Anal. Chem. 1999, 71, 379-383
Physical and Chemical Characterization of Atmospheric Aerosols by Atomic Force Microscopy David W. Lehmpuhl,† Kathryn A. Ramirez-Aguilar, Amy E. Michel, Kathy L. Rowlen,* and John W. Birks*
Department of Chemistry and Biochemistry and Cooperative Institute for Research in Environmental Sciences (CIRES), University of Colorado, Boulder, Colorado 80309
A new approach to the measurement of the size distribution of atmospheric particles as a function of chemical reactivity is demonstrated. In this method, particles are collected on an atomically flat mica substrate and imaged using atomic force microscopy. The particle size distribution is obtained from analysis of the image. The sample is then repeatedly exposed to a reactive gas such as ozone or oxygen atoms and reimaged. The dimensions of organic and graphitic (soot) particles are found to shrink at different rates due to formation of volatile products, while inorganic particles remain unchanged. Using this approach, it is possible to classify particles as organic, graphitic, or inorganic and to obtain the size distribution for each chemical class. In recent years, there has been great interest in the development of methods to simultaneously size and chemically characterize aerosol particles, especially those present in the atmosphere. Andreae and Crutzen recently reviewed the importance of aerosols on atmospheric chemistry and climate, emphasizing the roles of particles derived from biogenic sources.1 The authors pointed out the need to obtain “spatiotemporal variations in the concentration of aerosol particles and their physical and chemical properties”. Conventional methods of particle characterization using cascade impactors for size fractionation followed by various forms of chemical analysis have poor size resolution and require long sampling times. Differential mobility analyzers provide highresolution size distributions but no chemical information. A major innovation has been the development of single-particle analysis by mass spectrometry with laser ablation.2-13 Individual particles are sized by either light-scattering or hydrodynamic properties * Correspondence authors: (e-mail)
[email protected] and birks@ terra.colorado.edu. † Present address: Department of Chemistry, University of Southern Colorado, Pueblo, CO 81001. (1) Andreae, M. O.; Crutzen, P. J. Science 1997, 276, 1052-1058. (2) Sinha, M. P. Rev. Sci. Instrum. 1984, 55, 886-891. (3) McKeown, P. J.; Johnston, M. V.; Murphy, D. M. Anal. Chem. 1991, 63, 2069-2073. (4) Kievit, O.; Marijinissen, J. C. M.; Verheijen, P. J. T.; Scarlett, B. J. Aerosol Sci. 1992, 23, S301-S304. (5) Murphy, D. M.; Thomson, D. S. Aerosol. Sci. Technol. 1995, 22, 237-249. (6) Hinz, K. P.; Kaufmann, R.; Spengler, B. Anal. Chem. 1994, 66, 2071-2076. (7) Prather, K. A.; Nordmeyer, T.; Salt, K. Anal. Chem. 1994, 66, 1403-1407. (8) Nordmeyer, T.; Prather, K. A. Anal. Chem. 1994, 66, 3540-3542. (9) Carson, P. G.; Neubauer, K. R.; Johnston, M. V.; Wexler, A. S. J. Aerosol Sci. 1995, 26, 535-545. 10.1021/ac980849m CCC: $18.00 Published on Web 12/10/1998
© 1999 American Chemical Society
and the mass spectrum obtained upon partial vaporization and ionization by an intense laser pulse. This is a very powerful method for studies of atmospheric chemistry, but the cost and complexity of such instruments prevent their widespread application to global monitoring. We are particularly interested in developing techniques for obtaining vertical profiles of the chemical composition of the atmosphere using kites and balloons, where instrument packages are typically limited to 2-10 kg.14-17 These platforms and instrument packages can be deployed in remote locations and operated economically. As a potential means of measuring the physical and chemical properties of aerosol particles, we are investigating the use of atomic force microscopy (AFM) in conjunction with chemical reactivity. In vertical profiling applications, we would collect particles on a relatively flat substrate for analysis at the ground. AFM, which recently has been demonstrated for imaging of atmospheric particles,18,20 is preferred in this application over electron microscopy techniques because the images may be obtained under ambient conditions. Ko¨llensperger et al. have recently demonstrated that particles may be chemically characterized in AFM based on their reactions with liquid reagents.20 Here we demonstrate for the first time that reactive gases may be used in combination with AFM imaging to classify atmospheric particles as either “organic”, “graphitic”, or “inorganic” and to obtain the size distribution of each class. (10) Reents, W. J., Jr.; Downey, S. W.; Emerson, A. B.; Jujsce, A. M.; Miller, A. J.; Siconolfi, D. J.; Sinclair, J. D.; Swanson, A. G. Aerosol Sci. Technol. 1995, 23, 263-270. (11) Noble, C. A.; Prather, K. A. Environ. Sci. Technol. 1996, 30, 2667-2680. (12) Yang, M.; Reilly, P. T. A.; Boraas, K. B.; Whitten, W. B.; Ramsey, J. M. Rapid Commun. Mass Spectrom. 1996, 10, 347-351. (13) Reilly, P. T. A.; Gieray, R. A.; Yang, M.; Whitten, W. B.; Ramsey, J. M. Anal. Chem. 1997, 69, 36-39. (14) Balsley, B. B.; Birks, J. W.; Jensen, M. L.; Knapp, K. G.; Williams, J. B. Nature 1994, 369, 23. (15) Balsley, B. B.; Birks, J. W.; Jensen, M. L.; Knapp, K. G.; Williams, J. B.; Tyrrell, G. W. Environ. Sci. Technol. 1994, 28, 422A-427A. (16) Knapp, K. G.; Balsley, B. B.; Jensen, M. L.; Hanson, H. P.; Birks, J. W. J. Geophys. Res. 1998, 103, 13,399-13,411. (17) Knapp, K. G.; Jensen, M. L.; Balsley, B. B.; Bogner, J. A.; Oltmans, S. J.; Smith, T. W.; Birks, J. W. J. Geophys. Res. 1998, 103, 13,389-13,397. (18) Friedbacher, G.; Grasserbauer, M.; Meslmani, Y.; Klaus, N.; Higatsberger, M. J. Anal. Chem. 1995, 67, 1749-1754. (19) Ko ¨llensperger, G.; Friedbacher, G.; Grasserbauer, M.; Dorffner, L. Fresenius J. Anal. Chem. 1997, 358, 268-273. (20) Ko ¨llensperger, G.; Friedbacher, G.; Grasserbauer, M. Fresenius J. Anal. Chem. 1998, 381, 716-721.
Analytical Chemistry, Vol. 71, No. 2, January 15, 1999 379
THEORY A particle that reacts with a gas-phase reagent to form volatile products will shrink in each of its physical dimensions. The change in a dimension, such as height or diameter, as a function of time can be derived by assuming that the rate of volume change is proportional to the collision rate with the gas-phase reactant,
κγνnpMpcg dx dV dV dx ) -Am ) )A )-A dt dx dt dt 4FpNA
(1)
Here, A is the surface area perpendicular to the dimension x, v is the average molecular velocity, γ is the reaction efficiency (fraction of collisions that results in reaction), cg is the concentration of reactant gas, np is the number of moles of particle atoms (or molecules) vaporized per reactive collision, Mp is the molecular weight of the particle atoms (or molecules) vaporized, Fp is the density of the particle, and NA is Avogadro’s number. The parameter κ takes into account the degree of exposure of the particle to the reactive gas. For example, if opposite sides of a cube are both exposed to the reactive gas, the value of κ is 2 since the particle will shrink from both sides. However, since the bottom side of a cube is not exposed to the reactive gas, κ will be only 1 in the vertical dimension. Thus, even a cubic particle could shrink asymmetrically if the side of the particle next to the substrate is not accessible to the reactive gas. Perhaps more importantly, particles such as crystals may have different values of the reaction efficiency γ in different dimensions and shrink asymmetrically for that reason. Canceling A from both sides of eq 1 and integrating gives the simple result that the particle should shrink linearly in time for each dimension,
x ) xo - mt
(2)
The value of the reaction efficiency γ in a given dimension can be determined from linear regression of that dimension vs time, provided that the other physical parameters are known. The value of γ depends on the chemical composition of the particle. Inhomogeneous particles (e.g., an inorganic particle coated with an organic layer) may exhibit a value of γ that varies during the course of the reaction, thus providing additional information about the nature of the particle. EXPERIMENTAL SECTION Particle Collection. Atmospheric particles were collected by placing a freshly cleaved mica substrate under one of the jet orifices of the last stage of an eight-stage Anderson cascade impactor. The 50% cutoff diameter at this level of the impactor is 0.4 µm. Model particles included ∼150-nm silica nanospheres (derivatized with a 3-aminopropyl functional group; Bangs Laboratory, Inc., Fisher, IN), ∼50-nm polystyrene nanospheres (Duke Scientific Corp., Palo Alto, CA), and soot particles generated in a heptane flame. Silica and polystyrene nanospheres were deposited on a freshly cleaved mica substrate by dispersing the particles in methanol, depositing a drop of this solution on the mica surface, and allowing the solvent to evaporate. Soot particles were deposited by briefly exposing the mica surface to a heptane flame. AFM Imaging and Data Analysis. Two modes of imaging, contact and tapping, were used for particle sizing and chemical 380
Analytical Chemistry, Vol. 71, No. 2, January 15, 1999
characterization. A Molecular Imaging PicoAFM (Tempe, AZ) instrument was used for images obtained in contact mode, and a PicoAPEX environmental chamber was used with this instrument to expose the sample to ozone while imaging. The PicoAFM microscope was controlled using a Digital Instruments E controller and made use of etched silicon ESP probes (Digital Instruments, Santa Barbara, CA) having typical lever force constants of 0.020.1 N/m and a tip radius of curvature of 20 nm or less. A scan rate of 4.07 Hz was used. Tapping mode images were obtained using a Digital Instruments Nanoscope IIIa Multimode AFM. Tapping mode measurements made use of etched silicon TESP probes (Digital Instruments) with force constants of 20-100 N/m and radii of curvature less than 15 nm. Scan rates were e1 Hz. For tapping mode images, it was necessary to repeatedly remove the sample, expose it to the reactive gas, and then reposition the sample. To find the sample particles after each exposure, a locator TEM grid (Cu, 200 mesh, Ted Pella, Inc., Redding, CA) was placed on the backside of the mica substrate prior to adhering it to a stainless steel puck. The light microscope integrated into the AFM instrument could then be used to find the same set of particles.21 Reactive Gas Generation. A mixing ratio of 100 ( 12 ppbv ozone was produced by flowing air through a quartz tube irradiated by the 188-nm emission line of a low-pressure mercury lamp. This atmospherically relevant concentration of ozone had no measurable effect on the particles studied. Concentrations of ozone ∼3 orders of magnitude higher, 150 ( 5 ppmv, which had a large effect on organic particles such as polystyrene nanospheres but not on soot particles, were produced by dynamic dilution of ozone formed in an electrical discharge. Ozone concentrations were measured by flowing the reagent gas through a 10-cm absorption cell housed within a Hewlett-Packard 8451A diode array spectrophotometer. Oxygen atoms were produced within a flow tube reactor by passing a 0.5% O2/He mixture through a 2450-MHz microwave discharge at a flow rate of 100 cm3/min and a total pressure of a few Torr. The mica substrate was inserted ∼25 cm downstream from the discharge for exposure to O atoms. Nitric oxide was added downstream of the sample, and the bright chemiluminescence from the NO + O + M reaction was detected using a photomultiplier tube. The oxygen atom concentration was then measured by the well-characterized NO2 titration method.22, (Small air leaks can lead to error in measurement of the O atom concentration since N is rapidly exchanged for O in the reaction N + NO f N2 + O.) RESULTS AND DISCUSSION Qualitatively, it was found that organic particles, such as polystyrene nanospheres and organic particles formed in fires, could be vaporized upon exposure to 150 ppmv ozone for 1-3 h, while graphitic particles (i.e., soot) and inorganic particles such as silica remained virtually unreacted. Reactions of organic particles with 9 × 1012 oxygen atoms/cm3 were much faster, being complete in only a few minutes. Graphitic particles also vaporize upon exposure to oxygen atoms but at a slower rate. Thus, using AFM in combination with exposure to reactive gases, it is possible (21) Markiewicz, P.; Goh, M. C. Ultramicroscopy 1997, 68, 215-221. (22) Kaufman, F. In Progress in Reaction Kinetics 1; Porter, G., Ed.; Pergamon: New York, 1961; pp 1-39.
Figure 2. Particle height as a function of exposure time to ozone for eight atmospheric particles. After 135 min of exposure to 100 ppbv ozone, the ozone mixing ratio was increased to 150 ppmv.
Figure 1. AFM contact mode images of an atmospheric aerosol sample (A) before exposure to ozone, (B) after exposure for 135 min to 100 ppbv ozone followed by 67 min of exposure to 150 ppmv ozone, and (C) after exposure for an additional 168 min to 150 ppmv ozone. Repeated imaging of a control sample over a period of 280 min showed no change in particle height.
to classify atmospheric particles as being inorganic, organic, or graphitic. As an example of the use of ozone to chemically characterize particles, an atmospheric sample was obtained on November 19, 1996 on the campus of the University of Colorado, Boulder, during which time a juniper bush in the vicinity fortuitously caught fire. A mica substrate was located on the last stage of an Anderson
Figure 3. Fractionation of particles based on reactivity toward ozone for an atmospheric sample collected on the last stage of an Anderson cascade impactor: Size distribution for all particles (A), size distribution of particles that react with ozone (B), and size distribution of particles that are unreactive toward ozone (C).
cascade impactor (0.4-µm cutoff). Because of excessive particle accumulation, images could not be obtained directly under the impactor jets. Instead, a region in the periphery of the main impaction zone and containing very small particles in the range of 2-7 nm in height was imaged for the purpose of demonstrating the reaction of atmospheric particles with ozone. Many of these particles may be fragments of larger particle clusters that impacted with the mica surface. Images were obtained before ozone exposure and then imaged eight times over a period of 135 min Analytical Chemistry, Vol. 71, No. 2, January 15, 1999
381
Figure 4. AFM tapping mode images of silica nanospheres (left), polystyrene nanospheres (middle), and hexane flame soot particles (right) before and after indicated times of exposure to 9 × 1012 O atoms/cm3. Particle heights before O atom exposure are ∼150 nm for silica, ∼50 nm for polystyrene, and ∼30 nm for soot.
while being exposed to 100 ppbv ozone. The sample was subsequently exposed to 150 ppmv ozone for a period of 235 min, during which time 15 additional images were obtained. A small portion of three of the 7 µm × 7 µm images obtained are shown in Figure 1 for three different exposure times. Shrinkage and vaporization of the smaller particles are evident in these images. Plots of particle height vs exposure time for eight of the particles monitored are given in Figure 2. Particle heights rather than diameter are plotted since the particle height is not significantly affected by the shape and width of the AFM tip.23 Note that during the 135 min that the sample was exposed to 100 ppbv ozone, the particle height did not change significantly for any of the particles monitored. However, upon exposure of the same particles to 150 ppmv ozone, 22 particles decreased in height until they were completely vaporized, while the heights of 12 particles did not change within experimental error. These results allow us to classify each particle in the initial size distribution as either reactive or unreactive toward ozone and to produce size distributions for each of these classes of particles, as shown in Figure 3. (23) Ramirez-Aguilar, K. A.; Rowlen, K. L. Langmuir 1998, 14, 2562-2566.
382 Analytical Chemistry, Vol. 71, No. 2, January 15, 1999
As seen in Figure 1, particles that did not change upon ozone exposure are much larger on average and have a different morphology. Three particles plotted in Figure 2 do not change significantly upon exposure to either concentration of ozone. The height of the largest of the eight particles decreased rapidly by ∼2 nm upon exposure to 150 ppmv ozone and then remained relatively constant. The four remaining particles plotted in Figure 2 illustrate different behaviors in their reactions with ozone. Initially, the particles shrink approximately linearly in time, as predicted by theory. The rate of shrinking of three of the particles slows down, however, as time progresses, suggesting that either the cores of the particles have a different chemical composition or that products are formed on the surface that are less reactive. However, all four particles are nearly completely vaporized after 350 min of exposure. In previous work, we have shown that the rate of reaction of graphitic particles (carbon black or soot produced in flames) with ozone is very slow at room temperature.24 Thus, it (24) Stephens, S. L.; Calvert, J. G.; Birks, J. W. Aerosol. Sci. Technol. 1989, 10, 326-331.
is likely that the smaller, more reactive particles are organic, while the larger, unreactive particles are either inorganic or graphitic. Considering that the particles were produced in a fire, they probably are graphitic. The inability to distinguish between inorganic and graphitic particles using ozone as the reactant gas led us to investigate the use of a more reactive species, i.e., oxygen atoms, for vaporization of the particles. To characterize the relative reaction rates, we chose polystyrene nanospheres, silica nanospheres, and heptane soot particles as models of organic, inorganic, and graphitic particles, respectively. Images obtained before and after exposure of these particles to an oxygen atom concentration of 9 × 1012 atoms/cm3 are shown in Figure 4. The silica particles remain unchanged while the heights of the polystyrene and soot particles decrease at differing rates. On the basis of these results, it should be possible to distinguish among inorganic, organic, and graphitic particles by exposing the sample to ozone followed by O atoms. Three distinct size distributions, one for inorganic, one for organic, and one for soot, can be obtained as follows. The size distribution of the entire sample is obtained prior to any exposure. The sample is then exposed to ozone to determine which particles are organic; the organic particles shrink in size while the inorganic and soot particles remain unchanged. Finally, the sample is exposed to O atoms to identify those particles that are graphitic. Since the original size of all particles is known from the first image, construction of the three distinct particle distributions only requires that we expose the sample long enough to each of the reagent gases to detect changes in particle size. Validation of a full analytical technique for simultaneously sizing and chemically classifying particles will require studies of collection efficiency and reactivity as a function of particle dimensions and chemical composition. First, however, a quantitative method of particle sampling onto a flat substrate suitable for (25) Marple, V. A.; Willeke, K. In Fine Particles; Liu, B. Y. H., Ed.; Academic Press: New York, 1976; p 411. (26) De la Mora, J. F.; Halpern, B. L.; Kramer, M.; Yamashita, A.; Schmitt, J. In Aerosols; Liu, B. Y. H., Pui, D. Y. H., Fissan, H., Eds.; Elsevier: New York, 1984; p 109. (27) Morrow, P. E.; Mercer, T. T. Ind. Hyg. J. 1964, 25, 8. (28) Cheng, Y.-S.; Yeh, H.-C.; Kanapilly, G. M. Am. Ind. Hyg. Assoc. J 1981, 42, 605. (29) Weisweiler, W.; Gund, G. Fresenius J. Anal. Chem. 1991, 340, 534-539.
imaging by AFM will need to be developed. Promising methods include impaction and electrostatic precipitation. Various commercially available cascade impactors have been compared and problems associated with particle bounce and re-entrainment, interstage wall losses, and nonideal collection characteristics discussed.25 Low-pressure impactors are capable of extending the lower cut-size limit of typically 0.3 µm for conventional impactors to as small as 0.05 µm.26 However, particle bounce may be a severe problem due to the high particle velocities, and some particles may evaporate under the low pressures. Electrostatic precipitation has been developed as a means of collecting particles for examination by electron microscopy and is a promising technique for sample collection for AFM. A point-to-plane electrostatic precipitator is a simple device in which a corona discharge is created between a needle electrode at a negative voltage of a few kilovolts and a grounded collection plate.27 Collection efficiencies of 80-100% for particles in the 0.03-2-µm-diameter range have been demonstrated for flow rates below 100 cm3/min.28 This collection method requires a conducting substrate; highly doped silicon, which has a root-mean square roughness of less than 1 nm, is a good candidate. This work demonstrates some of the potential for using reactive gases such as ozone and oxygen atoms in combination with AFM for characterizing atmospheric particles. The application of other reagent gases to further chemically characterize particles with respect to hygroscopicity, acidity, basicity, etc., is possible. Similar procedures in which particles are deposited onto reactive films have been used in conjunction with electron microscopy.29 In addition to its analytical utility, the method described here provides a new means of measuring reaction efficiencies, γ, for gas-particle reactions that produce volatile products. ACKNOWLEDGMENT This work was supported by the National Science Foundation and the National Oceanic and Atmospheric Administration Office of Global Programs. K.A.R.-A. thanks Perkin-Elmer for an ACS Analytical Division graduate fellowship. Received for review July 31, 1998. Accepted October 27, 1998. AC980849M
Analytical Chemistry, Vol. 71, No. 2, January 15, 1999
383
