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Response to Comment on “Locating PCB Sources in Chicago: Receptor Modeling and Field Sampling” We welcome Oskouie et al.’s (1) comments on our paper (2) regarding PCB sources in Chicago. We appreciate their assistance in the project described in that paper and their ongoing assistance in investigating possible PCB emissions from sludge drying beds. Oskouie et al. (1) take issue with four areas of our paper. First, they claim that our potential source contribution function modeling (PSCF) cannot be used to pinpoint specific sources. We agree. As we state in our paper, PSCF results represent both potential source directions and locations because PSCF modeling evenly distributes weight along the path of trajectories. This even weighting results in a trailing effect so that areas upwind and downwind of sources are likely to be identified as sources as well, especially when sample numbers are limited as was the case in our study (2). Results from PSCF modeling need to be supported by sitespecific information. In our case, the site-specific information was upwind and downwind samples. Oskouie et al. (1) point to the PSCF results using data from the Lake Michigan Urban Air Toxics Study (LMUATS) collected at Kankakee, IL (Supporting Information of ref 2) as evidence that the highly industrialized Calumet Lake area is not a potential PCB source area. As we indicate in our paper, grid cells with small numbers of trajectory end points may have PSCF values that are incorrect, and thus, an end point filter was used to eliminate them. Since winds were not from the southwest during LMUATS, samples taken in Kankakee, IL, could not be used to draw any conclusions about the Lake Calumet area [that is why this area was not covered with either a either black (potential source) or open (unlikely to be a source) square in Figure 2 (Supporting Information of ref 2)]. The second point of Oskouie et al. (1) is that since we combined the analytical results from the filter and PUFs and since particles and gas-phase PCBs are transported differ* Corresponding author e-mail:
[email protected]. † Present address: California Air Resources Board, 101 I St., Sacramento, CA 95812.
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ently, the PSCF results presented are incorrect. We disagree. Our data (we extracted the PUFs and filters separately) and the data of others (for example, ref 3) show that approximately 90% of the airborne PCBs are in the gas phase. Since PSCF modeling concentrations are only used to stratify the data into above or below the criteria value (mean concentration in our study), using gas phase or total concentration would yield the same results. Third, Oskouie et al. (1) take issue with our use of a small lab-scale experiment to simulate what happens in full-scale sludge drying beds. As we point out in the paper, we acknowledge that there are significant differences between the two scenarios. However, since water loss and PCB loss from the sludge generally follow the same pathways, this experiment is a reasonable first attempt at estimating PCB losses. As we indicate in our paper, these laboratory measurements need to be verified in the field. This additional work in ongoing. Finally, Oskouie et al. (1) suggest that we have been premature in designating specific small areas of Chicago as major PCB sources. Although we believe the sources identified in our study are large relative to the sources included in PCB emissions inventories, our stated conclusion is that they probably only account for a small amount of the total PCB emissions to the Chicago atmosphere (2).
Literature Cited (1) Oskouie, A. K.; Lordi, D. T.; Sawyer, B.; Lanyon, R. Environ. Sci. Technol. 2003, 37, 5836. (2) Hsu, Y. K.; Holsen, T. M.; Hopke, P. K. Environ. Sci. Technol. 2003, 37, 681-690. (3) Simcik, M. F.; Zhang, H.; Eisenreich, S. J.; Franz, T. P. Environ. Sci. Technol. 1997, 31, 2141-2147.
Ying-Kuang Hsu† and Thomas M. Holsen* Department of Civil and Environmental Engineering Clarkson University Potsdam, New York 13699
Philip K. Hopke Department of Chemical Engineering Clarkson University Potsdam, New York 13699 ES030616Z
10.1021/es030616z CCC: $25.00
2003 American Chemical Society Published on Web 11/11/2003
