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A qualitative study of atmospheric aerosols and particles deposited on flat membrane surfaces by microscopy and other techniques
Powder Technology 161 (2006) 235 – 241 www.elsevier.com/locate/powtec
A qualitative study of atmospheric aerosols and particles deposited on flat membrane surfaces by microscopy and other techniques Carlos M. Romo-Kro¨ger * Basic Sciences Institute, Catholic University of Maule, Casilla 617, Talca, Chile Received 14 July 2005; received in revised form 25 August 2005; accepted 26 October 2005 Available online 5 January 2006
Abstract Samples of atmospheric particulate matter placed on Nuclepore\ filters were analyzed by microscopy and complementary techniques. Volcanic ash placed on glass surfaces and atmospheric material on Teflon\ filters were also studied by microscopy. The analyzed samples are related to volcanic eruptions, urban environments, and mining activity. Nuclepore filters are thin sheets with smooth surfaces on which particles are fixed almost uniformly after filtration, which makes them especially adequate for study by microscopy. Here a selection of samples analyzed by microscopy is presented, along with information on the shape and size of particles and their distribution on the surface of the filter. Hypotheses are advanced on the origin of particles, their crystalline states, and the physical mechanisms to trap particles on the filter surface. A morphological classification of the particles was carried out according to their appearance in the several samples analyzed. Other complementary analyses performed by PIXE (particle induced X-ray emission), by electron microprobe, by X-ray diffraction and by granulometry, are commented. The implications of the particles studied to human health are also discussed. D 2005 Elsevier B.V. All rights reserved. Keywords: Electron and optic microscopy; Atmospheric particles; Volcanic aerosols; Urban aerosols; Mining aerosols; Nuclepore\ filters
1. Introduction Aerosol particle analysis is thought of mainly as the determination of its chemical composition and gravimetric studies. However, several other physicochemical characteristics of air particles are major topics of study, complementary to the former analyses. Size, shape and crystalline state of aerosol particles have very important implications in the fields of surface deterioration, light extinction and in public health. Particle morphology is a main source of information in aerosol studies. It is directly related to the aerosol chemical composition, its sources and environmental effects. Optical and electron microscopy are combined to provide information on the structure of particles, sizes, sharpness of the borders, transparency, etc. It is known that certain compounds, such as salts and oxides, have specific geometric structures [1– 4]. Cubic, tetrahedral and other polygonal structures are observed in microscopic particles. Other particle shapes may be associated with the production of
* Tel.: +56 71 203383; fax: +56 71 242890. E-mail address: [email protected]. 0032-5910/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.powtec.2005.10.014
pumice and glasses in volcanic eruptions [5,6]. Particles of metallic origin, or those from oil combustion or from coal burning and refinery, appear as spherical fly ash or agglomerates of small particles [4,7 – 10]. Fibrous particles may arise from vegetables, asbestos or other materials. They are also the subject of systematic studies [11,12]. The aim of this work is to perform a morphological characterization of particles of the different studied environments, and to establish correlations between the structure of collected microparticles with their source, chemical composition, crystalline state and health risks. Particular attention is paid to specific structures observed in samples coming from the different environments. In urban environments it is possible to find almost any particle structure, while much more definite structures were found in environments near volcano eruptions and near a mine with rough material processing. 2. Sample description Samples were airborne particulates collected mainly on Nuclepore filters with pore diameters of 0.4 Am and 8.0 Am. Nuclepore filters are thin foils (10 Am thickness) of polycar-
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bonate resin with smooth surfaces and straight uniform diameter pores. After filtration, particles remain adhered to the filter surface, thus serving directly for microscopic or fluorescence analysis. Microscopy gives valuable information on the physical mechanisms taking place for trapping particles on filters during the filtration process. These mechanisms are impaction, interception and diffusion [13,14]. Electrical effects are also important in Nuclepore filtration, particles adhere to the filter surface by electrical and dipolar forces [15 –17]. Other samples included: airborne particles collected in the vicinity of a large copper smelter, urban aerosol filtered on Teflon filter mats and volcanic ash powder deposited on glass slides. Collection of particles was performed in the Chilean territory in the early 1990s, in zones near the last eruptions of the volcanoes Lonquimay and Hudson, the cities Santiago, Temuco and San Felipe, and adjacent to a copper mine with a crushing plant and rough material operations. Lonquimay volcano (38.37 S, 71.58 W) was active during all of 1989 and January 1990, affecting the nearby rural communities and the town of Lonquimay (10,000 inhabitants) located 20 km SE. It caused the death of many animals and human illnesses in the region [18]. The Hudson volcano is farther South in Chile (46.17 S, 72.92 W), it was active for several months in 1991. Its tephra fall was detected thousands of kilometers away [18] and it was reported to contribute importantly to dust fallout in the Argentinean Patagonia [19].
Santiago (29.5 S, 69.4 W) is the capital of Chile, with 5.5 million inhabitants. It has been recognized as a polluted city for a long time, due to a large number of vehicles and industries inside the city, and because it is surrounded by hills and mountains. San Felipe is a rather small city (80,000 inhabitants) in the central part of Chile (32.45 S, 70.44 W). It is surrounded by agricultural zones, with some industrial and commercial activity, and a large copper smelter 50 km to the West. Temuco (38.41 S, 72.35 W) is the major city of the southern part of Chile. It is characterized by vehicular flow, wood burning and some local industry. The other sampling location was inside the Lo Aguirre copper mine. It is located 25 km W of Santiago in a rural area. There are detonations in rocks, a crushing plant and leaching with sulfuric acid. It is a large generator of airborne dust particles in the zone. Samples were collected on Nuclepore filters at an air flow of 7 lt/min for 24 h, with stacked filter units containing two sequential filters. The inlet cutoff diameter for particles is 15 Am. In principle, particles with diameters between 15 and 2.5 Am remain on the first filter (coarse filter: 8.0 Am pore diameter) and particles with lesser diameters are collected on the second filter (fine filter: 0.4 Am pore diameter). Samples on Teflon filters were collected in the center of Santiago close to a main street (Providencia), in a city park. Volcanic ash was collected at ground level in remote zones near the
Fig. 1. SEM images of samples of volcanic origin. a, b and c correspond to fine filter samples (0.4 Am pore diameter) collected near the eruption of the Hudson volcano. d corresponds to a coarse filter sample (8.0 Am pores diameter) collected 20 km SE of the Lonquimay eruption. In e the sample was collected on a fine filter 1 km E of the Lonquimay eruption. Pores are seen as black circles and particles as lighter objects.
C.M. Romo-Kro¨ger / Powder Technology 161 (2006) 235 – 241
Lonquimay volcano eruption and transferred to glass slides for microscopic analysis. 3. Micrographs and other results For optical microscopy, the Nuclepore membrane filter is used directly as the object in the microscope. A LEITZ microscope with polarized light transmission was used. The light source, combined with an adjustable polarizer, underlies the object. This permitted definition of transparence and birefringence of particles and the filter. The size, shape, relief and distribution of individual particles on filter were determined from optical and SEM micrographs. Scanning electron microscopy (SEM) was performed in a PHILIPS EM 300 microscope. Pieces of the original filter (less than 1 cm2) were mounted on plastic and coated with a 30 nm gold film in a vacuum camera for the electron irradiation. The observation of micrographs permits the analysis of the morphology of particles, their physical state (solid, liquid, crystalline state, etc.) and the physical mechanisms for trapping them on filters. Figs. 1 –4 contain photographs obtained by SEM of the different samples on Nuclepore filters. The magnification is deduced by the pore size, in these photos pores are always
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visible as black circles. Filters of the different sampling localities were selected for this photograph sequence. Pores are seen in black, the backing is gray and particles are seen as light objects. The pore sizes are 8.0 Am for coarse filters and 0.4 Am for fine filters. Photos are fairly representative of the sampling sites. In certain cases the scanning was focused to special areas on the sample where interesting particles appeared. Fig. 1 corresponds to samples of volcanic origin. Figs. 1a, b and c correspond to fine filter samples collected near the eruption of the Hudson volcano. Fig. 1d corresponds to a coarse filter sample collected at the town of Lonquimay, 20 km SE of the Lonquimay volcano. The sample in Fig. 1e was collected at Portezuelo, 1 km E of the Lonquimay eruption. In both volcanic samples very well defined particulate objects are presented. Particles with acute edges and points can be observed, especially visible in photos with larger augment. They are probably originated in a fast cooling and shaking of the emerging material during the eruption. Melted granite, quartz and other materials can evolve to form micron and submicron size particles, which can fly kilometers of distance until where samples were collected. The maximum linear dimension of particles observed in the plane of the micrographs is as large as 13 Am in the coarse filter (Fig. 1d) and 7 Am in the
Fig. 2. SEM images of samples of urban origin. a to d correspond to coarse (b – d) and fine (a) filter samples collected in the city of Santiago. e to h are of samples from the city of San Felipe. e and h are for coarse filters and f and g are for fine filters. Pores are seen as black circles and particles as lighter objects.
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fine filter (Fig. 1b). Submicron particles can be observed even on the coarse filter, indicating more than 0% efficiency for these fine particles. Fig. 2 shows urban aerosols from two different cities. Fig. 2a to d correspond to coarse (Fig. 2b– d) and fine (Fig. 2a) filter samples collected in Santiago. In comparison, Fig. 2e to h are samples from the city of San Felipe. Fig. 2e and h are coarse filters and f and g are fine filters. Much more heterogeneous particles are seen in Fig. 2 for urban aerosols. These include irregularly shaped particles, spherical and wedged particles, and even vegetal objects such as pollen and spores (see Fig. 2h). This heterogeneity is due to the diversity of activities in the cities with potential to generate particles. Possible bacteria are seen as micrometric cylinders in urban samples (see Fig. 2g). Fig. 3 contains particles from Santiago (Fig. 3a to c) and of mining origin (Fig. 3d and e). Fig. 3a is of a fine filter, b and c are of coarse filters. Balled formations and agglomerations can be seen as the main objects in these photos. Also fly ash particles (spheres) are observed in Fig. 3c and fiber particles, possibly of asbestos, in Fig. 3a. In Fig. 3b an ethereal particle formation remains as a rigid body intercepted at the pore entrance, indicating strong bonds linking the minuscule components. More definite structures and sharpness are presented in particles of mining origin (Fig. 3d and e), including some clusters similar to the urban case. These
samples are on coarse filters. No special shapes (spheres or fibers) are observed in these samples. Fig. 4 includes the cities San Felipe, Temuco and Santiago. Fig. 4a is of a coarse filter collected in San Felipe. On the pore border is a 6 Am spherical particle with other minor sized particles on it, including an hexagonal particle, characteristic of some crystalline compounds. Also clusters and agglomerations are observed in this photo. Interesting formations of similar shape and size were observed distributed on the filter surface in a sample collected in Temuco. Fig. 4b shows one of these formations. These are thin films adhered to the backing surface, which can be seen as colored spots in the light microscope. These formations may have originated in some liquid particles in air crystallized by the low environmental temperature at night, then melted on the filter surface at higher temperatures, and finally dried during the evaporation for SEM analysis. In this city, temperatures below 0 -C are common in winter, when the sample was collected. Liquid particle sampling, under these conditions, could be an interesting topic for a future study. Photos of Fig. 4c to e are of samples on Teflon filter mats. Fig. 4c is a blank filter, showing its fibrous texture. Fig. 4d and e correspond to a regular sampling performed by the Chilean Health Ministry at the center of Santiago close to a large street inside a city park. Different shapes of particles are observed, as in other urban cases. Especially interesting are those round and oval particles of biological aspect, probably vegetal spores due
Fig. 3. a to c contain SEM images of particles from Santiago. d and e are SEM images of mining origin particles. a is of a fine filter, the rest of the images are of coarse filters. Pores are seen as black circles and particles as lighter objects.
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Fig. 4. SEM images of samples of three cities: San Felipe, Temuco and Santiago. a is of a coarse filter collected in San Felipe. On the pore border is a 6 Am spherical particle with other minor size particles on it, including a hexagonal particle. b is of a coarse filter collected in Temuco, a formation adhered to the filter surface is observed, possibly collected in the liquid phase. Pores are seen as black circles and particles as lighter objects. c to e are samples of Teflon filter. Both cover a field of 28 Am 21 Am. c is a blank filter, showing its fibrous texture. d and e correspond to Santiago, in a city park near a large street. Grains of vegetal origin may be observed.
to the presence of plants. Other cubic or tetragonal particles are observed in Fig. 4d on the left side. They could be of salts or oxides with origin in industrial operations in the city. Fig. 5 shows micrographs resulting from light microscopy. Each photo represents an area of 160 Am 110 Am. They were taken by transmission of polarized white light. Fig. 5a is a collection of ash particles on a microscope slide. This is volcanic ash settled at ground level 1 km from the Lonquimay eruption. Here the transparency of particles may be observed, with a glassy aspect and sharp points (knife-like points, such as the particle in the fourth quadrant). Their origin is probably in
the fragmentation of slag produced in the eruptive process, consisting of amorphous quartz and other materials. Fig. 5b corresponds to a sample collected on a coarse Nuclepore filter at the same site as the collected ash of Fig. 5a. Holes (pores) are seen as brilliant circles (most of them obstructed by particles). Particles are seen in dark, with red tones at pore sites, or transparent (see the two particles in the third quadrant). The background is the filter surface, seen in blue due to the suppression of some wave lengths by the birefringence of this resin. A certain structure or relief is appreciated on the filter surface by a lens effect of the transmitted light. In both samples
Fig. 5. Light microscopy micrographs taken by transmission of polarized white light. a corresponds to a portion of volcanic ash collected 1 km E of the town of Lonquimay and deposited on a microscope slide. Note the transparency of particles, their glassy aspect and sharp points. b corresponds to a sample collected on coarse filter at the same site. Pores are seen as brilliant circles (most of them obstructed by particles). Particles are seen dark or transparent (note the two transparent particles in the third quadrant). The filter surface is seen in blue due to a light polarization effect.
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(Fig. 5a and b), powder particles are well distinguished and with similar structure. Complementary analysis by PIXE (particle induced X-ray emission) provided information on the average elemental contents of particulate material collected on filters. A Nuclepore filter is an uncontaminated material used directly for particle irradiation in PIXE for elemental content determination. For the volcanic aerosols the elemental contents deter-
Table 1 Morphological classification of the objects in the material deposited in filters
Picture
Characteristics
Origin
• Fragmentation type particles • Acute edges
• Crushing plants
• Particle mask covering filler pores • Biluminous
• Urban
• Drying of soluble fraction of aerosols • Coating formations
• Urban
• Filiform particles • Transparent
• Copper mines • Crushing plants • Urban (rarely)
• Volcanos
mined by PIXE [20] were: Al (16%), Si (42%), S (8%), Ca (9%), Ti (3%), Fe (19%). The same percentages for Santiago were [21]: Al (11%), Si (29%), S (22%), K (6%), Ca (10%), Ti (9%), Fe (12%). Also Pb, Br and Zn were abundant in the Santiago’s samples. Important levels of As, Zn, Cu and S were found [22] among the components of particles from a large copper smelter in Chile (El Teniente). The PIXE analysis of the cellulose filters used by workers in the crushing plant of Lo Aguirre copper mine gave [23] important contents of Al, Si, K, Ca, Ti, Mn, Fe, Cu and Kr. Electron probe X-ray fluorescence gave a qualitative indication of the elemental contents of individual particles. This technique constitutes a complement to the ordinary PIXE analysis and presents advantages over the sophisticated micro-PIXE systems [24]. The volcanic ash material was also analyzed by crystallography and by granulometry. X-ray diffraction was performed in a PW 1050/80 diffractometer with graphite monochromator. Sodic calcic feldspar, (Ca, Na) (Si, Al)4O8, and also aluminic augite, (Ca, Mg, Al, Fe) Si2O6, were found in the material, with an abundance of calcium and iron, and also silicon and aluminum. These results are consistent with the mentioned PIXE data. Granulometry provided results on the mass percentages by particle sizes. The mass percentage of the ash material under 45 Am is 18%. A much greater value would be expected if the percentage was calculated in relation to the number of particles. Several factors may be directly related to the morphology of air particles. A morphological study provides information on particle origin, health risks and physicochemical properties. Classifications of particles according to their shapes have been performed to predict their dynamics in gases [25,26]. Table 1 is a morphological classification of the particles studied by microscopy in the different samples. It presents the typical pictures of the observed particles, some characteristics and the assigned origin for each type of particle. This table does not include geometric forms, such as spheres, cylinders, cubes or polyhedrons; which are frequently found in samples and are also associated with specific origins. 4. Discussion and conclusions
• Glass type particles • Transparent
• Copper mines • Volcanos
• Agglomerations • Melted metal type
• Copper mines • Copper smelters • Volcanos (rare)
Some micrographs resulting from the complete sampling campaign were presented here. As a result of these and other observations we may formulate some conclusions. Particles arising from volcanoes are well separated from one another, are of definite shapes, with marked borders and points. Much more heterogeneous shapes of particles are found in aerosols from urban environments. These are mainly amorphous and irregularly shaped particles, although they may include spheres, wedges, and other geometric forms. Even vegetal objects such as pollen and spores may be found. This situation may be explained by the presence in the cities of vehicle combustion, foundries, demolitions, vegetation, wood burning, etc. Definite structures characteristic of ore fragmentation are present in particles of mining origin. In these, some clusters and chain aggregates are observed, similar to the urban case.
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The behavior of particles is another matter of study through the observation of the micrographs. Nuclepore filters with pore diameters 8.0 Am are reported as 100% efficient to collect particles larger than 2.5 Am and 0% efficient for particles less than this. The simple observation of the photos shows a large number of submicron particles on the surface of this coarse filter (see Fig. 3b). Another observation in filter photos is that there are many particles adhered to the borders of the pores. The mechanisms of interception, diffusion and impaction are not sufficient to explain this phenomenon, and it is better explained by dipolar forces originated by strong gradients of electric fields occurred in the proximity of the border of pores [17]. Some formations, such as those in Fig. 3b and d, are aggregates of minor size particles held together by strong bonds. These kinds of formations behave as rigid bodies, similar to those observed in aerosols originated in flame reactors [27]. The former observations can shed light on the mechanisms of the filtration process and particle dynamics. Collection of atmospheric aerosols on Nuclepore membrane filters was an excellent method for further analysis by microscopy. Teflon filters are frequently used for further analyzes by gravimetry and by opacimetry. In this qualitative study, a Teflon filter was used as an alternative method of particle collection for observation by SEM (Fig. 4c– e). It presents some differences from the analysis by microscopy. While deposition of particles in Nuclepore membranes is mainly in a single plane, in Teflon particulate matter remains at different levels in the filter mat, limiting the observation of inner particles. On the other hand, Teflon presents a high capacity of particle retention at high volumes of blown air. Transparency and color observation, and some sense of depth, are advantages of optic microscopy in the study of microparticles, which complements electron microscopy. Having a morphological classification of particles, as a result of microscopic observation, is a useful means to determine particle behavior, origins and health risks. A certain correlation exists between the geometry of microparticles and macroscopic ones, compare the commonly observed crushed stones in gravel with those particles from volcanic origin (Fig. 1), or compare products of a quartz mine with the polygonal particle in Fig. 4a. Particles with a polyhedral aspect (cubic or other polygonal forms) are comparable to normal sizes mineralogical pieces, and also comparable with the reported crystallographic morphology of nanoparticles [28]. The glassy aspect of volcanic aerosol particles seems to be of amorphous quartz and similar to the volcanic scoria. These particles are mostly insoluble to any agent and with their acute cutting edges present potential for lung damage with a long permanency in lungs. Particles of vegetal origin, and even airborne bacteria, have potential health risks. Pollen grains [29] and fungi [30] in the air, as may be found in environments with vegetation, can induce allergies and airway inflammation.
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Acknowledgements Acknowledgements to the University of Chile, Sciences Faculty, Nuclear Physics Laboratory for performing the PIXE analysis, Electron Microscopy Unit for providing SEM analysis, and the Solid Physics Laboratory for the use of light microscope; and Physical and Mathematics Sciences Faculty, for the microprobe studies, to La Frontera University for collecting volcano samples, the Environment Health Service of the Metropolitan Region, Chile, for providing samples on Teflon, the Chilean Commission of Research, CONICYT, for supporting sampling in copper mines, and to the Chilean Commission of Nuclear Energy, for the X-ray diffractometry. References [1] M. Grasserbauer, Chp. 8 in Analysis of airborne particles by physical methods, in: H. Malissa (Ed.), CRC Press Inc., Florida, 1978, pp. 125 – 178. [2] I. Watt, Microsc. Anal. (57) (2002 (November)) 19 – 21. [3] D.J. Gaspar, et al., Appl. Surf. Sci. 231 – 232 (2004) 520 – 523. [4] P. Bacci, et al., J. Aerosol Sci. 14 (1983) 557 – 572. [5] H. Sigurdsson, Science 216 (1982) 1106 – 1108. [6] J.S. Fruchter, et al., Science 209 (1980) 1116 – 1125. [7] V.B. Stoyanova et al., J. Int. Res. Publ., Online, 2001/02, issue 2. no. 6, 2002. [8] C. Xiong, S.K. Friedlander, Proc. Natl. Acad. Sci. U. S. A. 98 (2001) 11851 – 11856. [9] S. Weinbruch, et al., J. Environ. Monit. 4 (2002) 344 – 350. [10] E.J. Schulz, R.B. Egdahl, T.T. Frankenberg, Atmos. Environ. 9 (1975) 111 – 119. [11] K.R. Spurny, et al., J. Air Pollut. Control Assoc. 26 (1976) 496 – 498. [12] K.R. Spurny, J. Aerosol Sci. 28 (Suppl. 1) (1997) S157 – S158. [13] K. Spurny, Environ. Sci. Technol. 3 (1969) 453 – 463. [14] K. Spurny, K.R. Spurny, Advances in Aerosol Filtration, CRC PressLewis Publishers, 1998. [15] C. Romo Kro¨ger, J.R. Rosales, Phys. Scr. 37 (1988) 270 – 273. [16] C.M. Romo Kro¨ger, J. Electrost. 25 (1990) 145 – 154. [17] C.M. Romo-Kro¨ger, V. Dı´az, J. Aerosol Sci. 27 (1996) 751 – 757. [18] Link to global volcanism network’s (Lonquimay Information, Cerro Hudson Information). [19] D.M. Gaiero, et al., Geochim. Cosmochim. Acta 67 (2003) 3603 – 3623. [20] J.R. Morales, M.I. Dinator, F. Llona, C.M. Romo-Kroger, J. Radioanal. Nucl. Chem. 172 (1993) 181 – 192. [21] C.M. Romo-Kro¨ger, Environ. Pollut. 68 (1990) 161 – 170. [22] C.M. Romo-Kro¨ger, et al., Atmos. Environ. 28 (1994) 705 – 711. [23] C.M. Romo-Kro¨ger, in: P.R. Cheremisinoff (Ed.), Encyclopedia of Environmental Control Technology, vol. 7, Gulf Publishing Co., Houston, 1995, pp. 389 – 403 Chp. 18. [24] C.M. Romo-Kro¨ger, Nucl. Instrum. Methods Phys. Res., B Beam Interact. Mater. Atoms 85 (1994) 845 – 848. [25] M. Hidy, Aerosols, an Industrial and Environmental Science, Academic Press Inc., Orlando, 1984, p. 21. [26] A. Ogawa, Separation of Particles from Air and Gases, CRC Press Inc., Florida, 1984, p. 121. [27] R. Bandyopadhyaya, A.A. Lall, S.K. Friedlander, Powder Technol. 139 (2004) 193 – 199. [28] J. Dundurs, L.D. Marks, P.M. Ajayan, Philos. Mag. 57 (1988) 605 – 620. [29] G D’Amato, Allergy 57 (2002) 30 – 33. [30] S. Rutherford, et al., J. Occup. Environ. Med., JOEM 42 (2000) 882 – 891.
A qualitative study of atmospheric aerosols and particles deposited on flat membrane surfaces by microscopy and other techniques Carlos M. Romo-Kro¨ger * Basic Sciences Institute, Catholic University of Maule, Casilla 617, Talca, Chile Received 14 July 2005; received in revised form 25 August 2005; accepted 26 October 2005 Available online 5 January 2006
Abstract Samples of atmospheric particulate matter placed on Nuclepore\ filters were analyzed by microscopy and complementary techniques. Volcanic ash placed on glass surfaces and atmospheric material on Teflon\ filters were also studied by microscopy. The analyzed samples are related to volcanic eruptions, urban environments, and mining activity. Nuclepore filters are thin sheets with smooth surfaces on which particles are fixed almost uniformly after filtration, which makes them especially adequate for study by microscopy. Here a selection of samples analyzed by microscopy is presented, along with information on the shape and size of particles and their distribution on the surface of the filter. Hypotheses are advanced on the origin of particles, their crystalline states, and the physical mechanisms to trap particles on the filter surface. A morphological classification of the particles was carried out according to their appearance in the several samples analyzed. Other complementary analyses performed by PIXE (particle induced X-ray emission), by electron microprobe, by X-ray diffraction and by granulometry, are commented. The implications of the particles studied to human health are also discussed. D 2005 Elsevier B.V. All rights reserved. Keywords: Electron and optic microscopy; Atmospheric particles; Volcanic aerosols; Urban aerosols; Mining aerosols; Nuclepore\ filters
1. Introduction Aerosol particle analysis is thought of mainly as the determination of its chemical composition and gravimetric studies. However, several other physicochemical characteristics of air particles are major topics of study, complementary to the former analyses. Size, shape and crystalline state of aerosol particles have very important implications in the fields of surface deterioration, light extinction and in public health. Particle morphology is a main source of information in aerosol studies. It is directly related to the aerosol chemical composition, its sources and environmental effects. Optical and electron microscopy are combined to provide information on the structure of particles, sizes, sharpness of the borders, transparency, etc. It is known that certain compounds, such as salts and oxides, have specific geometric structures [1– 4]. Cubic, tetrahedral and other polygonal structures are observed in microscopic particles. Other particle shapes may be associated with the production of
* Tel.: +56 71 203383; fax: +56 71 242890. E-mail address: [email protected]. 0032-5910/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.powtec.2005.10.014
pumice and glasses in volcanic eruptions [5,6]. Particles of metallic origin, or those from oil combustion or from coal burning and refinery, appear as spherical fly ash or agglomerates of small particles [4,7 – 10]. Fibrous particles may arise from vegetables, asbestos or other materials. They are also the subject of systematic studies [11,12]. The aim of this work is to perform a morphological characterization of particles of the different studied environments, and to establish correlations between the structure of collected microparticles with their source, chemical composition, crystalline state and health risks. Particular attention is paid to specific structures observed in samples coming from the different environments. In urban environments it is possible to find almost any particle structure, while much more definite structures were found in environments near volcano eruptions and near a mine with rough material processing. 2. Sample description Samples were airborne particulates collected mainly on Nuclepore filters with pore diameters of 0.4 Am and 8.0 Am. Nuclepore filters are thin foils (10 Am thickness) of polycar-
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C.M. Romo-Kro¨ger / Powder Technology 161 (2006) 235 – 241
bonate resin with smooth surfaces and straight uniform diameter pores. After filtration, particles remain adhered to the filter surface, thus serving directly for microscopic or fluorescence analysis. Microscopy gives valuable information on the physical mechanisms taking place for trapping particles on filters during the filtration process. These mechanisms are impaction, interception and diffusion [13,14]. Electrical effects are also important in Nuclepore filtration, particles adhere to the filter surface by electrical and dipolar forces [15 –17]. Other samples included: airborne particles collected in the vicinity of a large copper smelter, urban aerosol filtered on Teflon filter mats and volcanic ash powder deposited on glass slides. Collection of particles was performed in the Chilean territory in the early 1990s, in zones near the last eruptions of the volcanoes Lonquimay and Hudson, the cities Santiago, Temuco and San Felipe, and adjacent to a copper mine with a crushing plant and rough material operations. Lonquimay volcano (38.37 S, 71.58 W) was active during all of 1989 and January 1990, affecting the nearby rural communities and the town of Lonquimay (10,000 inhabitants) located 20 km SE. It caused the death of many animals and human illnesses in the region [18]. The Hudson volcano is farther South in Chile (46.17 S, 72.92 W), it was active for several months in 1991. Its tephra fall was detected thousands of kilometers away [18] and it was reported to contribute importantly to dust fallout in the Argentinean Patagonia [19].
Santiago (29.5 S, 69.4 W) is the capital of Chile, with 5.5 million inhabitants. It has been recognized as a polluted city for a long time, due to a large number of vehicles and industries inside the city, and because it is surrounded by hills and mountains. San Felipe is a rather small city (80,000 inhabitants) in the central part of Chile (32.45 S, 70.44 W). It is surrounded by agricultural zones, with some industrial and commercial activity, and a large copper smelter 50 km to the West. Temuco (38.41 S, 72.35 W) is the major city of the southern part of Chile. It is characterized by vehicular flow, wood burning and some local industry. The other sampling location was inside the Lo Aguirre copper mine. It is located 25 km W of Santiago in a rural area. There are detonations in rocks, a crushing plant and leaching with sulfuric acid. It is a large generator of airborne dust particles in the zone. Samples were collected on Nuclepore filters at an air flow of 7 lt/min for 24 h, with stacked filter units containing two sequential filters. The inlet cutoff diameter for particles is 15 Am. In principle, particles with diameters between 15 and 2.5 Am remain on the first filter (coarse filter: 8.0 Am pore diameter) and particles with lesser diameters are collected on the second filter (fine filter: 0.4 Am pore diameter). Samples on Teflon filters were collected in the center of Santiago close to a main street (Providencia), in a city park. Volcanic ash was collected at ground level in remote zones near the
Fig. 1. SEM images of samples of volcanic origin. a, b and c correspond to fine filter samples (0.4 Am pore diameter) collected near the eruption of the Hudson volcano. d corresponds to a coarse filter sample (8.0 Am pores diameter) collected 20 km SE of the Lonquimay eruption. In e the sample was collected on a fine filter 1 km E of the Lonquimay eruption. Pores are seen as black circles and particles as lighter objects.
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Lonquimay volcano eruption and transferred to glass slides for microscopic analysis. 3. Micrographs and other results For optical microscopy, the Nuclepore membrane filter is used directly as the object in the microscope. A LEITZ microscope with polarized light transmission was used. The light source, combined with an adjustable polarizer, underlies the object. This permitted definition of transparence and birefringence of particles and the filter. The size, shape, relief and distribution of individual particles on filter were determined from optical and SEM micrographs. Scanning electron microscopy (SEM) was performed in a PHILIPS EM 300 microscope. Pieces of the original filter (less than 1 cm2) were mounted on plastic and coated with a 30 nm gold film in a vacuum camera for the electron irradiation. The observation of micrographs permits the analysis of the morphology of particles, their physical state (solid, liquid, crystalline state, etc.) and the physical mechanisms for trapping them on filters. Figs. 1 –4 contain photographs obtained by SEM of the different samples on Nuclepore filters. The magnification is deduced by the pore size, in these photos pores are always
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visible as black circles. Filters of the different sampling localities were selected for this photograph sequence. Pores are seen in black, the backing is gray and particles are seen as light objects. The pore sizes are 8.0 Am for coarse filters and 0.4 Am for fine filters. Photos are fairly representative of the sampling sites. In certain cases the scanning was focused to special areas on the sample where interesting particles appeared. Fig. 1 corresponds to samples of volcanic origin. Figs. 1a, b and c correspond to fine filter samples collected near the eruption of the Hudson volcano. Fig. 1d corresponds to a coarse filter sample collected at the town of Lonquimay, 20 km SE of the Lonquimay volcano. The sample in Fig. 1e was collected at Portezuelo, 1 km E of the Lonquimay eruption. In both volcanic samples very well defined particulate objects are presented. Particles with acute edges and points can be observed, especially visible in photos with larger augment. They are probably originated in a fast cooling and shaking of the emerging material during the eruption. Melted granite, quartz and other materials can evolve to form micron and submicron size particles, which can fly kilometers of distance until where samples were collected. The maximum linear dimension of particles observed in the plane of the micrographs is as large as 13 Am in the coarse filter (Fig. 1d) and 7 Am in the
Fig. 2. SEM images of samples of urban origin. a to d correspond to coarse (b – d) and fine (a) filter samples collected in the city of Santiago. e to h are of samples from the city of San Felipe. e and h are for coarse filters and f and g are for fine filters. Pores are seen as black circles and particles as lighter objects.
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fine filter (Fig. 1b). Submicron particles can be observed even on the coarse filter, indicating more than 0% efficiency for these fine particles. Fig. 2 shows urban aerosols from two different cities. Fig. 2a to d correspond to coarse (Fig. 2b– d) and fine (Fig. 2a) filter samples collected in Santiago. In comparison, Fig. 2e to h are samples from the city of San Felipe. Fig. 2e and h are coarse filters and f and g are fine filters. Much more heterogeneous particles are seen in Fig. 2 for urban aerosols. These include irregularly shaped particles, spherical and wedged particles, and even vegetal objects such as pollen and spores (see Fig. 2h). This heterogeneity is due to the diversity of activities in the cities with potential to generate particles. Possible bacteria are seen as micrometric cylinders in urban samples (see Fig. 2g). Fig. 3 contains particles from Santiago (Fig. 3a to c) and of mining origin (Fig. 3d and e). Fig. 3a is of a fine filter, b and c are of coarse filters. Balled formations and agglomerations can be seen as the main objects in these photos. Also fly ash particles (spheres) are observed in Fig. 3c and fiber particles, possibly of asbestos, in Fig. 3a. In Fig. 3b an ethereal particle formation remains as a rigid body intercepted at the pore entrance, indicating strong bonds linking the minuscule components. More definite structures and sharpness are presented in particles of mining origin (Fig. 3d and e), including some clusters similar to the urban case. These
samples are on coarse filters. No special shapes (spheres or fibers) are observed in these samples. Fig. 4 includes the cities San Felipe, Temuco and Santiago. Fig. 4a is of a coarse filter collected in San Felipe. On the pore border is a 6 Am spherical particle with other minor sized particles on it, including an hexagonal particle, characteristic of some crystalline compounds. Also clusters and agglomerations are observed in this photo. Interesting formations of similar shape and size were observed distributed on the filter surface in a sample collected in Temuco. Fig. 4b shows one of these formations. These are thin films adhered to the backing surface, which can be seen as colored spots in the light microscope. These formations may have originated in some liquid particles in air crystallized by the low environmental temperature at night, then melted on the filter surface at higher temperatures, and finally dried during the evaporation for SEM analysis. In this city, temperatures below 0 -C are common in winter, when the sample was collected. Liquid particle sampling, under these conditions, could be an interesting topic for a future study. Photos of Fig. 4c to e are of samples on Teflon filter mats. Fig. 4c is a blank filter, showing its fibrous texture. Fig. 4d and e correspond to a regular sampling performed by the Chilean Health Ministry at the center of Santiago close to a large street inside a city park. Different shapes of particles are observed, as in other urban cases. Especially interesting are those round and oval particles of biological aspect, probably vegetal spores due
Fig. 3. a to c contain SEM images of particles from Santiago. d and e are SEM images of mining origin particles. a is of a fine filter, the rest of the images are of coarse filters. Pores are seen as black circles and particles as lighter objects.
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Fig. 4. SEM images of samples of three cities: San Felipe, Temuco and Santiago. a is of a coarse filter collected in San Felipe. On the pore border is a 6 Am spherical particle with other minor size particles on it, including a hexagonal particle. b is of a coarse filter collected in Temuco, a formation adhered to the filter surface is observed, possibly collected in the liquid phase. Pores are seen as black circles and particles as lighter objects. c to e are samples of Teflon filter. Both cover a field of 28 Am 21 Am. c is a blank filter, showing its fibrous texture. d and e correspond to Santiago, in a city park near a large street. Grains of vegetal origin may be observed.
to the presence of plants. Other cubic or tetragonal particles are observed in Fig. 4d on the left side. They could be of salts or oxides with origin in industrial operations in the city. Fig. 5 shows micrographs resulting from light microscopy. Each photo represents an area of 160 Am 110 Am. They were taken by transmission of polarized white light. Fig. 5a is a collection of ash particles on a microscope slide. This is volcanic ash settled at ground level 1 km from the Lonquimay eruption. Here the transparency of particles may be observed, with a glassy aspect and sharp points (knife-like points, such as the particle in the fourth quadrant). Their origin is probably in
the fragmentation of slag produced in the eruptive process, consisting of amorphous quartz and other materials. Fig. 5b corresponds to a sample collected on a coarse Nuclepore filter at the same site as the collected ash of Fig. 5a. Holes (pores) are seen as brilliant circles (most of them obstructed by particles). Particles are seen in dark, with red tones at pore sites, or transparent (see the two particles in the third quadrant). The background is the filter surface, seen in blue due to the suppression of some wave lengths by the birefringence of this resin. A certain structure or relief is appreciated on the filter surface by a lens effect of the transmitted light. In both samples
Fig. 5. Light microscopy micrographs taken by transmission of polarized white light. a corresponds to a portion of volcanic ash collected 1 km E of the town of Lonquimay and deposited on a microscope slide. Note the transparency of particles, their glassy aspect and sharp points. b corresponds to a sample collected on coarse filter at the same site. Pores are seen as brilliant circles (most of them obstructed by particles). Particles are seen dark or transparent (note the two transparent particles in the third quadrant). The filter surface is seen in blue due to a light polarization effect.
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(Fig. 5a and b), powder particles are well distinguished and with similar structure. Complementary analysis by PIXE (particle induced X-ray emission) provided information on the average elemental contents of particulate material collected on filters. A Nuclepore filter is an uncontaminated material used directly for particle irradiation in PIXE for elemental content determination. For the volcanic aerosols the elemental contents deter-
Table 1 Morphological classification of the objects in the material deposited in filters
Picture
Characteristics
Origin
• Fragmentation type particles • Acute edges
• Crushing plants
• Particle mask covering filler pores • Biluminous
• Urban
• Drying of soluble fraction of aerosols • Coating formations
• Urban
• Filiform particles • Transparent
• Copper mines • Crushing plants • Urban (rarely)
• Volcanos
mined by PIXE [20] were: Al (16%), Si (42%), S (8%), Ca (9%), Ti (3%), Fe (19%). The same percentages for Santiago were [21]: Al (11%), Si (29%), S (22%), K (6%), Ca (10%), Ti (9%), Fe (12%). Also Pb, Br and Zn were abundant in the Santiago’s samples. Important levels of As, Zn, Cu and S were found [22] among the components of particles from a large copper smelter in Chile (El Teniente). The PIXE analysis of the cellulose filters used by workers in the crushing plant of Lo Aguirre copper mine gave [23] important contents of Al, Si, K, Ca, Ti, Mn, Fe, Cu and Kr. Electron probe X-ray fluorescence gave a qualitative indication of the elemental contents of individual particles. This technique constitutes a complement to the ordinary PIXE analysis and presents advantages over the sophisticated micro-PIXE systems [24]. The volcanic ash material was also analyzed by crystallography and by granulometry. X-ray diffraction was performed in a PW 1050/80 diffractometer with graphite monochromator. Sodic calcic feldspar, (Ca, Na) (Si, Al)4O8, and also aluminic augite, (Ca, Mg, Al, Fe) Si2O6, were found in the material, with an abundance of calcium and iron, and also silicon and aluminum. These results are consistent with the mentioned PIXE data. Granulometry provided results on the mass percentages by particle sizes. The mass percentage of the ash material under 45 Am is 18%. A much greater value would be expected if the percentage was calculated in relation to the number of particles. Several factors may be directly related to the morphology of air particles. A morphological study provides information on particle origin, health risks and physicochemical properties. Classifications of particles according to their shapes have been performed to predict their dynamics in gases [25,26]. Table 1 is a morphological classification of the particles studied by microscopy in the different samples. It presents the typical pictures of the observed particles, some characteristics and the assigned origin for each type of particle. This table does not include geometric forms, such as spheres, cylinders, cubes or polyhedrons; which are frequently found in samples and are also associated with specific origins. 4. Discussion and conclusions
• Glass type particles • Transparent
• Copper mines • Volcanos
• Agglomerations • Melted metal type
• Copper mines • Copper smelters • Volcanos (rare)
Some micrographs resulting from the complete sampling campaign were presented here. As a result of these and other observations we may formulate some conclusions. Particles arising from volcanoes are well separated from one another, are of definite shapes, with marked borders and points. Much more heterogeneous shapes of particles are found in aerosols from urban environments. These are mainly amorphous and irregularly shaped particles, although they may include spheres, wedges, and other geometric forms. Even vegetal objects such as pollen and spores may be found. This situation may be explained by the presence in the cities of vehicle combustion, foundries, demolitions, vegetation, wood burning, etc. Definite structures characteristic of ore fragmentation are present in particles of mining origin. In these, some clusters and chain aggregates are observed, similar to the urban case.
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The behavior of particles is another matter of study through the observation of the micrographs. Nuclepore filters with pore diameters 8.0 Am are reported as 100% efficient to collect particles larger than 2.5 Am and 0% efficient for particles less than this. The simple observation of the photos shows a large number of submicron particles on the surface of this coarse filter (see Fig. 3b). Another observation in filter photos is that there are many particles adhered to the borders of the pores. The mechanisms of interception, diffusion and impaction are not sufficient to explain this phenomenon, and it is better explained by dipolar forces originated by strong gradients of electric fields occurred in the proximity of the border of pores [17]. Some formations, such as those in Fig. 3b and d, are aggregates of minor size particles held together by strong bonds. These kinds of formations behave as rigid bodies, similar to those observed in aerosols originated in flame reactors [27]. The former observations can shed light on the mechanisms of the filtration process and particle dynamics. Collection of atmospheric aerosols on Nuclepore membrane filters was an excellent method for further analysis by microscopy. Teflon filters are frequently used for further analyzes by gravimetry and by opacimetry. In this qualitative study, a Teflon filter was used as an alternative method of particle collection for observation by SEM (Fig. 4c– e). It presents some differences from the analysis by microscopy. While deposition of particles in Nuclepore membranes is mainly in a single plane, in Teflon particulate matter remains at different levels in the filter mat, limiting the observation of inner particles. On the other hand, Teflon presents a high capacity of particle retention at high volumes of blown air. Transparency and color observation, and some sense of depth, are advantages of optic microscopy in the study of microparticles, which complements electron microscopy. Having a morphological classification of particles, as a result of microscopic observation, is a useful means to determine particle behavior, origins and health risks. A certain correlation exists between the geometry of microparticles and macroscopic ones, compare the commonly observed crushed stones in gravel with those particles from volcanic origin (Fig. 1), or compare products of a quartz mine with the polygonal particle in Fig. 4a. Particles with a polyhedral aspect (cubic or other polygonal forms) are comparable to normal sizes mineralogical pieces, and also comparable with the reported crystallographic morphology of nanoparticles [28]. The glassy aspect of volcanic aerosol particles seems to be of amorphous quartz and similar to the volcanic scoria. These particles are mostly insoluble to any agent and with their acute cutting edges present potential for lung damage with a long permanency in lungs. Particles of vegetal origin, and even airborne bacteria, have potential health risks. Pollen grains [29] and fungi [30] in the air, as may be found in environments with vegetation, can induce allergies and airway inflammation.
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Acknowledgements Acknowledgements to the University of Chile, Sciences Faculty, Nuclear Physics Laboratory for performing the PIXE analysis, Electron Microscopy Unit for providing SEM analysis, and the Solid Physics Laboratory for the use of light microscope; and Physical and Mathematics Sciences Faculty, for the microprobe studies, to La Frontera University for collecting volcano samples, the Environment Health Service of the Metropolitan Region, Chile, for providing samples on Teflon, the Chilean Commission of Research, CONICYT, for supporting sampling in copper mines, and to the Chilean Commission of Nuclear Energy, for the X-ray diffractometry. References [1] M. Grasserbauer, Chp. 8 in Analysis of airborne particles by physical methods, in: H. Malissa (Ed.), CRC Press Inc., Florida, 1978, pp. 125 – 178. [2] I. Watt, Microsc. Anal. (57) (2002 (November)) 19 – 21. [3] D.J. Gaspar, et al., Appl. Surf. Sci. 231 – 232 (2004) 520 – 523. [4] P. Bacci, et al., J. Aerosol Sci. 14 (1983) 557 – 572. [5] H. Sigurdsson, Science 216 (1982) 1106 – 1108. [6] J.S. Fruchter, et al., Science 209 (1980) 1116 – 1125. [7] V.B. Stoyanova et al., J. Int. Res. Publ., Online, 2001/02, issue 2. no. 6, 2002. [8] C. Xiong, S.K. Friedlander, Proc. Natl. Acad. Sci. U. S. A. 98 (2001) 11851 – 11856. [9] S. Weinbruch, et al., J. Environ. Monit. 4 (2002) 344 – 350. [10] E.J. Schulz, R.B. Egdahl, T.T. Frankenberg, Atmos. Environ. 9 (1975) 111 – 119. [11] K.R. Spurny, et al., J. Air Pollut. Control Assoc. 26 (1976) 496 – 498. [12] K.R. Spurny, J. Aerosol Sci. 28 (Suppl. 1) (1997) S157 – S158. [13] K. Spurny, Environ. Sci. Technol. 3 (1969) 453 – 463. [14] K. Spurny, K.R. Spurny, Advances in Aerosol Filtration, CRC PressLewis Publishers, 1998. [15] C. Romo Kro¨ger, J.R. Rosales, Phys. Scr. 37 (1988) 270 – 273. [16] C.M. Romo Kro¨ger, J. Electrost. 25 (1990) 145 – 154. [17] C.M. Romo-Kro¨ger, V. Dı´az, J. Aerosol Sci. 27 (1996) 751 – 757. [18] Link to global volcanism network’s (Lonquimay Information, Cerro Hudson Information). [19] D.M. Gaiero, et al., Geochim. Cosmochim. Acta 67 (2003) 3603 – 3623. [20] J.R. Morales, M.I. Dinator, F. Llona, C.M. Romo-Kroger, J. Radioanal. Nucl. Chem. 172 (1993) 181 – 192. [21] C.M. Romo-Kro¨ger, Environ. Pollut. 68 (1990) 161 – 170. [22] C.M. Romo-Kro¨ger, et al., Atmos. Environ. 28 (1994) 705 – 711. [23] C.M. Romo-Kro¨ger, in: P.R. Cheremisinoff (Ed.), Encyclopedia of Environmental Control Technology, vol. 7, Gulf Publishing Co., Houston, 1995, pp. 389 – 403 Chp. 18. [24] C.M. Romo-Kro¨ger, Nucl. Instrum. Methods Phys. Res., B Beam Interact. Mater. Atoms 85 (1994) 845 – 848. [25] M. Hidy, Aerosols, an Industrial and Environmental Science, Academic Press Inc., Orlando, 1984, p. 21. [26] A. Ogawa, Separation of Particles from Air and Gases, CRC Press Inc., Florida, 1984, p. 121. [27] R. Bandyopadhyaya, A.A. Lall, S.K. Friedlander, Powder Technol. 139 (2004) 193 – 199. [28] J. Dundurs, L.D. Marks, P.M. Ajayan, Philos. Mag. 57 (1988) 605 – 620. [29] G D’Amato, Allergy 57 (2002) 30 – 33. [30] S. Rutherford, et al., J. Occup. Environ. Med., JOEM 42 (2000) 882 – 891.