Environ. Sci. Technol. 2003, 37, 2022
Comment on “Electrostatic Sampler for Semivolatile Aerosols: Chemical Artifacts” and “Filter and Electrostatic Samplers for Semivolatile Aerosols: Physical Artifacts” The studies of Volckens and Leith (1, 2) deal with an important topic, namely, the continuing need for aerosol sampling methods that minimize errors when attempting to distinguish between gas- and particle-phase fractions of a given compound of interest. These studies are valuable contributions to the field. The motivation for this comment here comes only from a difference over symbols and nomenclature. In particular, Volckens and Leith (1, 2) introduce the reader to their studies using a system of nomenclature that is not consistent with what has become the widely accepted terminology in the field of gas/particle partitioning. The result is a potential for confusion in the literature. In equation 1 as it appears in both papers, Volckens and Leith (1, 2) state that Kp is a dimensionless partitioning ratio between the gas and particle phases of the aerosol (i.e., that it is the ratio of the mass amount in the particle phase per unit volume of air, divided by the mass amount in the gas-phase per unit volume of air). While this is a very useful ratio that has been utilized in many other papers on this topic, including efforts from our group, it is not a thermodynamic partitioning constant, dependent as it is on the mass/volume ratio of the suspended particulate matter that is TPM (µg/m3). Specifically, it is also not Kp as this symbol has now come to be widely understood in the field of gas/ particle partitioning (e.g., refs 3-8). In particular then, for gas/particle partitioning work, we argue that Kp should be strictly reserved for the thermodynamic partitioning constant that gives the ratio of the equilibrium concentration within the particle phase cp (ng/µg) to the concentration in the gas phase cg (ng/m3):
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Kp (m3/µg) )
cp (ng/µg) cg (ng/m3)
Partitioning to soils or sediments from water is, analogously, widely described using the symbol Kd (mL/g) as the ratio in the particle phase (g/g) to the concentration in the water phase (g/mL) (e.g., ref 9). Thus, what Volckens and Leith (1, 2) give as cp and Kp are more typically and preferably cp × TPM and Kp × TPM, respectively. Indeed, then, although Volckens and Leith (1) cite Pankow (3) for their equation 2 for Kp for absorptive partitioning, their equation 2 gives what may be inferred from Pankow (3) to be, in fact, Kp × TPM.
Literature Cited (1) Volckens, J.; Leith, D. Environ Sci. Technol. 2002, 36, 46084612. (2) Volckens, J.; Leith, D. Environ Sci. Technol. 2002, 36, 46134617. (3) Pankow, J. F. Atmos. Environ. 1994, 28, 185-188. (4) Pankow, J. F.; Seinfeld, J. H.; Asher, W. E.; Erdakos, G. B. Environ. Sci. Technol, 2001, 35, 1164-1172. (5) Seinfeld, J. H.; Asher, W. E.; Erdakos, G. B.; Pankow, J. F. Environ. Sci. Technol. 2001, 35, 1806-1817. (6) Offenberg, J. H.; Baker, J. E. Atmos. Environ. 2002, 36, 12051220. (7) Cocker, D. R.; Mader, B. T.; Kalberer, M.; Flagan, R. C.; Seinfeld, J. H. Atmos. Environ. 2001, 35, 6073-6085. (8) Oh, J.-E.; Choi, J.-S.; Chang, Y.-S. Atmos. Environ. 2001, 35, 4125-4134. (9) Scharzenbach, R. P.; Gschwend, P. M.; Imboden, D. M. Environmental Organic Chemistry; Wiley: New York, 1993.
James F. Pankow* Department of Environmental and Biomolecular Systems OGI School of Science & Engineering Oregon Health & Science University P.O. Box 91000 Portland, Oregon 97291-1000 ES021046Q
10.1021/es021046q CCC: $25.00
2003 American Chemical Society Published on Web 04/04/2003
