by 1 to 2 nm over the surface. These differences are visible in the height image, but in the amplitude image, the substrate appears flat. The HOPG substrate varies more significantly in height over the sample due to the different atomic layers of the sample exposed. The boundaries between the atomic layers are visible, running roughly diagonal across the sample. For the palmitic acid coated ammonium sulfate particles on the nonpolar HOPG substrate, we observe particles with odd shapes (Figure 3a). Part of the particle retains a spherical structure, and additional material originating from the particle spreads out along the nonpolar HOPG surface. We show a larger picture of one of the structures and two line scans obtained from the height mode as insets. From these line scans, the spherical particle is observed to be a higher feature than the rest of the material. Because palmitic acid is hydrophobic and HOPG is nonpolar, the organic compound is likely to have a smaller contact angle with the substrate than the ammonium sulfate core. The images suggest that, upon impact with the HOPG surface, the ammonium sulfate core remains intact on the surface and the palmitic acid spreads along the HOPG. In contrast, in the image of palmitic acid coated ammonium sulfate on the polar silicon substrate (Figure 3b), the majority of particles are compact and roughly circular in shape. On close inspection, thin ridges can be seen around each particle. A line scan is shown through one particle with ridges around the particle indicated. The image suggests that the particles spread along the surface upon impact and then reformed spherical particles, due to the higher contact angle expected between the hydrophobic palmitic acid and the polar SiOx/Si(100) substrate. The attraction between the acid functional group and the polar SiOx/Si(100) substrate may cause the ridges. Note that, for this aerosol system, we are able to determine that the particles have a core-shell structure based only on the AFM images. The behavior of the aerosols upon deposition on the two different polarity substrates indicates that the palmitic acid is in the outer shell of the particles. Representative Raman spectra taken of a palmitic acid coated ammonium sulfate particle are shown in Figure 4. An image of the particle from which the spectra shown were acquired is displayed as viewed under 100× magnification in the Supporting Information (Figure S-1). As illustrated in Figure 2, the laser spot is scanned over the particle in the xy plane, and the spectra are displayed and numbered in the order of the scan with offsets for clarity. Spectrum 1 has signal only from the quartz substrate. In spectrum 6, features are visible from both palmitic acid and ammonium sulfate. Features are marked on the spectra corresponding to the broad ammonium N-H stretch (approximately 3100 cm-1), the sharp palmitic acid C-H stretch (approximately 2900 cm-1), and a sharp feature due to SO42- (approximately 980 cm-1). Spectra 2 and 3 show features resulting only from palmitic acid. Spectra 4-6 show features from both palmitic acid and ammonium sulfate. These spectra suggest that the outer layer of the particle is composed only of palmitic acid. The particles were produced to have core-shell structures, and like the AFM results, the Raman spectra confirm this structure. 7968

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Figure 4. Raman spectra of the circled particle in Figure S-1 (Supporting Information) obtained as described in Figure 2. The bottom spectrum (#1) is taken at the edge of the particle with the beam focused over the quartz substrate. The spectra are offset for clarity. Raman-active vibrational modes are marked for the ammonium N-H stretch, the palmitic acid C-H stretch, and for a vibration of sulfate.

Slightly Soluble Organic Compound: Succinic Acid. AFM images of particles composed of a mixture of succinic acid and ammonium sulfate on HOPG and SiOx/Si(100) are shown in Figure 5. We obtained images from samples with organic weight fractions of 0.0-1.0. In Figure 5, we show images obtained using an organic weight fraction of 0.75. We have previously shown through aerosol mass spectrometry, which measures the average composition of the particles, that the resulting aerosols have the same weight fraction as the atomized solution.23 On the nonpolar HOPG substrate, the particles impact on the surface and form conical structures. A line scan through the largest particle shows the shape of the particle. The succinic acid/ ammonium sulfate particles deform and restore but do not leave residual material on the surface due to the repulsion between the succinic acid and the nonpolar HOPG. The roughness of the HOPG surface may also affect the shape of the particles. Figure 5b demonstrates the effect of depositing the succinic acid/ammonium sulfate solution onto the polar SiOx/Si(100) substrate. In this figure, flat terraces with fluid-looking boundaries surround a circular core. A line scan through one of the particles shows the terraces. Some degree of heterogeneity in the sizes and presence or absence of cores and surrounding material is observed. Because the components of the particles are polar and the substrate is polar, we expect a small contact angle between the particles and the substrate. When the particles impact and deform on the surface, they stay spread along the surface due to the attraction between the components of the particles and the substrate. For all organic weight fractions including pure ammonium sulfate and pure succinic acid, we observe similar images, with only subtle differences. In other words, for all weight fractions, we observe spreading on the polar SiOx/Si(100) substrate and conical structures on HOPG. We cannot, therefore, easily differentiate whether succinic acid or ammonium sulfate is the component spreading on the polar SiOx/Si(100) substrate. (23) Freedman, M. A.; Hasenkopf, C. A.; Beaver, M. R.; Tolbert, M. A. J. Phys. Chem. A 2009, 113, 13584–13592.

Figure 6. Raman spectra of the succinic acid/ammonium sulfate system acquired as illustrated and described in Figures 2 and 4. The bottom spectrum (#1) is taken at the edge of the particle with the beam focused over the quartz substrate.

Figure 5. AFM images of the succinic acid/ammonium sulfate system at an organic weight fraction of 0.75 on (a) HOPG and (b) SiOx/Si(100). The 5 µm × 5 µm images are shown in amplitude mode. Line scans through representative particles that are marked in the image are also shown. In (b), terraces arising from an aerosol component that spreads along the SiOx/Si(100) substrate are marked in the line scan. A large particle is present in the lower left-hand corner of (b).

An image of succinic acid/ammonium sulfate particles at an organic weight fraction of 0.75 under 100× magnification is shown in the Supporting Information (Figure S-2). Raman spectra for an approximately 2 µm diameter particle with an organic weight fraction of 0.75 are shown in Figure 6. The same methods are used to obtain spectra as described for Figure 4. Succinic acid has a sharp feature due to C-H stretching at approximately 2900 cm-1. Spectra 1 and 2 have features only from succinic acid. Spectra 3-5 have features due to both succinic acid and ammonium sulfate. These results suggest that organic coatings are present on the particles at high organic weight fractions. We did not observe the presence of coatings at lower organic weight fractions due to resolution limitations. We tried several

methods for making particles to increase the thickness of the shell such as nebulization, and all gave similar results to the atomized sample. Soluble Organic Compound: Glutaric Acid. To investigate structure in aerosol particles with soluble organic compounds, we worked with mixtures of glutaric acid with ammonium sulfate. An AFM image of glutaric acid/ammonium sulfate particles at an organic weight fraction of 0.75 deposited on a nonpolar HOPG substrate is shown in Figure 7a. The particles remain circular in form due to the large contact angle between the hydrophilic glutaric acid and the nonpolar substrate. Two types of particles are observed on the substrate. Some particles, including the largest ones, have fluid boundaries (#1) and other particles have more solid-looking boundaries (#2). Figure 8b shows an AFM image of particles at an organic weight fraction of 0.50 on the polar SiOx/Si(100) substrate. Some components of the particles have spread along the surface, creating large bumps visible in line scan #1. Many particles also have numerous “satellite” particles surrounding them. Line scan #2 is taken through a particle with some spreading along the surface and satellite particles. The satellite particles may be due to splattering of wet particles on the surface as they impact. Images taken at an organic weight fraction of 0.75 show similar features. An image of the glutaric acid/ammonium sulfate particles at an organic weight fraction of 0.75 acquired under 100× magnification is shown in Figure S-3 (Supporting Information). Some of the particles in the image are more spherical in shape with dark gray edges and light gray centers (#1). Other particles are more uniform in color and less spherical in shape (#2). Raman microscopy shows that some of the particles deposited on the substrate are wet and others are dry. We show spectra of a typical dry particle (Figure 8a) and a typical wet particle (Figure 8b) at an organic weight fraction of 0.75. The wet particle has an additional peak at approximately 3450 cm-1 due to the O-H stretch of water. For the dry particle, spectra 1 and 2 have signal only from glutaric acid and spectra 3-6 have signal from both glutaric acid and ammonium sulfate, indicating that the particle has a shell of organic material. The wet glutaric acid/ammonium sulfate particles have Analytical Chemistry, Vol. 82, No. 19, October 1, 2010

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Figure 8. Raman spectra of (a) dry and (b) wet glutaric acid/ ammonium sulfate particles at an organic weight fraction of 0.75. Raman spectra are acquired and marked as described in Figures 2 and 4. The Raman-active vibrational mode arising from the water O-H stretch is indicated.

Figure 7. (a) An AFM image of glutaric acid/ammonium sulfate particles with an organic weight fraction of 0.75 on the nonpolar HOPG substrate. The two different types of particles observed on the substrate are indicated. (b) Particles with an organic weight fraction of 0.50 on SiOx/Si(100). Line scans through two particles are shown. A small particle surrounding a larger particle is indicated in the line scan.

homogeneous structures, where both glutaric acid and ammonium sulfate are visible in spectra 2-6. Highly Soluble Organic Compound: Sucrose. AFM images of sucrose and ammonium sulfate particles are shown in Figure 9. The images show isolated particles both on the nonpolar HOPG substrate (Figure 9a) and on the polar SiOx/ Si(100) surface (Figure 9b). On the HOPG substrate, the organic weight fraction of the particles is 0.75. Because sucrose is highly soluble and the HOPG substrate is nonpolar, we would predict similar results as for succinic acid/ammonium sulfate on HOPG. As expected, the particles did not spread along the surface. Some of the particles appear less homogeneous in their features than the succinic acid/ammonium sulfate system, having both fluidand solid-looking boundaries. 7970

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Figure 9b shows sucrose/ammonium sulfate particles at an organic weight fraction of 0.50 on a polar SiOx/Si(100) surface. The particles appear to have splattered onto the substrate as evidenced by each large particle being surrounded by many smaller particles. We have observed splattering in AFM images for other highly soluble organic aerosols as well. An image of sucrose/ammonium sulfate at an organic weight fraction of 0.75 under a magnification of 100× is shown in Figure S-4 (Supporting Information). Raman spectra are shown in Figure 10. The feature at approximately 3450 cm-1 suggests that water is present. The particles were subsequently further dried overnight, but they still retained water. Surprisingly, the particles are not homogeneous. Spectrum 1 shows background from the quartz substrate. Spectrum 2 has signal only from sucrose. Spectra 3-6 show both sucrose and ammonium sulfate. These results suggest that a shell of sucrose is present on the particle. Because the water feature is broad and low in intensity, it is unclear from the spectra shown whether water is also present in the shell. Summary of Results and Future Outlook. In short, our results from AFM and Raman microscopy demonstrate that we can observe core-shell structures in organic aerosols using AFM and Raman microscopy. AFM has given definitive information about structure in experiments with particles composed of palmitic acid/ammonium sulfate and was suggestive of the presence of water in the glutaric acid/ammonium sulfate and sucrose/ammonium sulfate systems. In addition, using AFM alone, we can identify the aerosol shape and the solubility of the

Figure 9. (a) AFM images of the sucrose acid/ammonium sulfate system at an organic weight fraction of 0.75 on HOPG. (b) Sucrose/ ammonium sulfate particles at an organic weight fraction of 0.50 on SiOx/Si(100).

Figure 10. Raman spectra of the sucrose/ammonium sulfate system, as described in Figures 2, 4, and 8.

organic compound based on its impaction onto polar and nonpolar substrates. Raman microscopy gave evidence for core-shell structures for the particles containing palmitic acid or sucrose, for high organic weight fractions of succinic acid/ammonium sulfate, and for the dry glutaric acid/ammonium sulfate particles. Surprisingly, even some of the most soluble organic compounds mixed with ammonium sulfate were found to have core-shell structures. In fact, the wet glutaric acid/ammonium sulfate particles were the only homogeneous structures observed. Our results suggest that we may obtain better predictions for aerosol properties if we assume that particles have core-shell rather than homogeneous structures. In future experiments, we will focus on further development of these techniques to assess the size of the coating and perform a more detailed examination of internal structure to determine whether additional heterogeneities are present. In particular, AFM imaging in phase mode may allow for more detailed information on internal structure. One area of the literature in which partitioning of the organic component has been considered is for determining properties of aerosol particles at high relative humidities (99-100%) and in the supersaturated regime. In order to model hygroscopicity and the critical supersaturation needed for the formation of clouds, both surface tension and the partitioning of the organic component to the surface of the aerosol need to be considered.24,25 Our results suggest that we may also find that results for aerosol optical properties and hygroscopicity in the subsaturated regime are better modeled if the partitioning of the organic component is included. Atmospheric Implications. We have demonstrated that, regardless of solubility, many organic compounds partition to the surface layer of the aerosol particles. To estimate possible differences in radiative forcing, we calculated the extinction cross sections and asymmetry parameters resulting from homogeneous and core-shell particles. The asymmetry parameter arises due to the difference in forward and backward scattering by a particle. For these calculations, we used MATLAB versions of Mie codes adapted from Bohren and Huffman.26,27 We assumed all the particles were dry. For the core-shell particles, we assumed that the organic component formed a shell around an ammonium sulfate core. We performed calculations for a wavelength of 532 nm and at particle diameters of 100-1000 nm for all the organic compounds in this study. Because sucrose and ammonium sulfate have similar refractive indices, however, the effect of having a homogeneous vs core-shell internal structure is minimal. The other three organic compounds have similar refractive indices and, therefore, give similar results for extinction cross section and asymmetry parameter. Figure 11a shows the difference between extinction cross sections for homogeneous particles with volumeweighted refractive indices and core-shell particles for a particle diameter of 700 nm as a function of organic weight fraction. In general, the differences between the extinction cross sections calculated for the two different internal structures increases with (24) Ruehl, C. R.; Chuang, P. Y.; Nenes, A. Atmos. Chem. Phys. 2010, 10, 1329– 1344. (25) Sorjamaa, R.; Laaksonen, A. Aerosol Sci. 2006, 37, 1730–1736. (26) Bohren, C. F.; Huffman, D. R. Absorption and Scattering of Light by Small Particles; Wiley-VCH; Weinheim, Germany, 2004. (27) Ma¨tzler, C. IAP Research Report, No. 2002-08 [Online]; Universita¨t Bern: Bern, Switzerland, 2002; http://Diogenes.iwt.unibremen.de/vt/laser/wriedt/ Mie_Type_Codes/body_mie_type_codes.html (accessed May 2008).

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where ∆FR is the ratio of radiative forcing for core-shell particles compared with homogeneous particles and gcs and gh are the asymmetry parameters for core-shell and homogeneous structures, respectively. We calculate up to a 6% difference in the radiative forcing ratio in comparisons of the two different internal structures. The core-shell structure leads to increased scattering. Because the asymmetry parameter is smaller for the core-shell particles than for the homogeneous particles, the numerator of eq 1 is greater than the denominator, leading to the enhancement in forcing caused by the core-shell particles. Figure 11b shows the results of eq 1 for the palmitic acid/ammonium sulfate system at several organic weight fractions. The ratio is greatest for the particles with the largest diameters.

Figure 11. (a) Extinction cross sections calculated for homogeneous and core-shell particles with a diameter of 700 nm. The solid lines indicate results for homogeneous particles, and the dashed lines are results for core-shell particles. The abbreviations shown in the legend are as follows: PA/AS is palmitic acid/ammonium sulfate, SA/AS is succinic acid/ammonium sulfate, suc/AS is sucrose/ammonium sulfate, and GA/AS is glutaric acid/ammonium sulfate. In all cases, the particles are assumed to be dry. (b) Radiative forcing ratios for the palmitic acid/ammonium sulfate system for organic weight fractions of 0.25, 0.50, and 0.75. The ratios were calculated as described in the text.

diameter, but due to the oscillations in the Mie scattering curves, the trend is not linear or monotonic. The sign of the difference also changes with particle size. For the organic compounds besides sucrose and for particles approximately 550 nm and smaller, the extinction cross section for the core-shell particles is larger than for the homogeneous particles. For sizes between 600 and 1000 nm, the extinction cross section is greater for the homogeneous particles. To determine the effect of the different internal structures on radiative forcing, we use an approach developed by Chylek and Wong.28 To compare particles with different extinctions, we use the equation

∆FR )

(1 - gcs /2) (1 - gh /2)

(28) Chylek, P.; Wong, J. Geophys. Res. Lett. 1995, 22 (8), 929–931.

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CONCLUSION Development of new, nonvacuum techniques to probe aerosol internal structure is essential for characterizing the structure of organic aerosols. With knowledge of structure, aerosol optical properties can be determined. Understanding aerosol optical properties will aid characterization of the aerosol direct effect. Our results test the effectiveness of the use of AFM and Raman microscopy to obtain information about aerosol internal structure. Using AFM images taken on polar and nonpolar substrates, we can identify the solubility of the organic component. In addition, a clear indication of core-shell structures was obtained when the components of the aerosol particle had large differences in solubility. With Raman microscopy, we observe core-shell structures for the majority of the systems studied. Our results suggest that core-shell structures are more prevalent in organic aerosols than previously assumed. Mie scattering theory showed up to a 6% difference in radiative forcing resulting from core-shell vs homogeneous internal structures, where core-shell structures result in additional scattering. ACKNOWLEDGMENT ThisworkwassupportedthroughNASAGrantNos.NNG06GE79G, NNX09AE12G, and NSF-ATM-0650023. Atomic force microscopy was performed at the University of Colorado Nanomaterials Characterization Facility. M.A.F. acknowledges support from the NOAA Climate and Global Change Postdoctoral Fellowship Program administered by the University Corporation for Atmospheric Research. K.J.B. received support from NASA (NESSF fellowship NNX08AU77H). SUPPORTING INFORMATION AVAILABLE Additional information as noted in text. This material is available free of charge via the Internet at http://pubs.acs.org.

(1) Received for review June 1, 2010. Accepted July 29, 2010. AC101437W