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Symposium Series 130; American Chemical Society: Washington, DC, 1980; pp 423-439. (15) Sinn, H.; Kaminsky, W.; Janning, J. Angew. Chem., Znt. E d . Engl. 1976, 15, 660. (16) Bracker, G. P. Conserv. Recycl. 1981, 4 , 161. (17) Gary, J. H.; Handwerk, G. E. In Chemical Processing and Engineering; Petroleum Refining: Technology and Economies; Marcel Dekker Inc.: New York, 1975; Vol. 5. (18) Midgley, T., Jr.; Henne, A. T. J. Am. Chem. SOC.1929,51, 1215. (19) Tamura, S.; Mukarami, K.; Kuwazoe, H. J . Appl. Polym. Sci. 1983, 28, 3467.
(20) Sakai,T.;Soma, K.; Sasaki, Y.; Tominaga, H.; Kunugi, T. In Refining Petroleum for Chemicals; Gould, R. F., Ed.; ACS Advances in Chemistry Series 97; American Chemical Society: Washington, DC, 1970; pp 68-91. (21) Gelling, I. R.; Loadman, M. J.; Sidek, B. D. J. Polym. Sci., Polym. Chem. 1979, 17, 1383.
Received for review February 11, 1991. Revised manuscript received April 16,1991. Accepted April 22, 1991. The financial support of Energie, Ressources Quebec and the Natural Science and Engineering Research Council Canada has made this study possible.
CORRESPONDENCE Comment on “Theoretical Analysis of Evaporative Losses of Adsorbed or Absorbed Species during Atmospheric Aerosol Sampling”
Literature Cited Zhang, X.; McMurry, P. H. Environ. Sci. Technol. 1991, 25, 456-459.
Coutant, R. W.; Brown, L.; Chuang, J. C.; Riggin, R. M.; Lewis, R. G. Atmos. Enuiron. 1988, 22, 403-409. Van Vaeck, L.; Van Cauwenberghe,K.; Janssens,J. Atmos. Environ. 1984, 18, 417-430.
SIR: The referenced paper by Zhang and McMurry (I) provides valuable insights into the effect of sampler pressure drop on evaporative losses of adsorbed species when either filter- or impactor-type samplers are operated under ideal conditions. However, in attempting to compare their theoretical predictions with the field sampling results of Coutant et al. (2) and Van Vaeck et al. (3),they have omitted the effects of variable temperature, which are always present in practical ambient air sampling for trace species such as PAH. As stated by Zhang and McMurry, the driving force for evaporative loss of adsorbed species from collected particulate matter is proportional to Po- P, where Pois the equilibrium vapor pressure of the adsorbed species and P is the vapor-phase pressure. Assuming equilibrium between the vapor and adsorbed phases at the sampler inlet, Zhang and McMurry explored the effect of reduced pressure within the sampler on P and concluded that losses due to this effect should amount to no more than 10% for a filter sampler. A potentially more significant effect due to temperature variation can occur in sampling for ambient levels of PAH. In this case, relatively high volume sampling (100-200 L/min for 12-24 h) is required for collection of sufficient quantities of trace PAH for analysis. Such efforts are almost always accompanied by wide swings in the ambient temperature. For example, in our work, temperature swings of up to 27 OC (10-37 “C) were experienced. Under such conditions, Po for anthracene would increase by a factor of 30. Thus, anthracene adsorbed on particles collected a t 10 OC and then subjected to a temperature of 37 OC would evaporate much more rapidly and completely than is predicted by Zhang and McMurry’s consideration of the pressure drop effect. A second point needs to be clarified concerning Zhang and McMurry’s comment that “the denuder used by Coutant et al. was only evaluated for naphthalene”. It is true that the performance of our denuder was validated by using naphthalene as a surrogate PAH, but denuder efficiencies were corrected for differences in diffusion coefficients for each species and for the effects of temperature and pressure on the diffusion coefficients.
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0013-936X/91/0925-1649$02.50/0
Robert W. Coutant Battelle Columbus, Ohio 43201-2693
SIR: We thank Dr.Coutant for his comments on the effect of temperature variations on sampling efficiencies. We agree that if temperature were to increase during sampling, then evaporation of semivolatile particulate compounds previously collected a t lower temperatures would occur. Similarly, if the temperature were to decrease systematically during sampling, then gas-phase species could be retained by particles on the filter. Thus, temperature will affect the measured filter and downstream adsorber loadings, and actual sampling efficiencies are likely to be different from that predicted by our steadystate sampling theory. The magnitude and direction of the effect depends on the time-dependent temperature during sampling. Under variable-temperature sampling conditions, evaporation a t high temperatures is likely to exceed uptake at low temperatures because the equilibrium partial pressure a t the deposit surface is an increasing function of temperature. Indeed, as we suggested toward the end of our paper and as Coutant points out in his comment, this may be the reason for the high losses observed by Coutant et al. (1) and Van Vaeck et al. (2). Our steady-state theory (3) does not apply to such transient effects. The treatment of transient phenomena would require more information than is currently available about rates of uptake and release of semivolatile compounds from particulate surfaces. Also, applying such a theory to ambient sampling would require information on time-dependent temperatures and concentrations. However, the theory can be used to estimate a minimum value for sampling efficiencies under transient sampling conditions. As an approximation, assume that adsorption of gases on collected particles does not occur to any appreciable extent when temperatures decrease, but that particles rapidly achieve equilibrium by volatilization when
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sampling temperatures increase. In this case, the retained particulate fraction collected during a given time interval is determined by the highest evaporative driving force (which will occur, for example, at the highest temperature under fixed total concentration conditions) encountered during the remainder of the sampling period. If the relationship between equilibrium gas-phase partial pressure and temperature is known, this approach can be used to estimate the maximum extent of evaporative losses. Coutant also points out that the diffusivities of PAHs were taken into account when evaluating the performance of their denuder with species other than naphthalene. In our view this is necessary but not sufficient. The atmosDheric aerosol contains a multicomDonent mixture of species (including water vapor) that competes with PAHs for the same adsorption sites. A complete understanding of denuder performance with such systems will require studies with more realistic systems.
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artifacts for adsorbed and adsorbed compounds should be small.
Literature Cited (1) Coutant, R. W.; Brown, L.; Chuang, J. C.; Riggin, R. M.; Lewis, R. G. Atmos. Environ. 1988, 22, 403-409. (2) Van Vaeck, L.; Van Cauwenberghe, K.; Janssens, J. Atmos. Environ. 1984, 18, 417-430. (3) Zhang, X. Q.;McMurry, P. H. Enuiron. Sci. Technol. 1991, 25. 456-459.
Peter H. McMurry," Xlnqul Zhangt Department of MechanicalEngineering University of Minnesota Minneapolis, Minnesota 55455
