Envlron. Scl. Technol. 1992, 26, 1967-1976
Structural Variations in Individual Carbonaceous Particles from an Urban Aerosol K. A. Katrlnak,*pt*fP. Rez,§ and P.
R. Buseckt,lI
Departments of Geology and Chemistry and Center for Solid State Science, Arizona State University, Tempe, Arizona 85287 ~
~~
rn We have used transmission electron microscopy with electron energy-loss spectroscopy (EELS) to characterize the structures of carbonaceous aerosol particles from Phoenix, AZ. The particles are 1 2 pm in diameter and consist of tens to hundreds of aggregated spherules. EELS reveals a range of carbon structures, from amorphous to graphitic, within aggregates. Structural variations occur among individual spherules, on a scale of approximately 0.05 pm. The energy-loss data suggest combinations of single, double, and triple carbon bonds, implying the presence of both organic and elemental carbon. We interpret the graphitic domains as part of the primary spherules and the amorphous areas as condensed hydrocarbons. Surface coatings occur on carbonaceous aggregates collected in the summer months. The coatings contain oxygen, nitrogen, and sulfur. Coated aggregates have carbon structural variations similar to uncoated aggregates. The coatings are interpreted as sulfates and nitrates deposited from the atmosphere as end products of photochemical reactions.
Introduction Carbonaceous particles comprise over half of the submicron mass in many natural aerosols. They have a large impact on visibility and climate and potentially affect public health as carriers of toxic chemicals. A major source of carbonaceous aerosol particles is combustion of fossil fuels. Primary carbonaceous particles form when incomplete combustion produces graphitic carbon spherules. Random collisions result in the formation of irregularly shaped aggregates, or soots, containing many individual spherules. Secondary carbonaceous particles form when gaseous hydrocarbons, emitted during combustion, are transformed in the atmosphere through gas-to-particle conversion. Forms of Particulate Carbon. Particulate carbon is commonly grouped into graphitic and amorphous types. These types have been reported both in laboratory-produced soots (1) and in natural aerosols (2, 3). Graphitic carbon is variously termed “soot”, “inorganic carbon”, ”elemental carbon”, or “black carbon” and is attributed to combustion sources (4). It is a primary aerosol, emitted in particulate form (5). In bulk aerosol samples, graphitic carbon is readily identifiable by its large optical absorption (6). In an individual-particle study of Arctic aerosol samples, combustion-type particles were characterized as graphitic by their polycrystalline diffraction patterns (3). Electron-diffraction patterns indicative of graphite have also been observed in diesel soot ( I ) . In the Phoenix aerosol, graphitic carbon averages 50% by mass of total carbon (7). The second major form of particulate carbon, amorphous or “organic” carbon, can be emitted directly through combustion processes as primary aerosol; it also forms in the Department of Geology. Current address: Energy and Environmental Research Center, University of North Dakota, Grand Forks, ND 58202-8213. *Center for Solid State Science. Department of Chemistry. f
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atmosphere when hydrocarbons condense as secondary particles (5,8,9). Amorphous carbon is typically measured indirectly, by subtracting graphitic carbon mass from the total carbon mass (6). In individual particles from the Arctic aerosol, organic-type carbon particles were classified as amorphous by the lack of peaks in their electron-diffraction patterns (3). Structural Complexities of Carbonaceous Materials. Although convenient, the separation of particulate carbon into graphitic and amorphous fractions does not accurately describe the complex nature of individual particles. Previous work has shown a wide range of ordering in carbonaceous combustion products. Transmission electron microscope (TEM) images of coke samples show a progression of structural change from amorphous to graphitic carbon with increasing temperature (10). Intermediate structural steps include isolated carbon layers and clusters of poorly stacked layers, approximately 1.0 and 1.4 nm in length, respectively (10). Such small-scale variations are not detectable in bulk analyses. Similar structural progressions from isolated crystallites, to distorted graphitic layers, to flat, perfectly ordered layers have also been identified in pitch and coal (11, 12). The structural progressions occur as temperature increases from 400 “C to over 2000 OC (11,121. TEM imaging has also shown structural complexities present in carbonaceous materials in metamorphic rocks (13, 14). Additionally, TEM images of evaporated carbon films reveal microcrystalline graphitic regions within the mostly amorphous film (15). Carbon films apparently undergo the same structural changes with increasing temperature as do combustion carbons (16). Compositionof Soot. Carbonaceous particles from the Phoenix aerosol resemble combustion soots produced in the laboratory. Similar compositions are thus anticipated. Soot is known to be a chemically complex mixture of amorphous polymerized organic material plus graphitic elemental carbon (17). The hexagonal graphitic layers of soot spherules contain defects and dislocations; there are thus many active sites to allow the incorporation of oxygen, hydrogen, and nitrogen (18). Comparison of thermal separation results for bulk samples of carbonaceous aerosols with graphite and organic carbon showed that ambient atmospheric carbon has an average H/C atomic ratio of 0.15 f 0.05 (19). Carbonaceousaerosol particles deposited in rural New York during winter storms consist of elemental carbon cores with adsorbed organic compounds (20). Electron Energy-Loss Spectroscopyof Carbon. The degree of structural ordering within carbonaceous materials is commonly measured by X-ray diffraction in bulk samples or by electron diffraction in “EM samples. Structural variations in carbon have also been investigated using electron energy-loss spectroscopy (EELS), based on the distinctive variations in shapes of spectra for diamond, graphite, and amorphous carbon, which result from differences in their electronic structures (21).EELS has also been used to detect structural variations within amorphous diamond films (22). In the present study, EELS is used to determine variations in carbon structure among dif-
0 1992 American Chemical SocleItY
Environ. Sci. Technol., Vol. 26, No. 10, 1992
1967
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Flgure 1. Electron energy-loss spectrum for evaporated carbon film,
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ferent spherules both within single aggregates and between aggregates. To our knowledge, this is the first use of EELS to determine structural details of carbonaceous aerosol particles. In EELS, an electron beam of known energy passes through a thin sample and enters a spectrometer, which measures the energy lost by the electrons during passage through the sample. The amount of lost energy is characteristic of the particular elements and the type of bonding present in the sample (23). An energy-loss spectrometer is commonly used as an attachment to a TEM, as in this study. The small probe size attainable in a TEM allows spatial resolution of 0.01 pm for EELS analysis. Energy-loss spectra can be separated into three parts (Figure 1). The sharp peak at zero energy loss represents unscattered, elastically scattered, and quasi-elastically scattered electrons. The low-loss section, between approximately 10 and 50 eV energy loss, consists of one or more broad peaks caused by the excitation of oscillations of the conduction and valence electrons. These collective excitations are called plasmons (23). The third section of the spectrum is the core-loss region, at >50 eV energy loss. The core-loss region has much less intensity than the preceding regions, and so an instrumental gain change in the spectrum is necessary (Figure 1). Core-loss transitions are caused by excitation of inner-shell electrons within the sample (23). These excitations are indicated by a rise in transmitted electron intensity just above the binding energy of the atomic shells. Core-loss transitions are called "edges" because of their abrupt rise in spectral intensity. Edges for K, L, and M atomic shells are commonly used for elemental analysis. It is difficult to measure the absolute abundance of a particular element in a sample using EELS; however, relative abundances of elements with peaks in the core-loss region can be readily calculated. Core-loss transitions are easily observed for light elements such as carbon because their edges are relatively large and distinct. In addition, the fine structure of the core-loss edge is sensitive to structural variations in the sample. Core-loss edges are thus especially useful in this study. Carbon structural variations can be determined by the relative amounts of A* and u* bonding, as revealed in the core-loss regions of energy-loss spectra. In graphite and amorphous carbon, the onset of the K-edge at 284 eV marks the K-shell transition from the 1s core state to the A* conduction band. The 1s transition to the u* conduction band occurs at 291 eV or higher (24). These 1988 Envlron. Scl. Technol., Vol. 26, No. 10, 1992
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Flgure 2. High-loss features of carbon electron energy-loss spectra, showing onset of carbon K-edge at 284 eV, A* peak at 286 eV, and 6. peak at approximately 291 eV. (a, top) The Tlconderoga graphite spectrum has a large, jagged u* peak, indlcating a fully ordered structure. A*/u* = 0.61. (b, middle) The carbon black spectrum has a smaller jagged u* peak, indlcatlng a partial graphitic structure. T * / U + = 0.56. (c, bottom) The evaporated carbon film spectrum has a broad, featureless u" peak, indicattng many randomly orlented bonding configuretlons as Is typical of an amorphous material. T * / u ' = 0.41.
transitions occur in both graphite and amorphous carbon, forming A* and CY* peaks with varied intensities in the core-loss spectra (Figure 2). Spectra with larger A*/u* ratios indicate more A bonding and thus structures with a greater graphitic character. In the graphite spectrum (Figure 2a), the u* peak is tall and sharp, indicating structural ordering (25). The carbon-black spectrum (Figure 2b) has a similar shape,owing to ita partly graphitic structure. Less sp2bonding occurs in amorphous carbon. The broad, featureless shape of the u* peak in the spectrum for evaporated carbon film (Figure 2c) is caused by the superposition of intensities from the many different
Table I. Sample Collection Periods for Phoenix Aerosol Particles" sample no.
date collected
1 2
July 6, 1989 August 3, 1989 December 14, 1989 January 11,1990 January 25, 1990 February 15, 1990 March 22, 1990
3
4 5 6 7
collection end start 6:45 a.m. 6:53 a.m. 725 a.m. 7:42 a.m. 7:42 a.m. 720 a.m. 726 a.m.
4:53 p.m. 4:45 p.m. 940 a.m. 956 a.m. 10:06 a.m. 1010 a.m. 9:33 a.m.
Samples were collected on the roof of the Maricopa County Air Pollution Control office, a two-story building at 1845 E. Roosevelt Avenue in Phoenix, AZ.
randomly oriented bonding configurations typical of an amorphous material (25).The peak height ratio a*/u*is thus indicative of the ratio of graphitic to amorphous structure for individual spherules within aggregate particles. Goals of This Work. Previous studies have suggested small-scale structural variations occur in carbonaceous aerosol particles. In this work, structural variations in individual carbonaceous aerosol particles have been investigated using transmission electron microscopy with EELS, a promising technique for detection of structural variations on a scale as small as 0.01 pm. The knowledge gained is used to evaluate processes contributing to the formation of particulate carbon.
Methods Sample Collection. Aerosol samples were collected on the roof of the two-story Maricopa County Air Pollution Control building at 1845 E. Roosevelt, Phoenix, AZ, by filtration of air through evaporated carbon film supported by 200-mesh Cu TEM grids. Each grid is 3.0 mm in diameter. There are irregular rounded holes (approximately 1 pm in diameter) throughout the carbon film. During collection, each grid rested on a polycarbonate filter containing 0.2-pm pores, mounted below a filter with 8-pm pores, all in a 47-nun filter holder. Approximately 1m3/h ambient air was pumped through each grid during the 2-10-h collection periods between July 1989 and March 1990. Samples were stored at room temperature in a desiccator following collection. Sample storage periods averaged approximately1year prior to acquisition of the data presented here. It is possible that some of the organic content of the particles was lost during storage, but comparison of the energy-loss spectra used in this study with spectra obtained in preliminary analyses conducted no later than 1 week after sampling showed no significant differences, suggesting little or no change in particle composition. To avoid altering the carbonaceous particles, samples were not carbon-coated prior to examination with the TEM. Samples were selected for analysis from collection dates spread among the different seasons of the year (Table I). Winter samples were chosen most frequently because of the polluted conditions associated with the Phoenix haze season (7).Lower particulate concentrations in the summer months necessitated collection periods of up to 10 h, compared with 2-3 h for winter samples. Samples of graphite and carbon black were prepared for comparison with the aerosol samples. The graphite sample was collected from a well-known mineral deposit near Ticonderoga, NY. The carbon black sample was purchased commercially. The graphite and carbon black were ground separately in acetone with an agate mortar and pestle and
dispersed on evaporated carbon film supported by TEM grids. TEM Analysis and Data Processing. Samples were analyzed at an accelerating voltage of 100 kV, using a Philips EM400ST TEM equipped with a field-emission gun. Attachments used for EELS were a Gatan Model 607 electron energy-loss spectrometer and Model 666 1000channel parallel detector. Spectral acquisition was controlled using a data acquisition system based on an LSI 11/73 microcomputer. The acquisition system was developed at Arizona State University. All energy-loss data were obtained at
