Environ. Sci. Technol. 1985, 79, 682-685
Quantitative Energy-Dispersive Analysis of Lead Halide Particles from the Phoenix Urban Aerosol Jeffrey E. Post* and Peter R. Buseck Departments of Chemistry and Geology, Arizona State University, Tempe, Arizona 85287
Quantitative energy-dispersive spectrometry X-ray analyses (& 10%) were performed on approximately 300 P b halide particles (0.2-2 pm in diameter) from the Phoenix aerosol. The most abundant species appears to be a-2PbBrC1.NH4C1,comprising about one-third of the total particles. Other compounds identified are PbBrCl, (PbO)2.PbBrC1,and probably 3Pb3(P04)2.PbBrC1. Particles having compositions different from previously reported atmospheric P b halide compounds might represent mixed phases or as yet unidentified species. Only about 5% of the particles have Br/C1 molar ratios > 1.0, and no particles with only Br and P b were observed. It is suggested that Br loss occurs from atmospheric P b halides having Br/C1 or Br/Pb > 1.0, while P b halides with Br/C1 < -1.0 experience little halide loss.
Introduction A major source of Pb in the atmosphere, particularly in urban areas, is automobile emissions (1). Prior to 1976, all gasoline burned by automobiles in the United States contained 1.5-3 g/gal(0.4-0.8 g/L) of Pb as an antiknock additive (2). The Pb was added in the form of (C2H,),Pb, and C2H4C12and C2H4Br2were added as P b scavengers. Since 1976, automobiles produced in the United States have been designed to use unleaded gasoline. However, there are still a large number of vehicles in use that burn leaded gasoline. Most P b emitted from automobiles is in the form of halide salts that can alter chemically in the air (3). Previous studies of Pb compounds in the atmosphere suggest that their compositions are varied and complex. From powder X-ray diffraction (XRD) measurements on automobile exhaust particles, Hirschler and Gilbert ( 4 ) concluded that PbBrCl, 2PbBrC1.NH4C1, and PbBrC1.2NH4C1 are major phases; if P is present in the gasoline as an impurity or additive, then as much as 20% of the P b compounds occur as 3Pb3(P04)2.PbBrC1.Habibi (5)using XRD and X-ray fluorescence suggested that the major P b salt is PbBrCl with large amounts of PbO occurring as 2PbO.PbBrCl; lesser amounts of PbS04 and Pb3(P04)2 were also reported. Habibi (5) noted that the larger particles (2-10 pm in diameter) are mainly PbBrCl while the predominant submicron species is 2PbBrC1.NH4C1. O’Conner et al. (6) using bulk analyses and chemical mass balance techniques concluded that PbBr2 was the major P b salt in the air over Perth, Australia. An XRD study by Biggins and Harrison (7) determined PbSO4-(NH4)2S04 to be the major aerosol P b compound at five locations in England. They also encountered PbS04, PbBrCl, PbBrC1.2NH4C1, a-2PbBrC1.NH4C1,and PbBrC1.(NH4),BrC1. Most of the above studies of atmospheric Pb-bearing particles have relied on bulk analytical methods, primarily XRD. However, it is normally not possible with bulk techniques to obtain detailed information about chemical speciation, elemental associations, and particle morphology. XRD is useful for identifying chemical compounds ~
*Address correspondence to this author at the Department of Mineral Sciences, Smithsonian Institution, Washington,DC 20560. 682
Environ. Sci. Technol., Vol. 19, No. 8, 1985
in aerosols, but it cannot detect amorphous or poorly crystalline substances or materials that are present in low abundances. Obviously, individual particle analysis methods might provide important and unique information about Pb-bearing particles in urban aerosols. Previous studies of individual Pb-rich particles using the electron microprobe or analytical scanning-electron microscope (ASEM) have yielded only qualitative (8) or, at best, semiquantitative results (3). During the past several years our research group has developed the capabilities to perform accurate analyses of individual particles larger than -0.1 pm. As part of a recent study of individual particles in the Phoenix urban aerosol (9, IO),we analyzed more than 8000 individual ambient aerosol particles, including several hundred Pb-bearing particles, most of which are P b halides emitted from automobiles. In this paper we present the results of our analyses of the individual P b halide particles with two purposes in mind: (1) to demonstrate that quantitative energy-dispersive X-ray analyses can be performed on individual atmospheric P b halide particles and (2) to provide information about speciation of P b halide particles (>0.2 pm) in the Phoenix aerosol.
Experimental Section The particle samples were collected onto Nuclepore filters at three sites in the Phoenix area on Feb 9 and 10, 1980. The sample collection and preparation procedures are described by Post (9) and Post and Buseck (10). Qualitative analyses using an ASEM (JEOL JSM 35 with attached PGT energy-dispersive X-ray analyzing system) revealed that about 90% of the Pb-bearing particles contain Pb halides as major components, commonly with minor amounts of P, Fe, Zn, S, and Si; particles showing only P b or P b with Fe, Zn, and S were also observed. Most of the Pb-bearing particles are smaller than 2 pm in diameter (measured from ASEM images). Poor resolution of ASEM images at operating conditions necessary for energy-dispersive spectrometry (EDS) analysis prevented analyses of particles smaller than about 0.2 pm in diameter. Quantitative EDS analyses of the atmospheric Pb halide particles were performed by using the Kcu X-ray lines for C1 and Br and the La line for Pb, at an accelerating voltage of 25 kV. During analysis, the electron beam was defocused so that the entire particle was scanned by the electron beam. Polished specimens of RbBr, NaC1, and PbCr04 were used as analytical standards. The X-ray intensities were corrected by using the particle analysis programs of Aden (11)and Aden and Buseck (12). One potential problem when analyzing for Pb using EDS methods is the overlap between the Pb Ma and S Ka lines. This is particularly of concern because (NH4)2S04, (NH4)2S04.PbS04and other sulfates are reported to be abundant in urban aerosols. We avoided this overlap problem when determining halide to P b ratios of particles using the P b La line. Also, for approximately 20 particles, we analyzed for both P b and S, using the intensity of the P b La line and subtracting the corresponding P b Ma line intensity from the combined Pb Ma, S K a peaks. In a few
0013-936X/85/0919-0682$01.50/0
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Table I. Pb Halide Compounds That Have Been Identified or Proposed To Exist in the Atmosphere no." 351 3.0
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compound PbBrCl-2NH4Cl PbBrC1.(NH4)zBrC1 cu-2PbBrC1.NH4Clc PbClzC PbBrz PbBrClc Pb(0H)Cl Pb(0H)Br PbOSPbBrCl PbO.PbBrC1.HzO (PbO)p*PbClp (PbO)z.PbBrz (PbO)z.PbBrCIC 3Pb3(P04)2-PbBrC1C
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cases, minor amounts of S were observed. We did not, however, analyze for S in most of the Pb halide particles represented in Figure 1. As a check of the analytical procedures, reagent-grade PbBr2 and PbC12were ground to yield particles between about 0.5 and 2 pm in diameter (the size range of Pb halide particles in the aerosol) and were analyzed in the same manner as the P b halide particles collected from the atmosphere. Analyses of the standard PbC12 particles showed relative errors of about f5% (typically, C1 was too high relative to Pb). Initial analyses of the PbBr2 particles were consistently low in Br by as much as 50% relative, suggesting that Br is lost from the particles during analysis. Particles analyzed for short counting periods ( -5%) decrease in the count rate for the Br X-rays. A possible explanation for the difference in Br loss between reagent PbBrz and ambient P b halides is discussed below. Quantitative EDS analyses were peformed on 60 P b halide particles from the Phoenix aerosol. Total X-ray counting time per particle was 200 s, and during that period the count rate for the Br Ka line was monitored to detect loss of Br from the sample. Br loss during analysis
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did not exceed -5% for any of the particles. The results of these analyses were used to prepare calibration curves for molar ratios of Cl/Pb and Br/Pb. Approximately 250 additional P b halide particles were analyzed for X-ray counting times of 30 s per particle. Cl/Pb and Br/Pb molar ratios were determined by using the calibration curves. As a check, several particles were analyzed, and halide to P b ratios were determined by using both the correction programs and the calibration curves. In all cases, the two methods yielded results that agreed within &lo%, which is about the estimated error for all analyses reported here. The results of the analyses are plotted in Figure 1. For each of the particles analyzed, the EDS spectrum was checked for the presence of elements (2 I 11)other than Pb, Br, and C1, and in the few cases where major amounts of other elements were detected (e.g., Fe, Zn, and Si), these analyses were excluded from Figure 1 (except in the case of P).
Results One of the problems with interpreting individual particle analyses, especially in the submicron size range, is determining whether a particle is monophasic or a mixture of two more species. For some of the larger particles (>1pm), SEM or BSE images are helpful in checking for homogeneity but are not always definitive. Our nominal assignment of species to Pb-halide-containing particles is based on comparison of experimentally measured halide to P b ratios to values for compounds known or reported to exist in urban aerosols (Table I). For example, particles showing Cl/Pb and Br/Pb of near 1.0 are consistent with PbBrCl; however, the particle might in fact be a mixture or reaction product formed from, e.g., PbBrCl and (NH4)2S04.It is also possible that a particle with the above halide to P b ratios could consist of two or more P b halides that as a mixture fortuitously yield ratios consistent with PbBrCl. Because of the problem of possible particle inhomogeneity, we refrain from making species assignments for an individual particle but rather look for clusters of particle analyses that have similar halide to P b ratios (Figure 1). The fact that the major concentrations of points in Figure 1 have P b to halide ratios near those for compounds that have previously been reported to occur in urban aerosols and that many of the larger particles appear monophasic in SEM images (many being spherical or rod shaped; Figure 2) gives us some confidence that we are recognizing at least the major Pb halide species in the >0.2 pm size range in the Phoenix aerosol. Even though identification of actual species is by necessity somewhat Environ. Sci. Technol., Vol. 19, No. 8, 1985
883
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!7gm2. SEM images of F% halide partkbs from the Roenix aerosol. Particles A-C have W P b = -1.3-1.5 and B r / b = -08: particle D yields relatively low X-ray intensities and is probably a mixture of Pb halide and soot. The scale bars represent 1 pm.
tentative, our analyses do provide information about the range of CI and Br to P b ratios in atmospheric particles. For example, Figure 1 shows only a few particles with BrjCl or Br/Pb values >1.0. The largest concentration of points in Figure 1is centered at about Br/Pb = 0.9 and CI/Pb = 1.4-1.5. The only known aerosol Pb compound with halide to P b ratios close to these values is a2PbBrC1.NH4C1 (Br/Pb = 1.0;Cl/Pb = 1.51, which is reported to be a common atmospheric P b halide in the submicron size range (5). Approximately one-third of the P b halide particles larger than -0.2 pm in the Phoenix aerosol have halide to P b ratios consistent with a-2PbBrCI.NH4C1. The scatter of points in Figure 1 about the ideal ratios might be due to analytical error, the presence of minor amounts of other elements (e.g., Fe, Zn, P, Mn, and Ca) in some particles that would alter the halide to P b ratios, and agglomeration of particles of diverse compositions. Also, the fact that most of the particles have halide to P b ratios lower than the ideal values suggests that losses of small amounts of halogen (especially Br) occurred before or during analyses. A second clustering of points occurs in Figure 1at about Br/Pb = 0.1-0.3 and CljPb = 0.1-0.5. The only known aerosol P b compounds that have halide to P b ratios that fall near these ranges are (PhO),.PbBrCl (Br/Pb = Cl/Pb = 0.33) and 3Pb3(P04),.PbBrCI (Br/Pb = CI/Br = 0.1). Several of the particles included in this second grouping contain P, which is commonly present as an impurity or additive in gasoline (5). The scatter of points probably occurs for the same reasons mentioned above. Most of the particles in this second grouping, however, are depleted in Br, in some m e s by more than 50%. relative to the ideal values for (PbO),.PbBrCI and 3Pb3(P04),.PbBrC1. Thus, either these particles have lost significant amounts of Br before or during analysis, are mixtures of two or more phases, or are P b halide compounds not previously reported from urban aerosols (e.g., with CI/Pb 0.3 and Br/Pb 0.1). In addition to the P b halides mentioned above, Table I lists several other Pb-hearing compounds previously observed in urban aerosols. Particles with compositions close to PbBrCI, PbCI2, and (PbO),.PbCl, were found in
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Environ. Sci. Technol.. VoI.
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the Phoenix samples; however, these do not appear to be abundant species in the Phoenix aerosol. Points in Figure 1 that do not correspond to compositions of Pb halide compounds in Table I probably represent agglomerates of two or more particles or might be compounds that have not yet been identified. There seem to be small concentrations of points in Figure 1centered at halide to P b ratios that do not correspond to known aerosol compounds, for example, at CI/Pb = -0.34.5, CI/Pb = -1.75, and Br/Pb = -0.7-0.75. These concentrations suggest that perhaps previously unreported Pb halides are present. Possibly these "new" compounds, if indeed they are that, are not abundant enough or are too poorly crystalline to be ohserved in XRD measurements, or perhaps they do not occur in the urban areas where earlier studies have been conducted. One obvious feature of Figure 1is that few particles have Br/Cl or Br/Pb molar ratios greater than 1.0. Only about 5% of the particles have Br/CI > 1.0, and no particles of PbBr, (or any combination of only P b and Br) were observed in the Phoenix aerosol samples. An obvious conclusion might be that Br-rich P b compounds are not produced by automobiles. This is the likely case since the most common lead additive composition has a Pb/Br/Cl rato of 1/1/2, making emission of PbBr, improbable. A second possibility is that Br-rich particles are generally smaller than -0.2 pm in diameter and therefore were not observed in this study. Alternatively, perhaps Br-rich P b compounds are present in fresh exhaust but are less stable in aerosols than are other P b halides and thus are not observed in aged samples (analyses were performed several weeks to months after sample collection). Presumably the depletion of Br from PbBr, or Pb(0H)Br results in the formation of compounds like Pb(OH),, PbC03, or PbO, all of which show only P b in EDS spectra. However, only a few such Pbrich particles were observed, indicating that Br-rich P b halides are probably not major species in automobile exhaust, a t least not in the size fraction studied here.
Discussion of Br Loss from Pb Halides In the last decade there has been much interest in the possible loss of Br from atmospheric P b halide particles. According to most studies (3,13,14), aerosols aged longer than a few hours show a depletion of Br. The exact mechanism for the Br loss is not understood. Mechanisms that have been proposed include acid-base reactions with CO,2- or OH- (3)and diffusion followed by volatilization (13).
Boyer and Laitinen (15) prepared and studied various P b halide particles, and in general, they found them to be stable toward halide loss by exchange with H 2 0 or COP. The only significant halogen loss they observed was for Br-rich halides exposed to ultraviolet light. They noticed that the degree of photodecomposition in air for Br and CI compounds increases exponentially with a linear increase in the Br/Pb ratio. After 30 h, more than 30% of the Br is lost from PbBr2 On the other hand, extrapolation of their data shows that for PbBrCl less than 0.5% of the halogen is lost after 30 h, and Pb halides with Br/Pb < 1.0 experience no significant loss of halogen. These results are consistent with the analyses of particles from the Phoenix aerosol: i.e., few particles with Br/Pb > 1.0 were detected. The explanation for why Br-rich P b halides are more unstable than the CI-rich isomorphs is not known, although work by Calingaert et al. (16) implies that this stability difference might result because of small differences in their crystal structures. XRD studies of the isomorphous com-
pounds PbC12and PbBr2 show that their unit cells possess two halogen sites with different volumes. As Br substitutes for C1 in PbCl,, it preferentially replaces the C1 from the larger sites until these C1 atoms are completely replaced; upon further substitution of Br for C1, the smaller of the halogen sites is occupied by Br. It seems more than coincidental that almost all of the apparently stable aerosol P b halides have Br/Cl < -1.0; perhaps the instability of the Br-rich P b halides arises from substitution of the Br into the smaller halogen position in the PbC12 unit cell, thereby creating a more expanded structure for the Br-rich relative to the C1-rich P b halides. The exact mechanism for Br loss is, however, not known.
Morphologies of Aerosol Pb Halides SEM images of several P b halide particles from the Phoenix aerosol are shown in Figure 2. The particles are spherical, cylindrical, irregularly shaped or, in some cases, occur as flattened "spongy-looking"mats. Commonly, the P b halides are associated with what appears to be soot (shows no elements in EDS spectra); this is not surprising since most of the non-lead-bearing particles in automobile exhaust are soot (5). The spongy-looking P b halide particles give low X-ray count rates relative to other P b halide particles and probably are a mixture of soot and Pb halides. Unfortunately, attempts to distinguish among the various P b compounds on the basis of morphology have, thus far, been unsuccessful. In Figure 2, for example, particles A-C have different morphologies but almost the same compositions (Br/Pb = -0.8; Cl/Pb = -1.3-1.5), probably corresponding to 2PbBrCl.NH4C1. Summary We have demonstrated that it is possible to perform quantitative EDS analyses (f10%) on aerosol P b halide particles larger than -0.2 ,urn in diameter. Although our data must be interpreted carefully because of possible particle inhomogeneity, our results provide a more detailed examination than has previously been possible of elemental associations and ratios that exist in Pb-bearing aerosol particles. Also, Figure 1does show concentrations of points centered near halide to P b ratios consistent with values of compounds known to exist in urban aerosols, suggesting that many of the particles we examined might be monophasic, at least with regard to the P b halide species present [e.g., we probably cannot distinguish between PbBrCl and a mixture of (NH4)2S04and PbBrCl]. Approximately one-third of the particles larger than -0.2 ,urn in diameter have halide to P b ratios near that of a-2PbBrC1.NH4C1, which is consistent with results from earlier studies ( 4 , 5 ) . Other common species probably include Pb02.PbBrC1, PbBrC1, and 3Pb3(P04)2.PbBrC1. P b halide compounds with a variety of other compositions were also encountered and might be mixtures of two or more phases, or Pb compounds not previously observed in the atmosphere. Only about 5% of the particles analyzed have Br/C1> 1.0, and none contain only P b and Br, suggesting that Br-rich P b halide compounds either are not prevalent in automobile emissions (at least not as particles larger than 0.2 pm in diameter) or are unstable in the atmosphere. P b halides with Br/Cl < -1.0 appear to be relatively stable in the atmosphere, supported by the fact that many yield analyses close to compositions of P b halide compounds known to exist in urban aerosols. These results are also consistent with the observation that PbBr, particles lose Br during EDS analyses whereas PbC12 and atmospheric P b halides
with Cl/Br > -1.0 experience little or no halogen loss. Thus, it is likely that the loss of Br observed for aerosol P b compounds occurs primarily from phases having Br/Cl or Br/Pb > -1.0. As mentioned above, in a recent XRD study of P b compounds from aerosols over five cities in England, Biggins and Harrison (7) report PbS04.(NH4)2S04to be the major P b phase along with PbS04, PbBrCl.(NH4)2BrC1, a-2PbBrC1.NH4C1, PbBrC1, and PbBrC1.2NH4Cl. They classify the first three compounds as formed by reactions with atmospheric sulfates and the latter three as vehicle emitted. We observed only a few Phoenix aerosol particles that showed P b and S in EDS spectra, and most were smaller than about 0.5 ,urn. Possibly the lead sulfate species reported by Biggins and Harrison are, in general, smaller than the size range examined here, or maybe the arid climate of Phoenix is not conducive to the reactions that they reported as producing lead sulfate in the more moist environment of England. Perhaps quantitative EDS analysis, using a transmission electron microscope, of P b halide particles smaller than those studied here can resolve this question.
Acknowledgments We thank Gary Aden, Ellen Thomas, Sherwood Idso, and John Bradley for assistance with sample collection and for insightful discussions. Registry No. a-2PbBrC1.NH4C1,72532-13-9; (PbO)2.PbBrC1, 11078-76-5;3Pb,(P04)2.PbBrC1, 96452-97-0;PbBrC1,13778-36-4; PbC12, 7758-95-4; (PbO)z*PbCl2, 12205-70-8.
Literature Cited (1) Marshall, E. Science (Washington, D.C.) 1982, 215, 1375-1378. (2) Provenzano. G. J . Air. Pollut. Control Assoc. 1978., 28.. 1193-1199. Ter Haar, G. L.; Bayard, M. A. Nature (London) 1971,232, 553-554. Hirschler, D. A.; Gilbert, L. F. Arch. Enuiron. Health 1964, 8,297-313. Habibi, K. Environ. Sci. Technol. 1973, 7, 223-234. O'Conner, B. H.; Kerrigan, G. C.; Thomas, W. W.; Pearce, A. T. Atmos. Enuiron. 1977, 11, 635-638. Biggins, P. D. E.; Harrison, R. M., Enuiron. Sci. Technol. 1979,13,558-565. Linton, R. W.; Natusch, D. F. S.; Solomon, R. L.; Evans, C. A., Jr. Enuiron. Sci. TechnoE. 1980, 14, 159-164. Post, J. E. Ph.D. Dissertation, Arizona State University, Tempe, AZ, 1981. Post, J. E.; Buseck, P. R. Enuiron. Sci. Technol. 1984,18, 35-42. Aden, G. D. Ph.D. Dissertation, Arizona State University, Tempe, AZ, 1981. Aden, G. D.; Bueeck, P. R. Microbeam Anal. 1983,18th, 159-201. Robbins, J. A.; Snitz, F. L. Enuiron. Sci. Technol. 1972,6, 164-169. Martens, C. S.; Wesolowski, J. J.; Kaifer, R.; John, W. Atmos. Enuiron. 1973, 7, 905-914. Boyer, K. W.; Laitinen, H. A. Enuiron. Sci. Technol. 1974, 8, 1093-1096. Calingaert, G.; Lamb, F. W.; Meyer, F. J. Am. Chem. SOC. 1949, 71, 3709-3720.
Received for review February 6,1984. Revised manuscript received January 17,1985. Accepted February 21,1985. This work was supported by Grants ATM-8022849 and ATM-8404022 from the Atmospheric Chemistry Division of the NSF.
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