Environ. Sci. Technol. 2004, 38, 6905-6906
Response to Comment on “Simulating the Influence of Snow on the Fate of Organic Compounds” We appreciate Carlson and Hites’ initiative to evaluate the performance of our model simulating the influence of a snow cover on the fate of organic chemicals in the environment by a comparison with measured data from the Integrated Atmospheric Deposition Network (IADN) (1). Unfortunately, their comparison is misguided and leads them to premature conclusions about the validity of our model. Our model calculations suggested that some semivolatile organic chemicals, such as PCB congener 28 and R-HCH may experience a temporary peak in air concentration during the melting of a seasonal snow cover that had been accumulating pollutants from the atmosphere over a period of several months (2). The hypothetical temperature course used in these simulations assumed a continuous period of freezing temperatures lasting 169 d (see Figure 1A). During this period of time, a hypothetical snowpack formed, that by the onset of melting had a height of 69 cm and a snow water equivalent of 30 cm. This hypothetical scenario describes typical winter conditions occurring in areas such as Manitoba, Northwestern Ontario, North Dakota, or Minnesota. Similar extended periods of snow accumulation also occur in high mountain regions and in the subpolar and polar regions. Such a situation, however, does not exist in the vicinity of the Laurentian Great Lakes, where the IADN is located. As a result of the warming effect of the lakes and, in the case of the lower Great Lakes, also lower geographical latitude, the winter there is characterized by a much shorter snow accumulation season and a succession of snow fall and melting as temperature repeatedly crosses the freezing threshold in the course of a winter. This is illustrated, for example, by the 1999 temperature profile measured at the IADN site of Sturgeon Point (Figure 1A), which is on average 10.4 deg warmer than our hypothetical scenario and only shows subzero temperatures of -1.3 °C in January. A review of the temperature recorded at Sturgeon Point over a longer period (1991-2000) shows that it rarely stays below freezing for 2 months at a time, and even during January and February the site experiences repeated freeze-thaw cycles. We fed the average monthly temperature profile measured at Sturgeon Point in 1999 in our model and calculated the seasonal variability of the air concentrations of PCB congeners 28 and 101. We assumed that 25% of the model region is forested. In agreement with the IADN data presented by Carlson and Hites (1), the simulated air concentration of PCB-101 now shows a summer peak (Figure 1B) coinciding with the temperature maximum. The summertime concentration of PCB-28 is controlled by the rate of atmospheric degradation and thus depends strongly on the uncertain model input parameters for the rate constant of the reaction with the OH radical and the OH radical concentration. If the default input values of 1 × 10-12 cm3/(molecules‚s) (3) for the reaction rate or 12 000-65 000 molecules/cm3 for the seasonally variable OH radical concentration are lowered by 1 order of magnitude, a time profile with a summer maximum is also obtained for PCB-28 (Figure 1B). For both chemicals a small rise in air concentrations is predicted to occur, concurrent with the melting of a shallow snowpack accumulating over a period of a few weeks of continuous subzero temperatures in January. Freeze-thaw cycles, which the model is currently not taking into account, could even 10.1021/es048518g CCC: $27.50 Published on Web 11/03/2004
2004 American Chemical Society
FIGURE 1. Annual temperature profile used in hypothetical simulations described in ref 2 and measured at the IADN station of Sturgeon Point in 1999 (A) and simulated annual concentration variability (normalized to annual mean) of PCB congeners 28 and 101 using the measured temperature profile and different assumptions concerning the rate of atmospheric degradation (B). prevent the occurrence of such a small air concentration peak. Among the IADN sites, Eagle Harbor on the Keweenaw Peninsula on the southern shore of Lake Superior has a climate that is most similar to the generic scenario described in the paper. Average temperatures are generally below freezing from late November until late March, resulting in a snow accumulation season of up to 4 months. Referring to 1991 and 1993 IADN data from Eagle Harbor, Hillery et al. (4) reported a seasonal air concentration profile for PCBs with a pronounced spring peak in addition to a summer time peak coinciding with the temperature maximum. Hornbuckle et al. (5) attributed the earlier peak to the release of contaminants stored up in the seasonal snow cover. Similarly, pronounced springtime peaks in air concentrations have been observed for HCHs and chlordane at a number of sites in the Canadian and Russian Arctic (6). There are at least two important lessons to be learnt from this. First, a meaningful model evaluation and comparison of observed and simulated data cannot rely on the results of simulations that are based on a hypothetical environmental scenario, in particular if this scenario is notably different from the environmental characteristics of the site where the observations were made. Instead, the model needs to be supplied with input parameters that are specific to this site. Equally important is the insight that relatively small temperature differences (such as between the two scenarios in Figure 1A) can result in large changes in the simulated fate of organic contaminants, if these differences are amplified by the effect of temperature on the water/ice transition. This VOL. 38, NO. 24, 2004 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
9
6905
is one of the primary incentives for trying to understand the role snow and ice play in the fate of organic pollutants. In conclusion, observations from Sturgeon Point and other IADN sites do not show a spring concentration peak because these sites do not experience the necessary combination of meteorological factors (i.e., extended periods of subzero temperatures and significant snow accumulation) that would allow for a sufficient accumulation of pollutants in a snowpack. We thus believe Carlson and Hites’ assessment of our model as “fundamentally flawed” (1) to be hasty and unsubstantiated and look forward to challenging our model with more meaningful comparisons with field observations (7).
Literature Cited (1) Carlson, D. L.; Hites, R. A. Comment on “Simulating the influence of snow on the fate of organic compounds”. Environ. Sci. Technol. 2004, 38, XXXX-XXXX. (2) Daly, G. L.; Wania, F. Simulating the influence of snow on the fate of organic compounds. Environ. Sci. Technol. 2004, 38, 4176-4186. (3) Anderson, P. N.; Hites, R. A. OH radical reactions: The major removal pathway for polychlorinated biphenyls from the atmosphere. Environ. Sci. Technol. 1996, 30, 1756-1763. (4) Hillery, B. R.; Hoff, R. M.; Hites, R. A. Atmospheric contaminant deposition to the Great Lakes determined from the integrated
6906
9
ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 38, NO. 24, 2004
atmospheric deposition network. In Atmospheric Deposition of Contaminants to the Great Lakes and Coastal Waters; Baker, J. E., Ed.; SETAC Press: Pensacola, FL, 1997; Chapter 15, pp 277-291. (5) Hornbuckle, K. C.; Jeremiason, J. D.; Sweet, C. W.; Eisenreich, S. J. Seasonal variation in air-water exchange of polychlorinated biphenyls in Lake Superior. Environ. Sci. Technol. 1994, 28, 1491-1501. (6) Hung H.; Blanchard, P.; Halsall, C. J.; Bidleman, T. F.; Stern, G. A.; Fellin, P.; Muir D. C. G.; Barrie, L. A.; Jantunen, L. M.; Helm, P. A.; Ma, J.; Konoplev, A. Temporal and spatial variabilities of atmospheric polychlorinated biphenyls (PCBs), organochlorine (OC) pesticides and polycyclic aromatic hydrocarbons (PAHs) in the Canadian Arctic: Results from a decade of monitoring. Sci. Total Environ. (in press). (7) Gouin, T.; Harner, T.; Daly, G. L.; Wania, F.; Mackay, D.; Jones, K. C. Variability of concentrations of polybrominated diphenyl ethers and polychlorinated biphenyls in air: Implications for monitoring, modeling and control. Atmos. Environ. (in press).
Gillian L. Daly and Frank Wania* Department of Physical and Environmental Sciences University of Toronto at Scarborough 1265 Military Trail Toronto, Ontario, Canada M1C 1A4 ES048518G
