Correspondence Comment on “Simulating the Influence of Snow on the Fate of Organic Compounds” Daly and Wania (1) recently published an organic compound environmental fate model which predicted that certain more volatile compounds would have maximum concentrations in the atmosphere in the spring as a result of the melting snowpack. Daly and Wania note that “the reasonability and accuracy of the model results would need to be confirmed by comparison with field measurements if it were to be used predictively”. On the basis of atmospheric concentrations of several persistent organic compounds measured at the U.S. sites of the Integrated Atmospheric Deposition Network (IADN), we have found no indication of significant springtime peaks for any analytes; therefore, we must conclude that the published model is flawed, casting doubt on the conclusions presented in the paper by Daly and Wania (1). The U.S. sites of IADN have been operated by Indiana University since the end of 1994. Gas-phase atmospheric samples are taken every 12 days at five sites around the Great Lakes, each of which has significant winter snowfall. The concentrations of PCBs, PAHs, and pesticides are quantified in each sample. More details on IADN have been published elsewhere (2) and online (www.smc-msc.ec.gc.ca/iadn/ index.html). Examples of data for two PCB congeners highlighted in Daly and Wania’s Figure 6 are shown in our Figure 1 for Sturgeon Point, a site on the eastern shore of Lake Erie. The vertical lines indicate April 1 of each year; the springtime concentration peak predicted in Daly and Wania’s Figure 6 should occur in the month of April. Clearly, there is no regular concentration peak (Figure 1A) at this time in the data for either PCB-28 or PCB-101, let alone an annual maximum. There are, on the other hand, regular annual maxima during the summer. The concentrations can be mostly explained by atmospheric temperature, as noted previously by our group (3). When concentrations are corrected for temperature using the slope of the Clausius-Clapeyron equation (3), no annual periodicity can be observed (Figure 1B), and there are no consistent concentration (or partial pressure) peaks in the spring. We chose to show data from Sturgeon Point because it is closest to the area used in Daly and Wania’s model, “south central Canada”, presumably the Toronto area. Data from the other U.S. IADN sites, some of which are colder and snowier than Sturgeon Point, also fail to show springtime peaks. The IADN data have a high enough temporal resolution (once in 12 days) to be able to capture a hypothetical 1- or 2-month springtime peak, which would presumably cover ∼5 consecutive samples. It is possible that a spring concentration peak exists but that it occurs over a few days
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FIGURE 1. Vapor phase data for PCB-28 and 101 at Sturgeon Point, near Buffalo, NY. (A) Concentrations and (B) temperaturecorrected partial pressures (3). Vertical lines indicate April 1 of each year. instead of over weeks or months. Daly and Wania addressed this issue, as shown in their Figure 7; short melting periods and short accumulation periods could result in peaks that are too inconsequential to be noticeable with a 12-day sampling regime. However, our data still do not agree with the predications shown in their Figure 7B, which shows a relatively constant concentration from the spring to fall for PCB-101. The final scenario, no snow at all, is also shown in their Figure 6; PCB-101 would have essentially unchanging concentrations over the year while PCB-26 would increase during the winter. Again, this is clearly not what is seen in our Figure 1. We conclude that their model is flawed, both with and without the inclusion of terms for snow. Their model should not be used for predicting the quantitative or qualitative behavior of organic contaminants until improvements are made that result in agreement between the model and observations.
Literature Cited (1) Daly, G. L.; Wania, F. Simulating the influence of snow on the fate of organic compounds. Environ. Sci. Technol. 2004, 38, 4176-4186. (2) Buehler, S. S.; Hites, R. A. The Great Lakes’ Integrated Atmospheric Deposition Network. Environ. Sci. Technol. 2002, 36, 354A-359A. (3) Buehler, S. S.; Basu, I.; Hites, R. A. Causes of variability in pesticide and PCB concentrations in air near the Great Lakes. Environ. Sci. Technol. 2004, 38, 414-422.
Daniel L. Carlson and Ronald A. Hites* School of Public and Environmental Affairs Indiana University Bloomington, Indiana 47405 ES048708R
10.1021/es048708r CCC: $27.50
2004 American Chemical Society Published on Web 11/03/2004
