one type apparently arises when the air in the city becomes stagnant, while another type is clearly associated with unusually large industrial emissions. The Pb/Br ratio is not a practical tool for measuring the relative amounts of pollution from industrial and automotive sources-at least in an area with no strong industrial sources of Pb. A cknow ledgmerit The authors would like to acknowledge the assistance of D. Yesso in preparing standard solutions, and of W. Strang of the Allegheny County Air Pollution Bureau who provided data on carbon monoxide and fine particulates in the Pittsburgh area.
Literature Citec! (1) Johansson, T . B., Akselsson, R., Johansson, S. A. E., Nucl.
Instr. Meth., 84,141 (1970).
(2) Herman, A. W., McNelles, L. A., Campbell, J . L., “International Journal of Applied Radiation and Isotopes,” Vol. 24, p p 677-88, Pergamon Press, Holland, 1973. (3) Rhodes, J. R., Pradzynski, A. H., Hunter, C. B., Enuiron. Sci. Technol., 6,922 (1972). (4) Crocker Nuclear Lab., University of California, Davis, “Final R e ~ o r tto the California Air Resources Board on Cont. ARB50i,” UCD-CNL 169. (5) “Chemical Technolow, An Encyclopedic Treatment,” Vol. 1, pp 241-6, Barnes and Noble, New York, N.Y., 1968. (6) Spurny, K . R., Lodge, J . P., Frank, E. R., Sheesley, D. C., Enuiron. Sci. Technol., 3, 453 (1969). (7) Comfort, J. R., “Informal Report PHY-1970 B (unpublished),” Physics Division, Argonne National Laboratory, 1970. (8) Spink, P., Erskine, J. R., ibid., PHY-1965 B, 1965. (9) Robbins, J . A., Snitz, F . L., Enuiron. Sci. Technol., 6, 164 (1972). (10) Lininger, R. L., Duce, R. A., Winchester, J . W., Matson, W. R., J . Geophys. Res., 71,2457 (1966). Received for review July 27, 1973. Accepted June 13, 1974. Work supported by National Science Foundation Grant No. GP-23318.
Lead HaliUe Aerosols Some Properties of EnvironmentalSignificance Kenneth W. Boyer’ and Herbert A. Laitinen*** School of Chemical Sciences, University of Illinois, Urbana, 111. 61801
rn The physical and chemical properties of laboratory pure lead halide aerosol particles have been investigated. Aerosols generated from a lead bromochloride, PbBrC1, melt had a temperature-dependent bromide ion excess and chloride ion deficiency with respect to stoichiometric PbBrCl. A linear departure from the stoichiometric unity PbBrCl was accompanied by a n exponential change in the rate of ultraviolet light-induced photochemical decomposition. When exposed to carbon dioxide and water vapor, the laboratory pure aerosols were quite stable toward halogen loss by hydrolytic exchange, especially compared to literature reports of rapid halogen loss by automobile exhaust particulates.
In studies with burned leaded gasoline (1-3), X-ray diffraction showed that lead bromochloride, PbBrCl, is the most abundant lead-containing compound in exhaust particulates, with lesser amounts of the alpha and beta forms of the double salt ammonium chloride-lead bromochloride, NH4C1.2PbBrCl. One controversial aspect of automobile exhaust particulates with burned leaded gasoline has to do with whether the particulates readily lose halogen. Pierrard ( 4 ) reported that chemically pure PbBrCl is decomposed by ultraviolet light, releasing free bromine and chlorine. Robbins and Snitz ( 5 ) reported that freshly exhausted particulates readily lose halogen, particularly bromine (up t o 70% in the first 20 min after being exhausted). even in the absence of sunlight. Ter Haar and Bayard (6) also reported a large decrease in the brominePresent address, U.S. Food and Drug Administration, Washington, D.C. 20204 Present address, Department of Chemistry, University of Florida, Gainesville, Fla. 32611.
to-lead ratio in exhaust particulates within the first hour after being exhausted into a large black bag to exclude sunlight. Moran (7, 8) did not report any change in exhaust particulate composition with time, but did report a significant portion of unidentified bromine- and chlorinerich forms of PbBrCl. Analysis for bromine-to-lead ratios in atmospheric particulates (9, 10) and in lake sediments (11) indicates little loss of bromine if it is assumed that PbBrCl in automobile exhaust particulates is the major source of bromine and lead coexisting in these samples. Thus, the question arises as to whether the changes in composition that have been reported are due to fundamental properties of the lead compounds themselves, are due to their formation in and interaction with the remainder of the automobile exhaust, or are due to reactions in the atmosphere. Experimental
Lead bromine and lead bromochloride aerosols were produced from controlled temperature heating of the pure compounds, as illustrated in Figure 1. The lead halide aerosols collected on the quartz slide were physically characterized with either a Cambridge Stereoscan scanning electron microscope (SEM), or a JEOL JSM-US SEM, operating a t 30 keV accelerating potential. The aerosols were exposed to various atmospheric conditions by placing the quartz slide with collected aerosol in a Pyrex chamber with optically flat faces inside a constant temperature control box. Here the samples were exposed to high humidity, high COz atmospheres in the dark, or were irradiated with ultraviolet light with dry C02 free air or nitrogen sweeping through the chamber, o r finally were exposed to filtered automobile exhaust with and without simultaneous uv irradiation. To determine if halide ion was lost during exposure to high COz, high humidity conditions, the sweep gas was passed into a 0.05N NaHC03/0.05N Na2C03 buffer. After Volume8, Number 13, December 1974
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Figure 1. Apparatus for
generation of lead halide aerosols
exposure to the high COz, high-humidity atmosphere was stopped, the total halide ion captured was determined by coulometric titration with silver ion using a hiamperometric end point. No attempt was made to determine free halogen lost during exposure to high COz, high-humidity conditions. Lead halide aerosols were photodecomposed in dry COz free air or nitrogen a t 35°C by irradiating them with two Black-Lite B-100 A lamps a t an intensity of 6.3 mW/cm2 between 3000 & . and 4000 A as measured with a Black Lite 5-221 Long Wave W meter. Free halogen lost during photodecomposition was determined by passing the sweep gas into standard methyl orange solutions a t p H 2.1 (12). During the photodecomposition experiments, no attempt was made to determine halogen lost as halide ion. Total halide ion remaining in the aerosol samples after photodecomposition was determined by coulometric titration with silver ion. Chloride ion in the presence of bromide ion was determined using a modification of a reported procedure (13) in which bromide ion interference was eliminated by oxidizing the bromide ion to free bromine with hydrogen peroxide, followed hy reaction of the free bromine with 8-
hkl 4
h K I I
004 200
032
123
1
MOLE ' I D B i 0 VOLE %Cl- 1 0 0
50 50
%a2
100
0
PbErCl
Figure 3.
PbBr2
Lead halide aerosol d-spacings vs halide composition
Left: 101, 020, and 111 planes. Right: 004, 200, 032, and 123 planes
hydroxyquinoline. The remaining chloride ion was coulometrically titrated with silver ion as for total halide, with the bromide ion content then being determined by the difference between total halide and chloride only. The total lead ion was determined by spectrophotometric titration with EDTA using xylenol orange as indicator. X-ray diffract ion patterns of lead halide aerosols were obtained with a Phillips Norelco Model 12045 X-ray diffraction unit and a 114.59-nm diameter powder camera.
Results and Discussion As shown in Figure 2A, aerosols generated from PbBrCl above the melting point consisted of well-defined spheres. Varying the generator crucible and reaction chamber temperatures resulted not only in changes in particle size distribution, but also in a varying bromide to chloride ratio in the aerosol. Pure PbBrCl has a melting point range of 425436°C (14). Table I, which lists the elemental composition of several aerosols and the generation temperatures a t which they were produced, shows that the Br-/Clratio increases with decreasing crucible or chamber temperature. Figures 2A through 2D show the progressive change in morphology with increasing Br-/Cl- ratio and decreasing temperature. With the crucible at 510"C, well above the melting point of PbBrC1, the aerosol particles are well-defined spheres with a mean particle diameter of about 3 pm. With the crucible a t 370°C, well below the
melting point of PbBrCl, the aerosol particles were mostly needle-like crystals with a cross-sectional diameter of 0.5 fim or less. From the literature (14), it is known that PbBrz and PbC12 form a complete series of solid solutions with a preferential formation of the isomorphic 50 mol 70 compound PbBrC1. The interplanar spacings of several of the aerosols listed in Table I1 are plotted in Figure 3 (left and
Table II. Exposure of Lead Halide Aerosols to Various Atmospheric Conditions Mole ratio PbBr,CI, Before Exposure conditions
1 2 3 4 5 6 7 8
505 500 510 505 450 412 366 370
450 450 370 365 450 403 370 318
Mole ratio, PbBr,Cly, Y
1.21 1.24 1.25 1.28 1.23 1.58 1.64 1.78
0.85 0.80 0.74 0.75 0.83 0.47 0.48 0.35
X + Y
2.06 2.04 1.99 2.03 2.06 2.05 2.12 2.13
x f y
x
After y
x f y
%
Halide lost
2.00
0.0%
(a).
(a)
2.02 1.33 0.68 2.01
0.5%
(a)
(a)
2.02 1.36 0.65 2.01
0.5%
(a)
(a)
1.97 (a)
(a)
1.96
0.5%
(a)
(a)
2.02 (a)
(a)
1.92
5.0%
(a)
(a)
1.96 (a)
(a)
1.85
7.3%
dry N , 32"C, 51.5 h r
Aerosol composition
X
y
2.00 0.00 2.00 2.00 (a)
Dark, 9% CO, 3% HQOvapor 2 5 T , 24 h r Dark, 7% CO, 3% H?O vapor 25"C, 24 h r Dark, 7% CO, 12% H 2 0 vapor 50"C, 90 h r Dark, 9% CO, 12% H z O vapor 5 0 T , 72 h r
U v light, dry C 0 2 free air 3 2 T , 50 h r U v light,
Table 1. Composition of Lead Halide Aerosols Produced a t Several Temperatures Generation temp, OC Aerosol _ _ _ - - ~ no. Crucible Chamber
x
U v light, dry CO, free air 3 2 T , 25 h r
2.00 0.00 2.00 1.40 (a)
Filtered unleaded auto exhaust
1.24 0.80 2.04 1.24 0.79 2.03
0.5%
1.21 0.85 2.04 1.20 0.84 2.02
1.0%
1.40 30.1%
105"C, 1 h r
Filtered leaded auto exhaust u v light, 105°C 2 hr a
Not analyzed.
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Figure 4. Photodecomposition of lead halide aerosols in air 1.0
right) along with the spacings of corresponding planes for PbBr2, PbC12, and PbBrCl obtained with the same X-ray diffraction equipment. These plots, which show a definite change in slope at the 50 mol 70 composition, are very similar to those obtained by Calingaert et al. ( 1 4 ) from fused, aqueous, and heat-treated preparations from mixtures of known initial PbBrz and PbCl2 composition. The mechanism of photodecomposition of lead halides has been described ( 4 , 15) as absorption of a photon to release an electron and a hole that diffuse to trapping sites within the crystal lattice. Neutralization of trapped electrons by lead ions forms photolytic lead, while neutralization of holes by bromide and chloride ions produces free bromine and chlorine atoms that subsequently may diffuse to the surface to escape. The most likely explanation of the photodecomposition behavior noted here is an exponential increase in the electron-hole production due to a linear increase in the absorptivity of lead halide for photons with increasing bromide ion concentration. Figure 4 shows photodecomposition in air vs. time for several of the nonstoichiometric lead halide aerosols listed in Table I, along with a similar curve for PbBr2. Figure 5 shows that the degree of photodecomposition (defined here as the YO of available halide lost as free halogen) increases exponentially with a linear increase in the Br-/ Pb2+ ratio. Table I1 shows the results of exposing lead halide aerosols to various combinations of COz, water vapor, ultraviolet light, and filtered automobile exhaust. The only exposure conditions which result in any significant halogen loss by laboratory pure lead halides are those which include uv light. Even exposure to automobile exhaust, which contains about 12% water vapor, 12% CO2, 3% CO, and a large number of other compounds in lesser concentrations that could react with the lead halides, did not produce any extraordinary halogen loss. Likewise the combination of uv light and automobile exhaust did not significantly increase halogen loss. Thus, “laboratory pure” lead halide aerosols are quite stable toward halogen loss by hydrolytic exchange with H2O or COS, and even toward irradiation by uv light, especially compared to the large halogen loss reported for fresh automobile exhaust particulates (5, 6). It must therefore be concluded that
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1.h
1.4
1.8 2.0 Bi/Fb’+ IN IRRADIATED A E R O W L
1.6
Figure 5. Photodecomposition dependence on Br - / P b 2 + the large halogen losses observed for automobile exhaust particulates are not a fundamental characteristic of lead halide aerosols formed in the absence of the automotive exhaust environment.
Literature Cited (1) Hirschler, D. A. Gilbert, L. F., Niebylski, L. M., Ind. Eng. Chem., 49,1131 (1957). (2) Hirschler, D. A., Gilbert, L. F., Arch. Environ. Health, 8, 297
(1964). (3) Habibi, K., Jacobs, E. S., Kunz, W. G., Jr., Pastell, D. L., “Characterization and Control of Gaseous and Particulate Exhaust Emissions from Vehicles,” presented a t the Air Pollution Control Association, West Coast Section, Fifth Technical Meeting, San Francisco, Calif., October 1970. (4) Pierrard, J. M., Environ. Sei. Technol., 3,48 (1969). (5) Robbins, J. A., Snitz, F. L., 6, 164 (1972). (6) Ter Haar, G. L., Bayard, M . A., Nature, 232,553 (1971). (7) Moran, J. B., Manary, 0. J., Fay, R. H., Baldwin, M. J., “Development of Particulate Emission Control Techniques for Spark-Ignition Engines,” Final Report to the Environmental Protection Agency, by the Dow Chemical Co., Midland, Mich., July 1971. (8) Moran, J. B., Baldwin, M . J., Manary, 0. J., Valenta, J. C., “Effect of Fuel Additives on the Chemical and Physical Characteristics of Particulate Emissions in Automotive Exhaust,” ibid., June 1972. (9) Payne, J. S., Lindgren, J. L., Environ. Sei. Technol., 6, 922 (1972). (10) Bowman, H . R., Conway, J. G., Asaro, F., ibid., p 558. (11) Shimp, N. F., Leland, H. V., White, W. A., “Distribution of Major, Minor, and Trace Constituents in Unconstituted Sediment from Southern Lake Michigan,” Environmental Geology Notes, Illinois State Geological Survey No. 32, Urbana, Ill., 1970. (12) Laitinen, H. A., Boyer, K. W., Anal. Chem., 44,920 (1972). (13) Prokopov, T. S., ibid., 42,1096 (1970). (14) Calingaert, G., Lamb, F. W., Meyer, F., J. Amer. Chem. Soc., 71,3709 (1949). (15) Kaldor, A., Somarjai, G. A,, J . Phys. Chem., 70,3538 (1966). Received for review February 20, 1974. Accepted August 16, 1974. This work was supported by National Science Foundation RANN grant 31605. T h e electron microscopy equipment used at the University of Illinois Center for Electron Microscopy was provided by NSF Grant GA 123946-32-53-358. K . W.B. held a n Environmental Protection Agency Predoctoral Fellowship in Air Pollution (U 910093).
