Combined Use of Optical and Electron Microscopic Techniques for the

Aug 10, 2010 - Hyo-Jin Eom , Dhrubajyoti Gupta , Xue Li , Hae-Jin Jung ... Zhangpeng Bai , Yuan Ji , Yiqun Pi , Kaixiang Yang , Li Wang , Yinqi Zhang ...
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Anal. Chem. 2010, 82, 7999–8009

Combined Use of Optical and Electron Microscopic Techniques for the Measurement of Hygroscopic Property, Chemical Composition, and Morphology of Individual Aerosol Particles† Kang-Ho Ahn,† Sun-Man Kim,† Hae-Jin Jung,‡ Mi-Jung Lee,‡ Hyo-Jin Eom,‡ Shila Maskey,‡ and Chul-Un Ro*,‡ Department of Mechanical Engineering, Hanyang University, Ansan, 425-791, Korea, and Department of Chemistry, Inha University, Incheon, 402-751, Korea In this work, an analytical method for the characterization of the hygroscopic property, chemical composition, and morphology of individual aerosol particles is introduced. The method, which is based on the combined use of optical and electron microscopic techniques, is simple and easy to apply. An optical microscopic technique was used to perform the visual observation of the phase transformation and hygroscopic growth of aerosol particles on a single particle level. A quantitative energydispersive electron probe X-ray microanalysis, named low-Z particle EPMA, was used to perform a quantitative chemical speciation of the same individual particles after the measurement of the hygroscopic property. To validate the analytical methodology, the hygroscopic properties of artificially generated NaCl, KCl, (NH4)2SO4, and Na2SO4 aerosol particles of micrometer size were investigated. The practical applicability of the analytical method for studying the hygroscopic property, chemical composition, and morphology of ambient aerosol particles is demonstrated. Studies on the heterogeneous chemistry of aerosol particles in the air have become important. Since airborne particles can react in the air with gaseous pollutants, such as SOx or NOx, and their physicochemical properties can be modified through heterogeneous chemical reactions, the modified chemical properties of the aerosol particles can change the climate through their modified direct radiative forcing properties and their modified effectiveness to serve as cloud condensation nuclei. In addition, the chemistry of the Earth’s atmosphere is influenced by reducing photolysis rates of important atmospheric gas-phase species through heterogeneous chemical reactions with the atmospheric aerosol particles. Hence, increasing attention has been devoted to the study of physicochemical characteristic changes of aerosol particles.1,2 † Part of the special issue “Atmospheric Analysis as Related to Climate Change”. * To whom correspondence should be addressed. Tel.: +82 32 860 7676. Fax: +82 32 867 5604. E-mail: [email protected]. † Hanyang University. ‡ Inha University. (1) Rossi, M. J. Chem. Rev. 2003, 103, 4823–4882.

10.1021/ac101432y  2010 American Chemical Society Published on Web 08/10/2010

Up to now, it has been well recognized that the hygroscopic properties of aerosols have played important roles in some heterogeneous reactions, for example, the reaction of CaCO3 particles to form nitrate.3-5 CaCO3 particles originally do not have significant amounts of water on the surface, and yet when Ca(NO3)2 species is formed on the surface as a reaction product between CaCO3 and HNO3, the surface of the particles becomes hygroscopic. Gaseous HNO3 species can dissolve easily, resulting in the total consumption of CaCO3 species and the completion of irreversible heterogeneous reaction. Thus, Ca(NO3)2 particles were observed as liquid droplets when they were formed in the air.6 Since the understanding of the hygroscopic properties of airborne particles is important in the study of physicochemical characteristic changes of aerosol particles, there have been many studies on the deliquescence and efflorescence behavior of aerosol particles.7-24 The sizes of hygroscopic particles in the atmosphere can vary depending on the ambient relative humidity (RH). In (2) Usher, C. R.; Michel, A. E.; Grassian, V. H. Chem. Rev. 2003, 103, 4883– 4940. (3) Laskin, A.; Wietsma, T. W.; Krueger, B. J.; Grassian, V. H. J. Geophys. Res. 2005, 110, D10106, 10.1029/2004JD005469. (4) Krueger, B. J.; Grassian, V. H.; Laskin, A. Atmos. Environ. 2004, 38, 6253– 6261. (5) Krueger, B. J.; Grassian, V. H.; Iedema, M. J.; Cpwin, J. P.; Laskin, A. Anal. Chem. 2003, 75, 5170–5179. (6) Hwang, H.; Ro, C.-U. Atmos. Environ. 2006, 40, 3869–3880. (7) Tang, I. N.; Munkelwitz, H. R. J. Appl. Meteorol. 1994, 33, 791–796. (8) Tang, I. N.; Munkelwitz, H. R. J. Geophys. Res. 1994, 99, 18801–18808. (9) Tang, I. N. J. Geophys. Res. 1996, 101, 19245–19250. (10) Tang, I. N. J. Geophys. Res. 1997, 102, 1883–1893. (11) Tang, I. N.; Tridico, A. C.; Fung, K. H. J. Geophys. Res. 1997, 102, 23269– 23275. (12) Tang, I. N.; Fung, K. H. J. Chem. Phys. 1997, 106, 1653–1660. (13) Cohen, M. D.; Flagan, R. C.; Seinfeld, J. H. J. Phys. Chem. 1987, 91, 4563– 4574. (14) Cohen, M. D.; Flagan, R. C.; Seinfeld, J. H. J. Phys. Chem. 1987, 91, 4575– 4582. (15) Cohen, M. D.; Flagan, R. C.; Seinfeld, J. H. J. Phys. Chem. 1987, 91, 4583– 4590. (16) Weingartner, E.; Gysel, M.; Baltensperger, U. Environ. Sci. Technol. 2002, 36, 55–62. (17) Gysel, M.; Weingartner, E.; Baltensperger, U. Environ. Sci. Technol. 2002, 36, 63–68. (18) Lee, C.-T.; Hsu, W.-C. J. Aerosol Sci. 1998, 29, 827–837. (19) Lee, C.-T.; Hsu, W.-C. J. Aerosol Sci. 2000, 31, 189–197. (20) Ge, Z.; Wexler, A. S.; Johnston, M. V. J. Colloid Interface Sci. 1996, 183, 68–77.

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other words, particles can grow by absorbing water with the increase of RH or can shrink when water evaporates with the decrease of RH. The size of a dry particle remains unchanged with the increase of RH until the deliquescence RH (DRH) is reached. At their DRH, the soluble particles become aqueous droplets, and they experience hygroscopic growth above their DRH. When RH is decreased, the concentration of salts in the aqueous droplets becomes dense and the inorganic salts can finally be crystallized at their efflorescence RH (ERH). It is also known that the ERH is sometimes significantly lower than the DRH, which is called hysteresis. Studies on the hygroscopic properties of particles with various chemical compositions have been carried out using many different analytical techniques, such as a single-particle levitation technique,7–15 the use of a humidified tandem differential mobility analyzer (HTDMA),16,17,24–27 the measurement of liquid water content using a thermal conductivity detector,18,19 a rapid singleparticle mass spectrometry,20,21 an environmental scanning electron microscopy (ESEM),22,23 an environmental transmission electron microscopy (ETEM),28 micro-Raman,29,30 and micro-FTIR.31 All the analytical techniques have proved to be very useful for studies on the hygroscopic properties of many different types of particles under controlled humidity. However, there have not been many hygroscopic and chemical compositional studies of real ambient aerosol particles on a single particle basis. Recently, an HTDMA system combined with a single-particle mass spectrometer showed its potential of simultaneously determining the hygroscopic property, chemical composition, and density of individual ambient particles.32 A similar technique was applied to determine the chemical composition as a function of the hygroscopicity of the ambient aerosol samples collected in Zurich and at an alpine research station Jungfraujoch.33 Indeed, the solid information on both the hygroscopic property and chemical composition of airborne particles can be obtained by using a single particle approach since the atmospheric particles are chemically heterogeneous. This multidimensional information on ambient single particles is valuable for understanding the fundamentals of ambient aerosols. Since the techniques provide growth factors at a certain RH for the same individual (21) Ge, Z.; Wexler, A. S.; Johnston, M. V. J. Phys. Chem. A 1998, 102, 173– 180. (22) Ebert, M.; Inerle-Hof, M.; Weinbruch, S. Atmos. Environ. 2002, 36, 5909– 5916. (23) Hoffman, R. C.; Laskin, A.; Finlayson-Pitts, B. J. J. Aerosol Sci. 2004, 35, 869–887. (24) Mikhailov, E.; Vlasenko, S.; Martin, S. T.; Koop, T.; Po ¨schl, U. Atmos.Chem.Phys. 2009, 9, 9491–9522. (25) Chan, M. N.; Chan, C. K. Atmos. Chem. Phys. 2005, 5, 2703–2712. (26) Vlasenko, A.; Sjogren, S.; Weingartner, E.; Gaggeler, H. W.; Ammann, M. Aerosol Sci. Technol. 2005, 39, 452–460. (27) Park, K.; Kim, J.-S.; Miller, A. L. J. Nanopart. Res. 2009, 11, 175–183. (28) Wise, M. E.; Semeniuk, T. A.; Bruintjes, R.; Martin, S. T. J. Geophys. Res. 2007, 112, D10224, 10.1029/2006JD007678. (29) Wang, L.-Y.; Zhang, Y.-H.; Zhao, L.-J. J. Phys. Chem. A 2005, 109, 609– 614. (30) Xiao, H.-S.; Dong, J.-L.; Wang, L.-Y.; Zhao, L.-J.; Wang, F.; Zhang, Y.-H. Environ. Sci. Technol. 2008, 42, 8698–8702. (31) Liu, Y.; Yang, Z.; Desyaterik, Y.; Gassman, P. L.; Wang, H.; Laskin, A. Anal. Chem. 2008, 80, 633–642. (32) Zelenyuk, A.; Imre, D.; Han, J.; Oatis, S. Anal. Chem. 2008, 80, 1401– 1407. (33) Herich, H.; Kammermann, L.; Gysel, M.; Weingartner, E.; Baltensperger, U.; Lohmann, U.; Cziczo, D. J. J. Geophys. Res 2008, 113, D16213, 10.1029/ 2008JD009954.

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particles,32,33 the full hygroscopic properties of the particles, such as DRH, ERH, and humidifying and dehydration curves, which provide detailed description of their hygroscopic properties, cannot be elucidated. Several applications for the simultaneous measurement of the hygroscopic property and chemical composition of individual ambient particles have been performed using ESEM or ETEM.22,23,34,35 The hygroscopic property of individual particles can be obtained from secondary/transmission electron images showing morphological change according to the controlled change of the water vapor contents. From X-ray spectral analysis, the elemental chemical compositions of individual particles can be obtained. These techniques are powerful in providing detailed information on the hygroscopic property and chemical composition of ambient aerosols. Without some practical difficulties in performing the experiment, the techniques would have a much wider application for ambient aerosol characterization. In this work, an analytical method for characterizing the hygroscopic property, chemical composition, and morphology of individual ambient aerosol particles is introduced. The method, which is based on the combined use of optical and electron microscopic techniques, is simple and easy to apply. An optical microscopic technique was used to perform the visual observation of the phase transformation and hygroscopic growth of aerosol particles on a single particle level. A quantitative energy-dispersive electron probe X-ray microanalysis (ED-EPMA), named low-Z particle EPMA, was used to perform a quantitative chemical speciation of the same individual particles after the measurement of the hygroscopic property. In order to validate the analytical methodology, the hygroscopic properties of artificially generated NaCl, KCl, (NH4)2SO4, and Na2SO4 aerosol particles of micrometer size were investigated. The practical applicability of the analytical method for studying the hygroscopic property, chemical composition, and morphology of supermicron ambient aerosol particles is demonstrated. EXPERIMENTAL SECTION Samples. To validate the analytical methodology, laboratorygenerated standard particles, such as NaCl, KCl, (NH4)2SO4, and Na2SO4 (all the chemicals used in this work, purchased from Aldrich, had >99.99% purity) were deposited by the nebulization onto 200 mesh Cu TEM grids (3.0 mm in diameter) coated with Formvar stabilized with carbon (Ted Pella, Inc.). Formvar is a registered trade name for polyvinyl formal, which is a family of polymers formed from polyvinyl alcohol and formaldehyde as copolymers with polyvinyl acetate. To measure both the hygroscopic property and chemical compositions of the same individual particles with an optical microscopic system and low-Z particle EPMA, it is necessary to work with the TEM grid as the same particles can be easily identified by using a center mark in the grid and grid patterns. An atomizer (HCT4810, single jet atomizer) was used to generate particles of micrometer size by nebulization using compressed N2 (99.999% purity). The solution droplets were dried by passing (34) Shi, Z.; Zhang, D.; Hayashi, M.; Ogata, H.; Ji, H.; Fujiie, W. Atmos. Environ. 2008, 42, 822–827. (35) Semeniuk, T. A.; Wise, M. E.; Martin, S. T.; Russell, L. M.; Buseck, P. R. Atmos. Environ. 2007, 41, 6225–6235.

Figure 1. Schematic diagram of the measurement setup for hygroscopic properties of individual particles.

through a silica packed diffusion dryer (residence time of ∼2 s, HCT4920 Diffusion dryer). Ambient aerosol particles were collected on the Cu TEM grid using a three-stage Dekati PM-10 cascade impactor. The Dekati PM-10 impactor has, at a sampling flow of 10 L/min, aerodynamic cutoffs of 10, 2.5, and 1 µm for stages 1-3, respectively. A stage 2 sample (2.5-10 µm size range) was used in this work. The sample was collected on January 15, 2010, on the roof of a campus building at Inha University (15 m above ground level), which is located in Incheon, Korea. Incheon is densely populated (population ) 2.7 million, area ) 427 km2) and has many different local emission sources, including seven industrial complexes, two seaports with ten wharfs, and one international airport. This sample was expected to be a heterogeneous mixture of coarsemode urban particles, which could be useful for investigating the practical applicability of the combined use of optical and electron microscopic techniques. Overall Description of Analysis Procedure. The hygroscopic property, chemical composition, and morphology of the same individual particles were obtained by the combined use of a “see-through” inertial impactor apparatus equipped with an optical microscope and a SEM instrument attached with an ultrathin window energy-dispersive X-ray (EDX) detector. In the see-through impactor apparatus, either ambient or generated aerosols were collected on an impact plate. N2 gas with controlled RH was supplied over the aerosol particles in the impactor. Optical images that contained information on the phase transformation and hygroscopic growth of individual particles on an image field according to RH change were recorded by using a digital camera through an optical microscope. The morphology and chemical compositions of the same individual particles were determined by the use of low-Z particle EPMA. The optical and secondary electron images were used to find the same individual particles. Hygroscopic Property Measurement System. Figure 1 schematically represents the measurement setup for the hygroscopic properties of individual particles, which is composed of three parts for (A) introducing generated aerosols on the impact plate of the inertial, see-through impactor, (B) introducing N2 gas with controlled humidity, and (C) observing the phase trans-

formation and hygroscopic growth of individual aerosol particles collected on the impact plate. For the collection of particles on the impact plate, 3-way valve was switched on to introduce either generated or ambient aerosols with a vacuum pump turned on. An atomizer was used to generate aerosol particles in the typical size range of 0.5-5 µm. An enlarged schematic of the impactor nozzle is shown in an inset of Figure 1. The jet-to-plate distance (S ) 0.6 mm) and the nozzle throat length (T ) 0.4 mm) for the given throat diameter (W ) 1.4 mm) were chosen for a light source to sufficiently illuminate the impact plate brightly. With a sampling flow of 4.0 L/min, the aerodynamic cutoff diameter of this impactor is 1.0 µm.36 Although this seethrough impactor can be used for the collection of ambient aerosols, ambient aerosol particles can also be collected on a collecting substrate using conventional cascade impactors, such as a Dekati PM-10 sampler. Then, the collecting substrate with particles can be put on the impact plate of the see-through impactor for the hygroscopic measurement. Once the collection of particles was finished, a 3-way valve was switched on to introduce N2 gas with controlled RH, that is, dry N2 gas passed through a bubbler containing deionized water that would become saturated with water. The saturated and dry N2 gases were mixed at different flow rates controlled by two mass flow controllers, resulting in a N2 gas flow with controlled RH, which went into the impactor. The RH, monitored by a digital hygrometer (Testo 645, ±1%), varied from ∼3% to ∼95% by 0.1-0.3% steps. The digital hygrometer was calibrated by using a dew-point hygrometer (M2 Plus-RH, GE), resulting in RH readings within 0.3% accuracy. By increasing and decreasing RH, humidifying and dehydration curves were obtained for all individual particles seen on an optical image field. To achieve a steady state for condensing or evaporating water, each humidity condition was sustained for two minutes. The particles collected on the impact plate were seen through a nozzle throat by the use of an optical microscope (Olympus, BX51M). The change in size of the individual particles resulting from the RH change was determined by analyzing the optical images (36) Yook, S. J.; Choi, Y. J.; Ahn, K.-H. Presented at the 6th International Aerosol Conference, September 9-13, 2002, Taipei, Taiwan, 239-240.

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Figure 2. Optical images of generated NaCl particles obtained at (A-D) RH ) 2.4%, 75.3%, 75.7%, and 94.0% during humidifying process and at (E-G) RH ) 47.7%, 47.6%, and 3.1% during dehydration process and (H) a SEI obtained after the hygroscopic measurement.

recorded by a digital camera (Canon EOS 5D, full frame, Canon EF f/3.5 L macro USM lens). The image size was 4368 × 2912 pixels, and the image recording condition was set as ISO200. The exposure time was 0.4 s, and the DOF was F/3.5. The optical images of the particles were processed using an image analyzing software (Matrox, Inspector v9.0). The size of an imaging pixel was calibrated using Olympus scale bars with a width of 10 µm, enabling the determination of a size of 0.1 µm (equivalent to 4 pixels of the images) change. Particles larger than ∼0.5 µm could be analyzed using this system. Low-Z Particle EPMA Measurement. After the hygroscopic measurement for individual ambient aerosols was performed, a quantitative single particle analytical technique, low-Z particle EPMA, was used to determine the chemical compositions of the particles on the image field. The low-Z particle EPMA allowed for the determination of the concentration of low-Z elements, such as carbon, nitrogen, and oxygen, as well as higher-Z elements, in individual particles of micrometer size.37-41 The quantitative determination of low-Z elements in individual environmental particles has improved the applicability of single particle analysis. For instance, many environmentally important atmospheric particles, such as sulfates, nitrates, ammonium, and carbonaceous particles, contain low-Z elements. For a decade, this technique has been successfully applied to the characterization of various types of atmospheric aerosol particles.42-49 The measurements were carried out using a Jeol JSM-6390 SEM equipped with an Oxford Link SATW ultrathin window EDX detector. The resolution of the detector is 133 eV for Mn KR X-rays. X-ray spectra were recorded under the control of Oxford INCA Energy software. To achieve optimal experimental conditions, such as a low background level in the spectra and high sensitivity for low-Z element analysis, a 10 kV accelerating voltage was chosen. The beam current was 1.0 nA for all measurements. To obtain statistically enough counts in the X-ray spectra while limiting the beam damage effects on sensitive particles, a typical measuring time of 20 s was used. A more detailed discussion on the measurement conditions is given elsewhere.41 The net X-ray intensities for the elements were obtained by nonlinear least8002

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squares fitting of the collected spectra using the AXIL program.50 The elemental concentrations of individual particles were determined from their X-ray intensities by applying a Monte Carlo calculation combined with reverse successive approximations.39,41 The quantification procedure provided results accurate within 12% relative deviations between the calculated and nominal elemental concentrations when the method was applied to various types of standard particles such as NaCl, Al2O3, CaSO4 · 2H2O, Fe2O3, CaCO3, and KNO3.40,49,51 RESULTS AND DISCUSSION Humidifying and Dehydration Curves of Inorganic Salts. Humidifying and dehydration curves of laboratory-generated inorganic salts, such as NaCl, KCl, (NH4)2SO4, and Na2SO4, which have been intensively studied by other techniques,8,27,31,52 were obtained for evaluating the hygroscopic measurement system. (37) Khan, M. S. I.; Hwang, H.; Kim, H.; Ro, C.-U. Anal. Chim. Acta 2008, 619, 14–19. (38) Ro, C.-U.; Kim, H.; Van Grieken, R. Anal. Chem. 2004, 76, 1322–1327. (39) Ro, C.-U.; Osan, J.; Szaloki, I.; de Hoog, J.; Worobiec, A.; Van Grieken, R. Anal. Chem. 2003, 75, 851–859. (40) Ro, C.-U.; Osan, J.; Szaloki, I.; Oh, K.-Y.; Kim, H.; Van Grieken, R. Environ. Sci. Technol. 2000, 34, 3023–3030. (41) Ro, C.-U.; Osan, J.; Van Grieken, R. Anal. Chem. 1999, 71, 1521–1528. (42) Geng, H.; Ryu, J.; Jung, H.-J.; Chung, H.; Ahn, K.-H.; Ro, C.-U. Environ. Sci. Technol. 2010, 44, 2348–2353. (43) Geng, H.; Park, Y.; Hwang, H.; Kang, S.; Ro, C.-U. Atmos. Chem. Phys. 2009, 9, 6933–6947. (44) Kang, S.; Hwang, H.; Kang, S.; Park, Y.; Kim, H.; Ro, C.-U. Atmos. Environ. 2009, 43, 3445–3453. (45) Kang, S.; Hwang, H.; Park, Y.; Kim, H.; Ro, C.-U. Environ. Sci. Technol. 2008, 42, 9051–9057. (46) Hwang, H.; Kim, H.; Ro, C.-U. Atmos. Environ. 2008, 42, 8738–8746. (47) Hwang, H.; Ro, C.-U. Atmos. Environ. 2006, 40, 3869–3880. (48) Ro, C.-U.; Hwang, H.; Kim, H.; Chun, Y.; Van Grieken, R. Environ. Sci. Technol. 2005, 39, 1409–1419. (49) Ro, C.-U.; Oh, K.-Y.; Kim, H.; Kim, Y. P.; Lee, C. B.; Kim, K.-H.; Osan, J.; de Hoog, J.; Worobiec, A.; Van Grieken, R. Environ. Sci. Technol. 2001, 35, 4487–4494. (50) Vekemans, B.; Janssens, K.; Vincze, L.; Adams, F.; Van Espen, P. X-Ray Spectrom. 1994, 23, 278–285. (51) Ro, C.-U.; Oh, K.-Y.; Kim, H.; Chun, Y.-S.; Osan, J.; de Hoog, J.; Van Grieken, R. Atmos. Environ. 2001, 35, 4995–5005. (52) Hu, D.; Qiao, L.; Chen, J.; Ye, X.; Yang, X.; Cheng, T.; Fang, W. Aerosol Air Quality Res. 2010, 10, 255–264.

Figure 3. Humidifying and dehydration curves for a typical NaCl particle collected on a TEM grid. Blank and solid circles are growth factor (G.F.) data obtained during the humidifying and dehydration processes, respectively. The growth factors were obtained by dividing areas of the particle at different RHs by that of the dry particle before starting the humidifying process. Hydration and dehydration curves, represented as growth factors in mass, are plotted in solid lines. Humidifying and dehydration curves from Tang et al.11 are also shown in dotted lines for comparison.

Figure 2 shows several optical images of generated NaCl particles on a TEM grid that were obtained at different RHs during humidifying and dehydration processes. These images are shown together with a secondary electron image (SEI) obtained after the hygroscopic measurement. The humidifying process started at RH ) 2.4%, where all of the 11 particles looked like rectangular solids (Figure 2A). The morphology of the 11 particles stayed unchanged before RH ) 74.5% when RH was increased from 2.4% by 0.1-0.3% RH steps. At RH ) 74.5%, the particles started to look more compact. At RH ) 75.3%, all of the particles looked circular as water was adsorbed on the particle surface, while NaCl solids were still in the core of the particles (Figure 2B). Above RH ) 75.3%, all of the particles started to rapidly absorb water vapor. At RH ) 75.7%, NaCl solids were completely dissolved, resulting in aqueous NaCl droplets (Figure 2C). Above RH ) 75.7%, the aqueous NaCl droplets continuously grew until RH ) 94.0% (Figure 2D). During this hydration process, some particles that were located close to each other became coagulated, that is, particles 3-1 and 3-2, 5-1 and 5-2, and 8-1 and 8-2. Thus, originally, 11 particles became 8 particles on the image field. The dehydration process started from RH ) 94.0% by decreasing RH. Until RH ) 48.2, the sizes of all the 8 NaCl droplets decreased as RH decreased. At RH ) 48.2%, particles 5 and 8 became NaCl soilds. Particle 1 recrystallized just at RH ) 47.7%, as shown in Figure 2E, where it can be seen that 6 particles are already NaCl solids, except for particles 2 and 4. Particle 4 recrystallized at RH ) 47.6% (Figure 2F) and particle 2 did at RH ) 47.2%. Figure 2G shows dry NaCl solids at RH ) 3.1%, which had been the same below RH ) 47.2%. The SEI of the image field clearly shows that all 8 particles are crystalline solids. From the 2-dimensional (2-D) area measurement of all the particles in the image field at each RH, the humidifying and dehydration curves for all the particles were obtained. Figure 3 shows the humidifying and dehydration data (displayed as blank and solid circles, respectively) for a typical NaCl

particle (particle 1 in Figure 2). The humidifying and dehydration data were obtained first by increasing RHs from 2.4% to 94.0% for the dry NaCl particle, and then by decreasing RHs to 3.1%. Growth factors were obtained by dividing areas of the particle at different RHs by that of the dry particle at RH ) 2.4% before starting the humidifying process. The 2-D area of the dry particle at RH ) 2.4 was 7.55 µm2. The NaCl particle looked rectangular on its 2-D optical image. Its size and shape stayed unchanged below RH ) 74.5% in the humidifying process. At RH ) 74.5%, its size and shape started to change and its 2-D area decreased by 3%. And further it decreased by 8% at RH ) 75.3%, where its shape looked circular as water was adsorbed on the particle surface, while a NaCl solid was inside. This shrinkage happened just before the deliquescence occurred because of the structural rearrangement of the NaCl crystal with the initial uptake of the moisture, which was also observed in another study.17 During the increase of RH from 75.3% to 75.7%, the NaCl solid completely dissolved to become an aqueous NaCl droplet, while its area increased 4.1 times. For this particle, its DRH was determined to be 75.5(±0.2)%. All the particles on the image field deliquesced almost at the same RH. On the basis of 11 individual NaCl particle data, the obtained DRH of NaCl was 75.5(±0.2)%, which matched very well with the reported values (75-76%).11,19,28,31,53,54 After its deliquescence, the particle kept growing as the uptake of the moisture continued when RH was increasing. During the dehydration process, where RH was decreased from 94.0% to 3.1%, the size of droplet #1 continuously shrank until RH became 47.8% with the decrease of its area by 3.7 times. At RH ) 47.7%, that is its ERH, its size and shape of droplet #1 suddenly changed by becoming a rectangular (on 2-D image) solid crystal with the decrease of its area by 2.4 times. Its size and shape (53) Joutsensaari, J.; Vaattovaara, P.; Vesterinen, M.; Ha¨meri, K.; Laaksonen, A. Atmos. Chem. Phys. 2001, 1, 51–60. (54) Tang, I. N.; Munkelwitz, H. R. Atmos. Environ. 1993, 27A, 467–473.

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remained unchanged by decreasing RH below its ERH until RH ) 3.1%. The volumes of crystalline particles should be the same before the humidifying process and after the dehydration process, but its 3-D structures before the humidification and after the dehydration can be different, resulting in its different 2-D sizes. However, for the exemplar particle, the before-and-after 2-D sizes were accidently similar (the area of the recrystallized solid was just 10% bigger than that of the original solid.). As the activation energy is required for the crystallization, the ERHs for inorganic salts are lower than the DRHs. On the basis of 8 individual NaCl droplet data, the obtained ERH was 47.7(±0.5)%, which matched very well with the reported values (45-48%).11,19,31 The ERH values for the individual droplets were somewhat scattered compared to their DRH values. The aqueous inorganic droplets effloresced at somewhat different RHs as the droplets contained the different amounts of nucleation seed particles in it or sat on the surfaces with different roughness, acting like a nucleation seed, of the TEM grid.55,56 Since the determination of particle size was based on 2-D images, the estimation of particle volume was inherently uncertain, which made it difficult to compare our humidifying and dehydration curves with those of other studies that provided growth factors based on particle mass or volume. However, the volume of water droplets that contain inorganic salts can be accurately estimated if the contact angle of the water droplets is known for collecting substrates, and furthermore, the mass of water droplets if the density of the water droplets is known. Using the contact angle data (∼150°) of water droplets on the TEM grids57 and the density data available,11 the hydration and dehydration curves, represented as growth factors in mass, for NaCl particle 1 were plotted in solid lines (Figure 3), together with the curves from Tang’s work11 (in dotted lines). Since the mass of dry crystals cannot be accurately estimated, the growth factor in mass at RH ) 94.0% was referenced to that of Tang’s. As shown in Figure 3, the almost perfect match between our and Tang’s hydration curves for droplet #1 was observed. However, the dehydration curve in our experiment was somewhat higher than that of Tang’s work, which was obtained from levitated single particles.11 In the dehydration process, water evaporated from the surface of the droplet, resulting in a decrease of its volume and mass. However, because of the surface tension between the droplet and the collecting substrate, its surface area did not decrease as much as its volume or mass did. When dehydration curves were obtained from levitated single particles, as previously done by Tang,11 this substrate effect was not present. Similar studies were performed on artificially generated KCl, (NH4)2SO4, and Na2SO4 particles. In Figure S1 of Supporting Information, several optical images of generated KCl particles on a TEM grid that were obtained at different RHs during the humidifying and dehydration processes are shown together with a SEI obtained after the hygroscopic measurement. In Figure S2 of Supporting Information, humidifying and dehydration data (displayed as filled and blank circles, respectively) for a typical KCl particle (particle 1 in Figure S2) are shown. The DRH and ERH of the KCl particles were 84.6(±0.3)% and 58.7(±0.7)%, respectively, which were obtained from 16 and 15 individual (55) Han, J. H.; Martin, S. T. J Geophys. Res. Atmos. 1999, 104, 3543–3553. (56) Pant, A.; Parsons, M. T.; Bertram, A. K. J. Phys. Chem. A 2006, 110, 8701– 8709. (57) Xiao, X.; Cheng, Y. T. J. Mater. Res. 2008, 23, 2174–2178.

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particles, that were very close to the reported values (i.e., 84-85% and 56-59%, respectively).11,13,21 Using the contact angle data of water droplets on the TEM grids, and the density data available, the hydration and dehydration curves, represented as growth factors in mass, for the KCl particle were plotted in solid lines together with the curves from Tang’s work11 (in dotted lines), where an almost perfect match between our and Tang’s hydration curve is seen. Figures S3 and S5 of Supporting Information show several optical images of generated (NH4)2SO4 and Na2SO4 particles, respectively, obtained at different RHs. Also, humidifying and dehydration data for typical (NH4)2SO4 and Na2SO4 particles are shown in Figures S4 and S6 of Supporting Information. The DRH and ERH of the (NH4)2SO4 particles obtained from 9 individual particles were 79.9(±0.3)% and 37.9(±0.5)%, which were very close to the reported values (i.e., 79-81% and 33-40%, respectively).8,11,18,24,53,54 Those of the Na2SO4 particles, which were obtained from 8 individual particles, were 84.4(±0.3)% and 58.5(±0.6)%, respectively, which were also very close to the reported values (i.e., 84-85% and 57-59%, respectively).8,11,18 Using the contact angle data of water droplets on the TEM grids and the density data available, the hydration and dehydration curves, represented as growth factors in mass for the (NH4)2SO4 and Na2SO4 particles were plotted in solid lines together with the curves from Tang’s work11 (in dotted lines) in Figures S4 and S6 of Supporting Information, respectively, where an almost perfect match between our and Tang’s hydration curves is also seen. The accurately obtained DRHs, ERHs, and hydration curves after DRHs for the four different inorganic salts strongly support the belief that this hygroscopic measurement system is reliable. As shown in Figures 2, 3, and S1-S6 of Supporting Information, the recrystallized solids and humidifying and dehydration curves of the four different inorganic compounds looked somewhat different from each other, reflecting their different crystalline and hygroscopic properties. However, one of our main aims of this work is to confirm the validity of our analytical methodology for the hygroscopic studies. Therefore, the detailed investigation of the crystalline and hygroscopic properties of various inorganic compounds has become one of our future studies. Application for the Study of Hygroscopic Property of Ambient Aerosol Particles. A coarse-mode ambient aerosol sample, collected on January 15, 2010 on the roof of a campus building at Inha University, located in Incheon, Korea, was used to demonstrate that the method based on the combined use of optical and electron microscopic techniques could be applied to determine the hygroscopic properties, morphologies, and chemical compositions of the same individual airborne particles. Overall, 25 particles on two image fields (13 particles on the first image field and 12 on the second) were analyzed. The optical images of the first image field obtained at RH ) 4.7% before the start of the humidifying process and after the end of the dehydration process are shown in Figures S7A and S7B of Supporting Information, respectively, together with the SEIs (Figures S7C and S7D of Supporting Information) of the same field obtained before and after the hygroscopic measurement. The optical images and SEIs had different image resolutions and contrasts. However, the location patterns of particles in the fields, together with the aid of markers in the TEM grid, allowed us to find the same image

Figure 4. Optical images of ambient aerosol particles obtained at (A-E) RH ) 4.7%, 44.3%, 64.4%, 76.3%, and 94.3% during the humidifying process and at (F-J) RH ) 48.4%, 48.2%, 48.1%, 45.5%, and 4.7% during the dehydration process.

fields easily. The particles on the image field looked somewhat different before and after the hygroscopic measurement, as the shape of the particles changed during the humidifying and dehydration processes. Before the start of the humidifying process, particles 11-1 and 11-2 were different particles. As RH increased above their DRHs, the particles 11-1 and 11-2 grew very large and coagulated to particle 11 (also see Figure 4). Also, particle 2 looks bigger in Figure S7D than in Figure S7C because it was coagulated during the hydration process with a particle located outside Figures S7A and S7C of Supporting Information. Some particles in Figures S7B and S7D, such as particles 1, 3, 5, 6, 7, and 8, look broader and more irregular than those in Figures S7A and S7C because of the presence of the surface tension between the TEM grid and the droplets. When the particles became solids from the crystallization, the surface tension present between them somewhat influenced the crystallization of the droplets. The optical images and SEIs of the second image field, where particles 14-25 sit, are given in Figure S8 of Supporting Information. As shown in Figure 4, when RH increased from 4.7% to 94.3% and decreased from 94.3% to 4.7%, the sizes of the particles on the image field individually changed based on their hygroscopic property. Figure 4A shows 14 particles on the image field at RH ) 4.7%. When RH was increased to 44.3%, the morphology of particles 1, 8, and 11-1 changed (Figure 4B). At RH ) 64.4%, particles 2, 3, 7, and 11-2 became aqueous droplets (Figure 4C). At RH ) 76.3%, particles 5 and 13 became dissolved, while the other dissolved particles grew to bigger droplets (Figure 4D). At RH ) 94.3%, all of the droplets were very big so that particles 11-1 and 11-2 were seen to have been coagulated. The morphology of particles 4, 9, 10, and 12 did not change because they were not hygroscopic. Particles 4 and 9 are elemental carbon fly ashes, and particles 10 and 12 are CaCO3 and SiO2, respectively. While RH decreased from 94.3% to 48.4%, particles 1 and 2 had crystallized, and at RH ) 48.4%, particles 7 and 13 just crystallized (Figure 4F). Particles 5, 3, and 11 became solids at RH ) 48.2%, 48.1%, and 45.5%, respectively (Figure 4G-I). Because the optical images were recorded using the digital camera by varying RH in 0.1-0.3% steps, the humidifying and

dehydration curves for all of the particles in the field were obtained after the processing of the image data. Using the low-Z particle EPMA technique, the chemical compositions and chemical species of all 25 particles were determined, which are shown in Table S1 of Supporting Information. Among the 25 particles, 11 particles are genuine sea-salts, whereas 3 particles are (partially or fully) reacted sea-salts, most probably with Na+ and Mg2+ as cations and Cl-, NO3-, and SO42- as anions, that is, notated as (Na,Mg)(Cl,NO3,SO4). The number of Ca-containing particles is 5, including CaCO3, Ca(CO3,NO3)/C, Ca(CO3,SO4), Ca(CO3, NO3,SO4)/C, and Mg(Cl,NO3)/AlSi/C particles, where AlSi and C denote aluminosilicates and carbonaceous species, respectively. The numbers of aluminosilicates, SiO2, and fly ash particles are 2, 2, and 2, respectively. According to the low-Z particle EPMA results, 11 particles (2, 3, 5, 7, 11, 13, 14, 19, 20, 22, and 24) are genuine sea-salts (see Table S1). The SEI of an exemplar genuine sea-salt particle (particle 7) shows a bright solid in the center (even showing a charging as it is a nonconductive solid such as NaCl) and a feature surrounding the central solid (Figure 5A). The atomic concentration data indicates that the NaCl species is the majority of the center of particle 7 and the chemical species at the surrounding area is most probably (Na,Mg,Ca,K)(Cl,SO4) (Figure 5C and D). This result indicates that the NaCl solid of the particle was fractionally crystallized when RH decreased to 48.4% (see Figure 5B where the humidifying and dehydration data for particle 7 are shown.). The humidifying curve (see the data points of blank circles in Figure 5B) shows a gradual increase of the growth factor around 65-75% RHs, which is different from the abrupt increase of the growth factor at 75.5% for pure NaCl particles. As sea-salt is a multicomponent system, this kind of gradual increase due to NaCl deliquescence has been reported.11,31 Several reports also observed that two or more component systems showed phase transformations at a lower RH than those of single component systems.21,58,59 For example, a two-component system of KCl-NaCl showed just one multicomponent DRH (MDRH) at 73.8%, which (58) Wexler, A.; Seinfeld, J. Atmos. Environ. 1991, 25, 2731–2748. (59) Freney, E. J.; Martin, S. T.; Buseck, P. R. Aerosol Sci. Technol. 2009, 43, 799–807.

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Figure 5. (A) SEI of a genuine sea-salt (particle 7), (B) its humidifying and dehydration data, and (C, D) X-ray spectral data obtained from points 1 and 2 on the SEI, respectively. Growth factors were obtained by dividing areas of the particle at different RHs by that of the particle before starting the humidifying process. Humidifying and dehydration curves from Tang et al.,11 represented as growth factors in mass, are also shown in dotted lines for comparison.

was lower than the DRHs of a single component NaCl and KCl system (75.3% and 84.2%, respectively).60 More hygroscopic magnesium, calcium, and potassium salts than sodium salts were reported to have dissolved at 30-40% RHs, well below the DRH of NaCl.11,31 However, in our system, the increase of the growth factor below RH ) 30-40% was not observed, which was probably due to the small amount of magnesium and potassium salts. In the dehydration process, a phase transition at RH ) 48.4% was observed (see data points of solid circles in Figure 5B), which was very close to the ERH of pure NaCl particles. It was also reported that in multicomponent systems, the multicomponent ERH (MERH) are usually lower than the ERHs of each component constituting the multicomponent system.59 For example, a twocomponent system of KCl-K2SO4 showed just one MERH at 52%, which was lower than the ERHs of a single component KCl and K2SO4 system (56% and 60%, respectively).59 In our work, it seemed that almost pure NaCl moiety fractionally crystallized near the ERH of NaCl at the central part of particle 7. Also, the observation of the abrupt decrease of the growth factor at RH ) 48.4% with a similar magnitude of efflorescence step to that of the deliquescence step strongly supports that the NaCl solid fractionally crystallized. After the crystallization of NaCl, the magnesium, calcium, and potassium salts effloresced as a small, gradual decrease of the growth factor was observed between RH ) 48.4-20.0%, although no clear step was seen, which was also observed in other studies.11,31 For comparison, the humidifying and dehydration curves of sea-salts from the work of Tang et al.11 are shown in Figure 5B. (60) Tang, I. N.; Munkelwitz, H. R.; Davis, J. G. J. Aerosol Sci. 1978, 9, 505– 512.

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The humidifying and hydration curves of genuine sea-salt particles 2, 3, 5, and 14 were simialr to those of particle 7 (Figures S10, S11B, and S12B, Supporting Information), whereas those of particles 13, 19, 20, and 24 were similar to each other, but somewhat different from those of particle 7 (Figures S11D, S12D, and S13, Supporting Information). The growth factors of particles 13, 19, 20, and 24 in the humidifying process gradually increased from ∼55-60% to ∼65-70% and then somewhat rapidly increased from ∼75%. On the basis of the low-Z particle EPMA data, the chemical compositions of particles 13, 19, 20, and 24 were different from those of particles 2, 3, 5, 7, and 14, in that particles 2, 3, 5, 7, and 14 had a considerable amount of carbonaceous species, but particles 13, 19, 20, and 24 did not (Table S1, Supporting Information). This carbonaceous species might be humic substance or humic-like-substance (HULIS) of marine origin.61 It was reported that organic species mixed with inorganic salts can significantly influence the hygroscopic property of inorganic salts.62,63 It is difficult to discuss the hygroscopic property of particles 11 and 22 in relation to their chemical compositions as the particles were coagulated during the humidifying process and X-ray spectral data of particles 11-1, 11-2, 22-1, and 22-2 were not acquired before the hygroscopic measurement. The potential electron beam damage on beam-sensitive particles was avoided by not performing X-ray spectral measurements before the hygroscopic measure(61) Kriva´csy, Z.; Kiss, G.; Ceburnis, D.; Jennings, G.; Maenhaut, W.; Salma, I.; Shooter, D. Atmos. Res. 2008, 87, 1–12. (62) Parsons, M. T.; Knopf, D. A.; Bertram, A. K. J. Phys. Chem. A 2004, 108, 11600–11608. (63) Gao, Y.; Yu, L. E.; Chen, S. B. Atmos. Environ. 2008, 42, 4433–4445.

Figure 6. SEIs and X-ray spectral data (A and C) and their humidifying and dehydration data (B and D) of two aged sea-salts (particles 17 and 18, respectively). Growth factors were obtained by dividing areas of the particle at different RHs by that of the particle before starting the humidifying process.

ment. As shown in Figure S14, Supporting Information, particle 11 seems to have been coagulated by two droplets with different chemical compositions because the humidifying curves of particles 11-1 and 11-2 are somewhat different. However, it is difficult to determine the chemical compositions of particles 11-1 and 11-2 based on the X-ray spectral data obtained from the coagulated particle 11, although X-ray spectra obtained at 3 different regions of particle 11 indicate different chemical compositions. Particles 17, 18, and 23 are reacted (aged) sea-salts. These particles are reaction products between sea-salts and nitrogen and sulfur oxides species.64 On the basis of the X-ray spectral data, the major chemical species of particle 17 are most probably (Na,Mg)(NO3,Cl,SO4)/C at its central area (Figure 6A) and (Mg,Na)Cl at its surrounding area. The humidifying curve of particle 17 showed a small decrease of the growth factor between RH ) 45.6 and 48.5%, which was probably because of the rearrangement of the particle with moisture adsorption on its surface, and a gradual increase between RH ) 48.5 and 70.7% (Figure 6B). The gradual increase seems to have been the result of NaCl and MgCl2 dissolution. The dehydration curve of particle 17 showed one decrease at RH ) 28.8%, which seems to have been from MgCl2 crystallization (ERH of pure MgCl2 is ∼33%65). The chemical compositions of particle 17 are quite complicated because it is a mixture of several inorganic and (64) Laskin, A.; Isema, M.; Inchkovich, A.; Graber, E. R.; Taraniuk, I.; Rudich, Y. Faraday Discuss. 2005, 130, 453–468, 10.1039/b417366j. (65) Chan, C. K.; Ha, Z.; Choi, M. Y. Atmos. Environ. 2000, 34, 4795–4803.

organic compounds while a hydroscopic study on this kind of complicated particle has never been performed on a single particle basis. Further studies on artificially generated individual particles of multicomponent systems need to be carried out. On the basis of the X-ray spectral data, the chemical composition of particle 18 is (Na,Mg)(NO3,SO4)/C with the most abundant NaNO3 species at its central and surrounding areas (Figure 6C). The humidifying curve of particle 18 showed two increases between 58.7 to 63.5% and 69.2 to 73.6% (Figure 6D). The cause of the first increase of the growth factor might have been due to the dissolution of the NH4NO3 species. The low-Z particle EPMA cannot identify the NH4NO3 species just based on nitrogen content, but the DRH of NH4NO3 was reported to be ∼62%.27,54 As the major chemical species is NaNO3 for particle 18, the second increase at RH ) 69.2-73.6% is probably related to the NaNO3 deliquescence, that is, the DRH of NaNO3 was reported to be 74.5%.8,12 The monotonous decrease of the growth factor during the dehydration process matches well with the observation of no efflorescence of NaNO3 during the dehydration process.31,66 On the basis of the X-ray spectral data, particles 1, 6, 8, 10, and 16 are Ca-containing particles. Particle 10 is CaCO3, which did not absorb or desorb water as RH changed (Figure S17). Particle 6 is a Ca(SO4,CO3) particle with minor nitrate (66) Elizabeth, R.; Gibson, K. P.; Grassian, V. H. J. Phys. Chem. A 2006, 110, 11785–11799.

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Figure 7. SEIs and X-ray spectral data (A and C) and their humidifying and dehydration data (B and D) of two reacted Ca-containing particles (particles 6 and 8, respectively). Growth factors were obtained by dividing areas of the particle at different RHs by that of the particle before starting the humidifying process.

species (Figure 7A). CaSO4 and CaCO3 are not hygroscopic. However, the humidifying and dehydration curves for particle 6 (Figure 7B) indicate that the particle is hygroscopic without clear DRH and ERH, which is probably because of the presence of Ca(NO3)2 species, which was reported to be hygroscopic without any clear phase transformations.67 As shown in Figure 7B, the 2-D area of particle 6 decreased as RH was decreased from 94.3% to ∼85% in the dehydration process. As RH was decreased from ∼85% to 17%, the volume of the droplet decreased, but the area did not because the surface tension between the droplet and the TEM grid made its 2-D shape maintained when water evaporated from the surface of the droplet. Visual observation by naked eyes during the measurements could discern this minute change. At RH ) 17%, the surface tension of the droplet overcame the surface tension between the droplet and the TEM grid, such that the growth factor gradually decreased below RH ) 17%. The Cacontaining particles 1 and 8 showed the similar behavior (Figures S18B and 7D). Although the content of Ca(NO3)2 species is minor (the N content of particle 6 is just 1.5% in atomic fraction.), the hygroscopic property of particle 6 seems to be determined by the presence of the hygroscopic chemical species. Particle 8 is a particle mixed with Ca(CO3,NO3,SO4) with minor aluminosilicates (Figure 7C). Its SEI shows a little bright part inside the circular particle, which may be insoluble CaCO3, CaSO4, and aluminosilicates. The circular shape and its humidifying and dehydration (67) Liu, J. Y.; Zhu, T.; Zhang, Z. F. Atmos. Chem. Phys. 2008, 8, 7205–7215.

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curves (Figure 7D) indicate that its hygroscopic property is mainly determined by the hygroscopic Ca(NO3)2 moiety. Indeed, its humidifying and dehydration curves very much resemble with those of particle 6. Particle 1 has a peculiar morphology, that is, there is an elongated part at the bottom left area, some crystalline part in the center, and with the exception of the bottom left area, a hemicircular form (see its SEI in Figure S18, Supporting Information). The X-ray spectra obtained from the three regions indicate that the elongated and central parts are Ca(CO3,NO3)/AlSi/C, whereas the hemicircular moiety is mainly Ca(NO3)2/C (see Figures S18A, C, and D, Supporting Information). Its humidifying and dehydration curves are also similar to those of particles 6 and 8, strongly indicating that Ca(NO3)2 moiety predominates the hygroscopic property of particle 1. The chemical compositions of particle 16 are (Mg,Ca)NO3/AlSi/C, of which the humidifying and dehydration curves are also mainly for nitrate species (Figure S19, Supporting Information). Becausethe hygroscopic chemical species seems to dominate the hygroscopic property of particles containing nonhygroscopic chemical species, which has also been reported by others,35,67 it is worth investigating what the minimum content of hygroscopic chemical species controlling the hygroscopic property of particles with major nonhygroscopic species is. Two particles were identified as aluminosilicate particles (particles 15 and 21), two as SiO2 particles (particles 12 and 25), and two as fly ash particles (particles 4 and 9), which were based on low-Z particle EPMA data. The fly ash particles,

classified by their circular and bright morphology and chemical compositions, are elemental carbon aerosols as the carbon contents are >90% with minor nitrogen and oxygen contents. All of the six particles were not hygroscopic and their sizes did not change with the change of RH. Their SEIs, X-ray spectra, and humidifying and dehydration curves are given in Figures S20-S22, Supporting Information.

Up until now, laboratory generated particles of one component have been studied extensively, and yet studies on individual ambient aerosol particles of two or three components are scarce. Our future studies will include multicomponent systems of environmental relevance that use this approach in order to understand hygroscopic properties of the ambient aerosols in more detail.

CONCLUSIONS This work demonstrates that the optical microscopic system for hygroscopic studies on a single particle level can provide DRHs, ERHs, and also humidifying and dehydration curves for inorganic salts. The preliminary results on the ambient aerosol particles clearly demonstrate how practical this method is for the characterization of the hygroscopic properties related to their chemical compositions and how complicated the ambient aerosols are in the full understanding of their hygroscopic properties. In many cases, individual ambient aerosols are multicomponent systems with different concentrations of various chemical species.

ACKNOWLEDGMENT This work was supported by the Korea Science and Engineering Foundation (KOSEF) grant funded by the Korea government (MOST) (ROA-2007-000-20030-0). SUPPORTING INFORMATION AVAILABLE Additional data are provided in 20 figures. This information is available free of charge via the Internet at http://pubs.acs.org. Received for review June 7, 2010. Accepted July 27, 2010. AC101432Y

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