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Heuss, J. M. Research Publication GMR-1802, General Motors Corp., Warren, MI, Feb 1975. Altshuller,A. P.; Kopczynski, S. L.; Wilson, D.; Lonneman, W.; Sutterfield, F. D. J . Air Pollut. Control Assoc. 1969, 19, 787-790. Altshuller,A. P.; Kopczysnki, S. L.; Wilson, D.; Lonneman, W.: Sutterfield, F. D. J . Air Pollut. Control Assoc. 1969, 19,’791-794. . Trijonis, J.; Hunsaker, D. EPA Report 600/3-78-019; U.S. EnvironmentalProtection Agency: Research Triangle Park, NC, Feb 1978. Research Triangle Institute, EPA Report 450/3-75-036;U.S. EnvironmentalProtection Agency: Research Triangle Park, NC. Mar 1975. SinghrH. B.; Martinez, J. R.; Hendry, D. G.; Jaffe, R. J.; Johnson, W. B. Environ. Sci. Technol. 1981,15,113-119. Atkinson, R.; Lloyd, A. C. J. Phys. Chem. Ref. Data, in press. Arnts, R. R.; Gay, B. W., Jr. EPA Report 600/3-79-081;U.S. EnvironmentalProtection Agency: Research Triangle Park, NC, Sept 1979. Martinez, J. R.; Singh, H. B. SRI Project 6780-8,SRI International, Menlo Park, CA, 1979.
quantitative fraction as reactive as the alkenes. As noted earlier, the HC/NO, ratio is very important. The HC/NO, ratio also increases as an air mass moves downwind to rural areas. Thus, the ozone-forming potential of LSHCs increases as the air mass moves downwind. In rural areas, highly reactive olefins such as terpenes emitted from vegetation may even act as sinks for ozone. In these situations, the alkanes would be infinitely more reactive than the alkenes. In a complex mixture, synergistic effects occur, and normally slow reacting hydrocarbons at moderate HC/NO, ratios become more reactive. Our experimental and modeling results indicate that all hydrocarbons of molecular weight greater than ethane will generate ozone at significant levels if irradiated at an appropriate HC/NO, ratio. The selective control of hydrocarbons above ethane, therefore, may have little beneficial effect on an urban area if the downwind environment as well as the urban area is considered. Registry No. 03, 10028-15-6;NO, 10102-43-9;trans-2-butene, 624-64-6; propane, 74-98-6; butane, 106-97-8;propene, 115-07-1.
Literature Cited (1) Kopczynski, S. K.; Kuntz, R. L.; Bufalini; J. J. Enuiron. Sci. Technol. 1975, 9, 648-653.
Received for review February 5, 1982. Revised manuscript received December 22, 1982. Accepted January 25, 1983.
CORRESPONDENCE Comment on “Calculation of Evaporative Emissions from Multicomponent Liquid Spills” SIR: The recent paper by Drivas (1)’presented a theoretical approach for estimating the time-varying rates of evaporation of individual components of a multicomponent liquid mixture from a spill. Model estimates were compared with experimental data from other investigators. This paper would have benefited from a short discussion of the limitations of the model as a result of the simplifying assumptions used in its formulation. The model assumes that the liquid solutions are ideal, following Raoult’s law. This assumption is likely to be correct only when the mixture’s components are quite similar in molecular structure (2). For mixtures of common classes of organic solvents, the activity coefficient at infinite dilution, a measure of nonideality, usually ranges from just less than 1to somewhat greater than 10, with some values as high as 300000 (3). Bishop et al. ( 4 ) found errors in estimating the ratios or organic vapor concentrations from several binary mixtures by Raoult’s law ranging from 10% to greater than 1000%. They showed that much better estimates of vapor ratios could be obtained by use of a computer program that estimates activity coefficients based on the contributions of functional groups within the molecules of mixture components. Thus, while the model proposed by Drivas may produce reasonable agreement for oil spills, considerable care should be used in application to other mixtures. A less significant source of error is the determination of the diffusion coefficient of vapor in air for the calculation of the mass transfer coefficient. Only a limited number of diffusion coefficients are available from ex0013-936X/83/0917-0311$01.50/0
perimental observations. Theoretical estimation is quite arduous by hand, but these calculations can be computerized. The range of diffusion coefficients of common volatile substances is less than 1order of magnitude (5), so that anticipated errors would be considerably less than a factor of 10 for volatile materials. Of course, the potential error from an incorrect diffusion coefficient value is less than directly proportional to the error in the diffusion coefficient itself. Nevertheless, some improvement in model accuracy would be expected by use of separate diffusion coefficient values for each of the evaporating compounds. For materials of low volatility, estimation of pure compound vapor pressures for many classes of materials is still problematic, and large errors in both experimental and correlative estimates are common (6). The agreement between Drivas’ model predictions and long-term crude oil experiments shown in his Figure 2 is quite good, but one must suspect that this is partially the result of the assumption that 50% of the total oil weight was evaporable. No justification of this assumption was given. If the percent evaporable material was selected on the basis of the fit between theoretical and experimental points in Figure 2, much less significance should be attached to the agreement obtained. Literature Cited (1) Drivas, P. J. Environ. Sei. Technol. 1982, 16, 726. (2) Reid, R. C.; Prausnitz, J. M.; Sherwood, T. K. “The Properties of Gases and Liquids”;McGraw-Hill: New York, 1977; p 296. (3) Walsham, J. G.; Edwards, G. D. J . Paint Technol. 1971, 43, 64. (4) Bishop, E. C.; Popendorf, W.; Hanson, D.; Prausnitz, J. Am. Ind. Hyg. Assoc. J . 1982, 43, 656.
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(5) Lugg, G. A. Anal. Chern. 1968,40, 1072. ( 6 ) Mackay, D.; Bobra, A.; Chan, D. W.; Shiw, U. Y. Enuiron. Sci. Technol. 1982, 16, 645.
( 5 ) Matsugu, R. S. M.S. Thesis, Department of Chemical Engineering and Applied Chemistry, University of Toronto, Toronto, Canada, 1973.
Charles E. Felgley Department of Environmental Health Sciences School of Public Health University of South Carolina Columbia, South Carolina 29208
SIR: I wish to thank Feigley for discussing the important limitation of the assumption of an ideal solution in my procedure for multicomponent evaporation calculations ( 1 ) . An earlier version of my paper contained a discussion of the calculation procedure for nonideal solutions, which was deleted for the sake of brevity. To derive equations for nonideal solutions, a modified Raoult’s law (2) can be easily applied: pi
= yixip:
where yi is the liquid activity coefficient of component i . This formulation leads to the following expressions for eq 4 and 5 in the paper:
and N
pi = x:yip;e-h&P,’t/ E [~?e-kyz~,’t] i=l
These expressions can be used to more accurately estimate evaporation rates for a nonideal solution, assuming that information on the activity coefficients yi is available. The ideal solution assumption (yi = 1)is appropriate primarily for liquid solutions containing compounds of similar molecular structure (e.g., straight-chain hydrocarbons). The estimation of the molecular diffusivities and vapor pressures for pure compounds should cause only minor errors in the evaporation calculations. Equations for the estimation of the molecular diffusivity of gases are reasonably accurate (3), and experimental vapor pressures exist for most compounds of environmental significance (4).
The assumption of 50% by weight evaporable oil in the long-term evaporation comparison (Figure 2 of the paper) was not arbitrary but was based completely on the experimental data recorded by Matsugu (5) for that particular evaporation test. After continuous evaporation of a crude oil sample in a wind tunnel for 20 days, 265 g of crude oil remained from the initial sample of 500 g, or 53% by weight nonvolatile compounds. Because the evaporation rate after 20 days was almost negligible, it was assumed that 50% by weight was a reasonable estimate of the nonvolatile fraction. Literature Cited (1) Drivas, P. J. Environ. Sci. Technol. 1982, 16, 726-728. (2) Denbigh, K. “The Principles of Chemical Equilibrium”; Cambridge University Press: London, 1968. (3) Lyman, W. J.; Reehl, W..F.; Rosenblatt, D. H. “Handbook of Chemical Property Estimation Methods”; McGraw-Hik New York, 1982. (4) Verschueren, K. “Handbook of Environmental Data on Organic Chemicals”; Van Nostrand-Reinhold: Princeton, NJ, 1977. 312
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Peter J. Drivas Energy Resources Co./California La Jolla, California 92037
Comment on “Characterization of Organic Contaminants In Envlronmental Samples Associated with Mount St. Helens 1980 Volcanic Eruption” SIR: Pereira et al. ( l a ) report the detection of monoand dichlorobenzoic acid methyl ester isomers and pentachlorobiphenyl isomers (P,CBs) in samples of fly ash taken from the Mt. St. Helens ash cloud deposits and suggest “that aromatic compounds produced as a result of the pyrolysis of plant and soil organic matter, in the presence of vaporized inorganic chloride salts, were chlorinated in the high-temperature eruption zone.” They cite the work of Olie et al. ( I b ) in support of their hypothesis, while acknowledging that the ash samples could have become contaminated with PCBs. I do not believe that the data suggest de novo synthesis of chloroaromatic compounds from cellulose carbon and inorganic chloride salt vapors. Olie et al. did not establish that a de novo synthesis of polychlorinated aromatic molecules occurs in the trace chemistries of fire via experiment; rather they attribute such an experiment to Lunde and co-workers based on a personal communication. Subsequently Ahling, Bjerrseth, and Lunde (2) published the results of a study demonstrating the de novo synthesis of polychlorinated aromatics from the incomplete combustion of polyvinyl chloride. In fact Olie et al. noted that “The presence of relatively large amounts of chlorophenols in flue gas condensate makes it likely that these compounds are the precursors of chlorodibenzodioxins in the incinerator process.” The de novo formation of aromatic chlorine compounds was considered possible. Support for this route of chlorodioxin synthesis came from the relatively large quantities of polychlorinated benzenes in the fly ash samples examined, according to the authors. The formation of polychlorinated dibenzofurans (PCDFs) and polychlorinated benzenes from the pyrolysis of PCBs has been demonstrated by Buser et al. ( 3 ) . Lustenhouwer and co-workers (4) identified the most probable precursors of polychlorinated dibenzo-p-dioxins (PCDDs) found in municipal incinerator fly ash as polyvinylchloride, polychlorinated benzenes, polychlorinated phenols, PCBs, and 2,4,5-T derivatives but could not rule out the de novo synthesis of PCDDs and PCDFs from organic carbon and inorganic chloride. On the basis of a spot survey of various combustion sources, Bumb et al. (5)concluded that PCDDs in general (2,3,7,8-TCDD) and the 2,3,7,8-tetrachlorodibenzo-p-dioxin isomer in particular are generated de novo in the trace chemistries of fire from organic carbon and inorganic chloride vapors. Unfortunately, the fuel feedstocks and precombustion air were not analyzed for the precursor molecules identified above. More recent studies conducted by Dow Chemical Co. scientists identified 2,3,7,8-TCDD in the soot, tar, and ash of stoves fired with allegedly uncontaminated wood (6). Nevertheless, because the in-
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