ASTM Standards; Including Tentatives,” Part 19, pp 25-6, Philadelphia, Pa., 1969b. Association of Official Analytical Chemists in “Official Methods of Analysis of the Association of Official Analytical Chemists,” 11th ed., pp 25-6, Washington, D.C., 1970a. Association of Official Analytical Chemists in “Official Methods of Analysis of the Association of Official Analytical Chemists,” 11th ed., pp 16-20, Washington, D.C., 1970b. Barker, J. E., Mott, R. A., Thomas, W. C., Fuel, 34, 303-16 (1955). Cohen, I. R., Allen, R. L., “Sampling and Sample Preparation in Incinerator Evaluation,” presented at the 160th ACS Meeting, Chicago, Ill., September 13-18, 1970 (Abstr., p 204). Glasstone, S., Lewis, D., in “Elements of Physical Chemistry,’’ 2nd ed., pp 91--3, D. VanNostrand, Princeton, N.J., 1960.
“Handbook of Chemistry,” N. A. Lange, Ed., rev. 10th ed., p 1588, McGraw-Hill, New York, N.Y., 1967a. “Handbook of Chemistry,” N. A. Lange, Ed., rev. 10th ed., p 1618, McGraw-Hill, New York, N.Y., 1967b. Parr Instrument Co., in “Oxygen Bomb Calorimetry and Combustion Methods; Technical Manual No. 130,” Parr Instrument Co., Moline, Ill., 1966. Schmidt, A. X., List, H. L., in “Material and Energy Balances,’’ Chap. 10, Prentice-Hall, Englewood Cliffs, N.J., 1962a. Schmidt, A. X., List, H. L., in “Material and Energy Balances,” p 217, Prentice-Hall, Englewood Cliffs, N.J., 1962b. Wilson, D. L., ENVIRON. SCI.TECHNOL., 5 (7), 609 (1971).
Receiced ,for recieiv April 10, 1972. Accepted August 7 , 1972.
Particles Containing Lead, Chlorine, and Bromine Detected on Trees with an Electron Microprobe Gary H . Heichell and Lester Hankin Departments of Ecology and Climatology and Biochemistry, The Connecticut Agricultural Experiment Station, Box 1106, New Haven, Conn. 06504
Analysis of the bark of trees with an electron probe microanalyzer revealed that lead, chlorine, and bromine are associated with particles, about 7 pm in diam, on or embedded in the bark. The similarity in elemental content is noted between the lead-bearing particles on trees and some of the compounds emitted in automobile exhaust.
T
he lead concentration on or in plants near highways suggests that the lead burden of plants is increased principally by deposition onto plant surfaces of lead-bearing residues from vehicle exhausts (Cannon and Bowles, 1962; Motto et al., 1970; Smith, 1971). The principal constituents of lead compounds in vehicle exhaust are the lead, chlorine, and bromine from the additives in gasoline (Hirschler and Gilbert, 1964). However, the form of the residues on plants has not been unequivocally demonstrated. We now report that at least a part of the lead on or embedded in the bark of trees growing near roads is combined with chlorine and bromine in particles similar in size and composition to particles in automobile exhaust. Experimental Procedure
Branches 5 mm in diam of the current year’s growth were sampled from limbs 1.75 meters above the ground on a white pine (Pinus strobus) growing in a dense roadside screen 2 to 3 meters west of the curb of a densely and speedily traveled highway in Hamden, Conn. Bark samples that averaged 4 mm To whom correspondence should be addressed.
in thickness were collected 0.75 meter above the ground from an elm (Ulmus americana) growing 1 meter from the curb of a densely but slowly traveled city street adjacent to the green in downtown New Haven, Conn. The pines had been established for 25 years, but the elm was transplanted to the downtown location 3 years before sampling. The total lead content was determined on oven-dry subsamples by atomic absorption spectrophotometry. The yearold pine twig contained 120 pg lead/g dry wt and the 3-yearold elm bark contained 1080 pg lead/g. The total lead in the year-old pine twig is less than in the 3-year-old elm bark probably because of the briefer exposure of the pine twig to contamination. Analysis of total lead, however, does not reveal the form and composition of the lead compounds deposited on the surface. For analysis with an electron probe microanalyzer (Sawhney and Zelitch, 1969; Ter Haar and Bayard, 1971) sections of surface bark 3 mm square and 1 mm thick were cut with a clean razor blade, compressed between two sheets of filter paper, and dried for 48 hr over silica gel. The sections were then mounted on a carbon block with Apiezon N stopcock grease, covered with a thin carbon coating in a vacuum evaporator, and stored in a desiccator. Individual spectrometers were tuned at the wavelengths corresponding to the K , Xrays of bromine, the K , X-rays of chlorine, and the L , X-rays of lead. The microprobe was operated at a beam voltage of 25 kV and a sample current 100 nA on brass. Particles containing lead were located and identified by scanning the surface of the bark with a 1-pm-diam spot beam at an instrument magnification of 250X until a rapid count rate was detected on the lead spectrometer. Areas of bark containing lead were scanned at an instrument magnification of 1OOOX with a 1-pm-wide electron line, and the characVolume 6, Number 13, December 1972
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Table I. Diameter of Lead-Bearing Particles on the Surfacesof Trees Location
Sample
New Haven
1
6
2 3 4 5 6 7 8 9 10
5
Mt. Carmel
Mean
=tsee.
Diarn, prn
10 5 5 3 9 13 5 6 6.7 + 1
teristic lead X-radiations were displayed on an oscilloscope. The electron image displayed on the cathode-ray tube of the oscilloscope was photographed to facilitate measurement of the dimensions of the particles.
Results and Discussion Particles containing lead held on the rough surface or embedded in the surface cells of the bark were found on both pine and elm. Within ten regions selected from bark of the two trees, the diameter of the particles that we examined containing lead ranged from 3 to 13 pm, with a mean of 6.7 + 1 pm (Table I). Particles on the 3-year-old elm bark were apparently poorer in lead than those on the year-old pine twigs. The count rates of particles on pine and elm having comparable dimensions were 4800 to 10,500 counts per 30 sec on the pine and 1200 to 2250 counts per 30 sec on the elm. These observations suggest that a few of the large particles on the elm bark that had been exposed for 3 years might be more strongly weathered than those on the pine. Lead, chlorine, and bromine in particles were analyzed by focusing the spot electron beam and simultaneously measuring on three spectrometers the emitted X-radiations as counts per 30 sec. Background measurements were made on areas of bark devoid of lead particles. The levels of chlorine and bromine associated with lead in the particles were significantly above background (Table 11). The count rates for lead are lower than those reported above because an alternate spectrometer was used. Although the lead content of the particles that we examined varied over fourfold, the ratio of lead/chlorine/bromine among the particles was similar except
1122 Environmental Science & Technology
Table 11. Analysis of Particles on Bark of Pine Element, counts/30 sec c1
Sample
Pb
1 2 3 4 5 6 7 Mean + s.e. Background : Mean + s.e. of 10 samples
2040 2305 2511 2647 1078 599 2113 1899 =k 160 66 i 7
443 350 449 370 314 316 379 374 + 21 258 f IS
Br
999 927 908 1063 657 587 862 858 i 66 306 rt 11
for sample six. Compounds of the elements that we observed in particles on the bark of trees are also present in automobile exhaust (Hirschler and Gilbert, 1964; Ter Haar and Bayard, 1971). Other lead-bearing compounds like oxides and carbonates have been detected in automobile exhaust (Ter Haar and Bayard, 1971), but the presence of those compounds in particles on vegetation has yet to be confirmed. Knowledge of the form and composition of lead residues on or in plants is clearly important in determining how lead affects plants. In humans, for example, some forms of lead are more toxic than others (Bryce-Smith, 1972). We have now shown that lead retained on or embedded in plant surfaces is associated with at least two other elements, chlorine and bromine.
Acknowledgment We thank Edward Weber, Geology and Geophysics Department, Yale University, for his assistance with the electron microprobe and Richard Botsford, Analytical Chemistry Department, The Connecticut Agricultural Experiment Station for the atomic absorption spectrophotometry.
Literature Cited Bryce-Smith, D., Chem. Brit., 8, 240 (1972). Cannon, H. L., Bowles, J. M., Science, 137,765 (1962). Hirschler. D. A.. Gilbert. L. F.. Arch. Enciron. Health., 8,. 297 (1964). ’ Motto, H. L., Daines, R. H., Chilko, D. M., Motto, C. K., ENVIRON. S a . TECHNOL.. 4.231 (1970). Sawhney, B. L., Zelitch, I.,khnt Phq’siol., 44, 1350 (1969). Smith, W. H., Forest Sci., 17,195 (1971). Ter Haar, G. I-., Bayard, M. A., Nature, 232,553 (1971). Receiced for reoiew May 19, 1972. AcceptedJuly 24, 1972.
