photographed a t magnifications up to l O O O O x , with point to point resolution of better than 80 nm. Figures 1 and 2 show identical fields as seen by the optical microscope and the SEM. A comparison of the two micrographs shows that no particles are lost during the etching and coating process. Since it is unlikely that particles are lost during the clearing stage, it has been concluded t h a t particle losses for the overall process are negligible. The regions marked 1 and 2 on Figures 1 and 2 are shown a t higher magnification in Figures 3 and 4, respectively. Measurement of fiber A (Figures 3 and 4) gives values of 0.3 ym for the diameter and 13 y m for the length; the fiber labeled B has a length of 3.5 y m and a diameter that varies along its length between 0.35 and 0.18 ym. Fiber A is clearly visible in Figure 1, while fiber B is hardly discernible from the background. The visibility of particles under phase contrast is dependent on several factors, including the absorption of the phase rings, the difference in refractive indices between the mounting medium and the particle, and the angle subtended by the particle and the eye ( I 5 ) .Under the conditions described (65% absorption phase ring, refractive index of the mounting medium 1.48, and optical magnification 500X), fibers of diameter greater than 0.3 pm are visible.
Conclusion A method is described of preparing asbestos fibers collected on Gelman DM filters for examination with the SEM. The filters show no sign of radiation damage, even a t the highest accelerating voltage and beam current obtainable on most SEMs. T h e available evidence indicates t h a t particle losses from the filter are negligible. I t has been possible to compare directly the same field of view as seen by the optical microscope with phase contrast and by the SEM. The studies show that fibers of diameter greater than 0.3 y m are readily visible with the optical microscope under the conditions described.
Work is in progress on other applications of the technique, including the isokinetic sampling of particles of small aerodynamic diameter (such as fibers) in fast-moving airstreams and the identification of small particles. The latter is possible only with particles that are unaffected by the etching process. However, since these particles are no longer embedded in a matrix of fixed refractive index, they can be readily identified by various optical microscopic techniques ( I O ) or X-ray energy dispersive analyzers.
Literature Cited (1) Spurny, K., Pich, J., I n t .
J . Air Water Pollut., 8, 193-6 (1964). ( 2 ) Liu, B. Y. H., Lee, K. W.,Enuiron. Sci. Technol., 10, 345-50 (1976). (3) De Nee, P . B., Proc. S y m p . Electron Microsc. Microfibers, 68 (1976). (4) Stewart, I. M., Proc. Symp. Electron Microsc. Microfibers, 93 (1976). (5) Beaman, D. R.,Walker, H. J., Proc. S j m p . Electron Microsc. Microfibers, 98 (1976). (6) Zumwalde, R. D., Dement, J. M., Proc. S j m p . Electron Microsc. Microfibers, 139 (1976). ( 7 ) Ortiz, L. IT., Isom, B. L., Proc. Electron Microsc. Soc. Am., 32nd, 554 (1974). ( 8 ) Parker, R. D., Buzzard, G . H., Dzubay, T. G., Bel1,J. P., Atmos. Enuiron., 11,617-21 (1977). (9) Spurny, K. R., Stober, W., Ackerman, E. R., Lodge, J. P., Spurny, K., J . Air Pollut. Control Assoc., 26,496-8 (1976). (10) LeGuen, J. M. M., Rooker, S. J., Vaughan, N. P., Health & Safety Executive, Internal Report No. IR/L/FD/80/23 (available from authors). (11) Van Duijn, C., Microscope, 11,301-9 (1957-8). (12) \’on Gies Heidermanns, G., Staub-Reinhalt. Luft, 38(10), 423-5 (1978). (13) Asbestosis Research Council, “The Measurement of Airborne Asbestos by the Membrane Filter Method”, Technical Note No. 1. Rochdale, Lancashire, U.K., 1971. (14) Galvin, S., LeGuen, J. M. M.:Ann. Occup. Hyg., in press. (15) LeGuen, J. M. M., Proc. Workshop Warmensteinach, 68 (1977). Received f o r reuiew October 19, 1979. Accepted April 21, 1980
CORRESPONDENCE
SIR: An article on the budgeting of polychlorinated biphenyl (PCB)fluxes in and out of Lake Superior by Eisenreich e t al. ( I ) has a computational error concerning the direction and magnitude of an estimated PCB flux. The error is in the equation chosen to describe the transfer of PCB in the vapor phase from the atmosphere to the lake water. This equation is of the form: flux = K,C,
(1)
where K , and C,, respectively, are the mass transfer coefficient and concentration of atmospheric PCB in the vapor phase. T h e formulation of this equation is credited to Bidleman e t al. (21,who derived it as a special case of the general equation t h a t describes the rate of mass transfer in the two-film model (3). Two assumptions were made, namely, that the exchange of chlorinated hydrocarbons between air and water is gas-phase controlled and that the water surface acts as a “perfect absorber” of these substances. Neither of these as0013-936X/80/0914-1011$01.00/0
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1980 American Chemical Society
sumptions appears to be valid, as laboratory and field studies have demonstrated the volatilization of PCB from aqueous solutions and that this process is liquid-phase controlled ( 4 , 5). Using the values provided by Eisenreich et al. for C, ( I ) and the values given by Mackay and Leinonen for the Henry’s law constants of the different PCB groups (61, it can be shown that in the case of Lake Superior the atmospheric concentration of PCB can be assumed to be negligible with respect to the aqueous PCB.Furthermore, it can be assumed that a negligible portion of the aqueous PCB is adsorbed to suspended sediments ( 7 ) .With these approximations in mind the general mass transfer equation of the two-film model can be simplified to: flux = -KIC1
(2)
where K1 is the mass transfer coefficient of PCB in the lake water and C1 is the aqueous PCB concentration. Note that the direction of the flux is from the water to the atmosphere. Volume 14, Number 8, August 1980
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A value for K1 can be estimated by the following method. Paris e t al. have reported that the ratio of the mass transfer coefficients of Aroclor 1242 vs. molecular oxygen is 0.22 ( 5 ) . For large lakes and oceans a value of 0.2 m h-I is commonly used for the mass transfer coefficient of molecular oxygen ( 3 ) . Consequently, a reasonable estimate for K1 is 0.044 m hkl. Using this value and the average of the range of C1 reported by Eisenreich et al. ( I ) , 4 pg mP3,Equation 2 yields a mass flux of 0.18 kg m-2 h-’ or 1541 pg m-* year-’ leaving Lake Superior via volatilization. This flux is an order of magnitude larger than the sum of all known inputs, and the resulting discrepancy in the budget is very similar to that found by Schwarzenbach e t al. in their budgeting of 1,4-dichlorobenzene in Lake Zurich ( 8 ) . In the case of Lake Zurich the investigators chose to resolve the discrepancy by assuming that their inventory of measurable fluxes and concentrations was correct and that the estimated magnitude of the volatilization flux could be reduced by lowering their estimate of K1 ( 8 ) .Perhaps this procedure would be valid in the case of Lake Superior, but such a solution does not lay to rest the possibility of incorrect budgeting procedures or the need to modify the two-film theory as applied to large bodies of water.
Literature Cited (1) Eisenreich, S . J., Hollod, G. J., Johnson, T. C., Enciron. Sci.
Technu/.. 13,569-73 (1979). ( 2 ) Bidleman, T. F., Rice, C. P., Olney, C. E . in “Marine Pollutant Transfer”, Windom, H. L., Ed., Lexington Books, Lexington, Mass., 1976, p p 323-52. (:I) Liss, P. Slater, P . G., Nature (London). . 247., 181-4 11974). (4) Mackay, D., in “Aquatic Pollutants: Transformations and Biological Effects”, Huitninger, O., Ed., Pergamon Press, Elmsford, N.Y.. 1978. DD 175-85. (5) Paris, D.’F:, Steen, W.C., Baughman, G. L., Chemosphere, 7, 319-25 (1978). (6) Mackay, I)., Leinonen, P. J., Enciron. Sci. Techno/., 9,1178-80 (1975). (7) Pavlou, S. P., Dexter, R. N., EnLiron. Sci. Techno!., 13, 65-71 (1979). (8)Schwarzenbach, R. P., Molnar-Kubica, E., Giger, W.. Wakeham. S. G., Enuiron. Sci. Techno/., 13, 1367-73 (1979).
s.,
Roger R. Greenburg 1772 Emerson St. Palo Alto, Calif. 94301
SIR: R. R. Greenburg’s correspondence, in which he discusses the atmospheric deposition calculations of Eisenreich e t al. ( I ) for PCB input to Lake Superior, has established some poignant questions. He suggests that our assumptions that there is gas-phase control for exchange of chlorinated hydrocarbons across the air-sea interface and that the lake surface acts as a “perfect absorber” are incorrect. Firstly, the steady-state transfer across the air-sea interface can be described (2) by: flux = KoL(C - P I H )
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Environmental Science & Technology
where KI. and K , are liquid and gas-phase mass transfer coefficients, KO[.is the overall liquid-phase mass transfer coefficient, H is Henry’s law constant, C is the solute concentration in the liquid phase, P is the solute partial pressure, T is absolute temperature, and R is the gas constant. Mackay et al. ( 2 ) have shown that for H I5 X atm m,’i-mol-’, resistance t o mass transfer lies 95% in the liquid phase, and for H 5 5 X atm m:3.mol-1,resistance t o mass transfer occurs 95% in the gas phase. H values reported for Aroclor mixtures by Mackay and Leinonen (3) suggest that PCB transfer is liquid phase controlled. However, H was calculated from saturation vapor pressures determined on liquid Aroclor mixtures for which individual isomers are solids a t room temperature. Recent data reported by Doskey and Andren ( 4 ) show that H values are in the range of 1 0 V and 10-7 atm m:’.rnol-’. If true, many, but perhaps not all, of the PCB isomers would experience gas-phase control. Note that this allows for only minimal volatilization. Secondly, the PCB concentration for which the masstransfer model is appropriate applies to that proportion in the gas phase. Recent field data and arguments by Junge ( 5 ) suggest that PCBs over Lake Superior are 90 to 100% in the gas phase. The field studies (6, 7 ) show an atmospheric PCB concentration of -1.5 ng rn+. Thirdly, the concentration of water-borne PCBs ranges from -0.3 to 2 pg m-’j, of which >95% is thought to be nonparticulate. Fourthly, gas-phase control seems justified with important consequences; we have constructed a detailed mass balance for PCBs in Lake Superior showing that the atmosphere represents -90% of all inputs (similar to Lake Michigan; 4 ) , with the major sinks being the water column and sediments in that order of importance. Thus, although the questions raised by R. R. Greenburg are important, they appear not to influence greatly the conclusions of our previous paper. Admittedly, the approach used t o estimate atmospheric deposition of PCBs was simplistic.
Literature Cited ( 1 ) Eisenreich, S.J., Hollod, G . J., Johnson, T. C., Enciron. Sci. T e c h n ~ ~ i13,569-73 ., (1979). ( 2 ) Mackay, D., Shiu, W.Y., Sutherland, R. P., Enciron. Sci. Techno/., 13,333-7 (1979). ( 3 ) Mackay, D., Leinonen, P.
