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 t h a n 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 t h e discrepancy by assuming t h a t 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. H e suggests that our assumptions t h a t 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 a t m 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 t h a t 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 t h a t 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.
