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Response to Comment on “Particle/Gas Concentrations and Distributions of PAHs in the Atmosphere of Southern Chesapeake Bay” SIR: We offer the following comments in rebuttal to those of Goss regarding our paper (1). With respect to point a of the comment, Goss correctly points out that there are thermodynamic reasons why the slopes of plots of log Kp vs log p°L may not be close to unity. These are discussed in detail in the references cited (2, 3). A necessary condition for the expectation of a slope of unity for such plots is that the difference between the enthalpies of desorption and vaporization within a compound class remains nearly constant (2, 3). This has been demonstrated for atmospheric gas-particle distributions of PAHs at environmental temperatures (2, 4). Consequently, deviations of the slope from unity for regressions of log Kp vs log p°L for our PAH data were interpreted based on non-thermodynamic factors (i.e., kinetics and potential sampling artifacts) as described by Pankow and Bidleman (2). Nonetheless, the fact that thermodynamic considerations can affect the slopes of plots of log Kp vs log p°L must be kept in mind when interpreting gas-particle distribution data in general. In response to point b of the comment regarding relative humidity effects on the slope of plots of log Kp vs log p°L, Goss has misinterpreted our statements. We did not infer that relative humidity (RH) does not have an effect on partitioning energetics, but rather indicated only that we were examining the potential for RH to alter our TSP measurements and hence introduce an experimental artifact (error) into our assessment. Changing TSP by some arbitrary amount does not influence the slope of the plots of log Kp log p°L since each Kp value for a particular event will be changed by the same factor. Only the intercept of log Kp vs log p°L plots will be offset by RH effects on TSP. This is evidenced by the data of Storey et al. (5) for partitioning of both n-alkanes and PAHs between the gas phase and quartz fiber filters; the surface area-normalized data of Dorris and Gray (6) for n-alkane adsorption to silicasupported water surfaces was also included in their analysis. The data clearly demonstrate decreased intercepts of plots of log Kp vs log p°L with increasing relative humidity; however, slopes of these plots remain near -1 in all cases. Storey et al. (5) further concluded that adsorption of semivolatile organic contaminants to mineral/oxide surfaces is not as important to determining atmospheric gas-particle distributions of these compounds as sorption to the organic carbon fraction of aerosol particles, particularly in urban environments. Nonetheless, in rural/remote atmospheric environments where aerosol particles may consist predominantly of continental dust, the relative importance of organic contaminant sorption to mineral surfaces is yet to be demonstrated (5). In reference to point ii in the comment, we used the term ‘condensation’ within the paper to refer to similarities between the cold weather deposition and warm weather volatilization we are hypothesizing for PAH terrestrial-atmospheric cycling and the ‘global distillation’ or ‘cold condensation’ hypothesis presented by Wania and Mackay (7, 8). Since the term condense means “to cause molecules of (the same or different substances) to combine” as well as “to make more dense or compact; to reduce the volume of” (9), use of the term in reference to removal of molecules from the gas phase to surfaces, surface films, or even water bodies seems appropriate. The terms adsorption and absorption, are of course, proper descriptors for actual transfer processes; however, in
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many incidences the actual mechanism leading to gas to condensed phase transfer of the organic molecules is unknown. Regarding Goss’s point iii, subpoint a, pertaining to the enthalpies of sorption, once again our statement has been misinterpreted. As stated in our original manuscript (1) and noted above, it is the difference between the enthalpy of desorption and vaporization, which is expected to vary little for the PAHs. Gas-particle partitioning data from numerous field studies (10-14) support this expectation for PAHs, which is borne out by the previously noted studies of Pankow and co-workers (2-4). Secondly, as noted by Goss, desorption enthalpies are expected to vary within compounds classes, thus the averaging of these values is debatable. Derived enthalpies of desorption for PAHs from urban data sets (13, 14) tend to increase with MW. Our data from Norfolk, VA (see ref 1, Table 2, Elizabeth River), also demonstrate this trend; however, the seasonal gas-particle distribution data from the semiurban and rural Southern Chesapeake Bay sites exhibit much less, if any, increase in desorption enthalpy with size of the PAH (see ref 1, Table 2). This may be due, in part, to determination of these values by simple linear regression of log Kp vs 1/T rather than a common y-intercept approach (4). Additionally, if our hypothesis with respect to terrestrial/urban surface mediation of gas phase PAHs (1) is correct, this reservoir will act as a third partitioning phase that undoubtedly also influences desorption enthalpies derived from data collected throughout various seasons. Moreover, aerosol particle types may vary between seasons as well as sites. This may be much more important at rural sites as compared with urban areas. For example, our Haven Beach site may receive urban particulate matter when winds are out of the south in summer, terrestrial/continental particulate matter when winds come from the northwest in winter, marine aerosols when there is a sea breeze, and pine/ oak pollen is known to comprise a large fraction of the TSP during spring. In contrast, atmospheric particulate matter in urban environments are most likely due to automotive and industrial sources with less seasonal variability. Thus, the practice of determining enthalpies of desorption from field data collected for various events when aerosol particles may be substantially different and the terrestrial reservoir influence on gas phase concentrations of semivolatile chemicals varies is probably questionable. With respect to point iii, subpoint b, enthalpies of desorption calculated for our rural Haven Beach site are indeed higher than any previously reported values, which are typically for urban air. Nonetheless, enthalpies of desorption for PAHs on a variety of model sorbents including carbon, NaCl, alumina, and silica range from 75 to 209 kJ/ mol when calculated via simple linear regression of log Kp vs 1/T (15). Our values fall within this range and may indicate, as noted above, a shift in the predominant aerosol particle type between the rural and urban areas studied. Thus, we agree with Goss in surmising that the characteristics of the particles measured in this study, particularly in rural areas, may be completely different than those for which experimental data has been presented before (typically in urban areas). Finally, we would like to reiterate that the air sampling methods used in this study have been used widely (10-14, 16-22). We have assessed sampling artifacts (1, 22) and determined that these cannot explain the deviation of our data from previously reported data collected in the same manner; albeit mostly urban and not resultant from a full seasonal sampling as conducted within this study. We propose that there are significant differences in aerosol
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particle characteristics between urban and rural sites and over seasonal cycles. Thus, future work characterizing atmospheric particle properties and the difference in partitioning energetics between remote and urban sites over seasonal cycles is crucial in advancing our understanding of contaminant distributions and transport in the environment.
Literature Cited (1) Gustafson, K. E.; Dickhut, R. M. Environ. Sci. Technol. 1997, 31, 140-147. (2) Pankow, J. F.; Bidleman, T. F. Atmos. Environ. 1992, 26A, 10711080. (3) Pankow, J. F. Atmos. Environ. 1994, 28A, 185-188. (4) Pankow, J. F. Atmos. Environ. 1991, 25A, 2229-2239. (5) Storey, J. M. E.; Luo, W.; Isabelle, L. M.; Pankow, J. F. Environ. Sci. Technol. 1995, 29, 2420-2428. (6) Dorris, G. M.; Gray, D. G. J. Phys. Chem. 1981, 85, 3628-3635. (7) Wania, F.; Mackay, D. Ambio 1993, 22, 10-18. (8) Wania, F.; Mackay, D. Environ. Sci. Technol. 1996, 30, 390A396A. (9) Webster’s New World Dictionary of the American Language, 2nd college ed.; Guralnik, D. B., Ed.; William Collins Pub., Inc.: New York, 1979. (10) Cotham, W. E.; Bidleman T. F. Environ. Sci. Technol. 1995, 29, 2782-2789. (11) Foreman, W. T.; Bidleman, T. F. Atmos. Environ. 1990, 24A, 2405-2416. (12) Hoff, R. A.; Chan, K.-W. Environ. Sci. Technol. 1987, 21, 556561. (13) Yamasaki, H.; Kuwata, K.; Miyamoto, H. Enivron. Sci. Technol. 1982, 16, 189-194.
(14) Bidleman, T. F.; Billings, W. N.; Foreman, W. T. Enivron. Sci. Technol. 1986, 20, 1038-1043. (15) Storey, J. M. E.; Pankow, J. F. Atmos. Environ. 1992, 26A, 435443. (16) Keller, C. D.; Bidleman,T. F. Atmos. Environ. 1984, 18, 837845. (17) Gardner, B.; Hewitt, C. N.; Jones, K. C. Environ. Sci. Technol. 1995, 29, 2405-2413. (18) Hoff, R. M.; Muir, D. C. G.; Grift, N. P. Environ. Sci. Technol. 1992, 26, 266-275. (19) Leister, D. L.; Baker, J. E. Atmos. Environ. 1994, 28, 1499-1520. (20) Lugar, R. M.; Harless, R. L.; Dupuy, A. E.; McDaniel, D. D. Environ. Sci. Technol. 1996, 30, 555-561. (21) Panshin, S. Y.; Hites, R. A. Environ. Sci. Technol. 1994, 28, 20012007. (22) Dickhut, R. M.; Gustafson, K. E. Mar. Pollut. Bull. 1995, 30, 385-396.
Kurt E. Gustafson* Sarasota Bay National Estuary Program 53333 N Tamiami Trail, Suite 104 Sarasota, Florida 34234
Rebecca M. Dickhut School of Marine Science The College of William and Mary Gloucester Point, Virginia 23062 ES9720117
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