Environ. Sci. Technol. 1992, 26, 276-283
Annual Cycle of Polychlorinated Biphenyls and Organohalogen Pesticides in Air in Southern Ontario. 2. Atmospheric Transport and Sources Raymond M. Hoff,*rt Derek C. G. Muir,$ and Norbert P. Grift$
Centre for Atmospheric Research Experiments, Atmospheric Environment Service, Rural Route 1, Egbert, Ontario, Canada LOL 1LO and Department of Fisheries and Oceans, Freshwater Institute, 501 University Crescent, Winnipeg, Manitoba, Canada R3T 2N6
w In the second of a pair of papers describing the measurements of over 90 polychlorinated biphenyl congeners and over 30 organohalogen pesticides, instances of seasonally high concentrations are analyzed in terms of meteorological transport. The seven cases of highest concentrations of the pesticide species all point to long-range transport from the south, especially the southern United States and the Caribbean. The data are also analyzed in terms of the temperature dependence of the air concentration. It is shown that the PCB concentrations, which are only weakly dependent on transport paths, are strongly dependent on temperature through the vapor pressure of the compound. A similar effect is seen in general for the organohalogens with a more strongly apparent influence of air transport path. a-Hexachlorocyclohexane (HCH), pentachloroanisole, and heptachlor show no temperature-concentration dependence. The results indicate that modeling predictions of the concentration of PCBs in air can be made by knowing the slope of the log (vapor pressure) vs inverse temperature curve (Antoine equation) as well as expected air concentrations of particulate matter. For other organochlorines, especially those with a more recent or current-use pattern, regional scale and long-range transport from areas with higher concentrations can be expected and further work to obtain samples in those arem is needed.
Introduction In the accompanying paper (I), we described the measurement techniques and presented data on the annual cycle of the air concentrations of polychlorinated biphenyls (PCBs) and organohalogen insecticides in air in southern Ontario. The annual cycles of these chemicals were modeled by a Lorentzian function over a 1-year period. This procedure yields a useful way of parameterizing the large variation in air concentration as a function of time. The data were also shown predominantly as monthly averages in order to bring out the large-scale variation in the data rather than the day-to-day “noise”. In this paper, we examine that “noise”, which contains much of the transport-related information. While there is true noise in the data due to measurement and analytical variability, these are generally small compared to the fluctuations in air concentrations that are related to regional or synoptic scale variations in the origin of the air being sampled. Even on the annual scale, it was apparent in the first paper that temperature effects are very important in determining the air concentration of these chemicals. Volatilization is indicated as the primary control of their air concentration and this paper will examine that process as well. Analysis of Several Episodes of High and Low Air Concentrations Examination of each air sample in terms of the air trajectory taken by the chemical before arrival at the Centre t Atmospheric Environment Service. *Freshwater Institute.
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for Atmospheric Research Experiments (CARE) in Egbert, ON (44’14’ N, 79’47’ W, 251 masl), has been carried out. The air trajectories have been obtained using a program developed by Olson et al. (2) and run on the Atmospheric Environment Service CRAY XMP computer at Dorval, PQ. Each trajectory is computed at three levels (1000,925, and 850 mbar; 10 mbar = 1 kPa) and the latitudes and longitudes of each 6-h time step backwards for 5 days (120h) are plotted. The expected accuracy of the 1000mbar trajectory is less than those aloft since the trajectory often contacts the earth’s surface and must be adjusted to surface height. The 1000-mbar trajectory usually is plotted as a “limit” or bound on the lateral extent of the spreading of the trajectories. The 925- and 850-mbar trajectories are more representative of tropospheric flow. These trajectories are expected to be accurate in position to -1 km/h of backward motion (2),and thus 120 h back in time, the trajectory position would have an error the size of states in the United States, for example. The accuracy of the trajectories is not solely relied on as an indication of transport. Synoptic charts and satellite photos have also been studied to confirm the most likely air transport paths, to determine whether major systems (tropical systems, for example) have contributed to the air inflow, and to ensure that the transport has occurred during relatively moisture free regimes. The latter criterion is important since a long transport path within a major precipitating low-pressure system might indicate longrange transport while, in fact, wet removal might decrease the amount of organic chemicals in the air. In this section, we will examine several cases during which seasonally high or low concentration events were observed. By this means, the sources of these high and low concentrations may be located. In the discussion that follows, the reader should be aware that sampling, which starts on July 10, for example, is initiated at 3 p.m. local time, which is 17 UTC (coordinated univeral time, previously called Greenwich time) in summer and 18 UTC in winter. T h e air trajectory which would most closely agree with that time will be the 00 U T C trajectory on the following date. July 10-15,1988. The initiation of sampling during this study (June 10-11) occurred at the very end of a significant high-pressure stagnation over eastern North America. On July 7, the air concentration of many pollutants including ozone and sulfates reached concentrations where industrial shut downs were requested. The series of panels in Figure la-d and Figure 2 show the concentration and trajectory sequences for this period. The concentration of PCBs in air (Figure la) was >2 ng m-3 on July 10-11, which was a factor of 2 higher than the following 2-day period. Air trajectories for the fiist sample (1-daysampling time) in Figure ICshowed anticyclonic flow around the back side of the stagnant high-pressure system. This trajectory corresponds to the sixth hour of the sample period. The sum of the hexachlorocyclohexane (HCH) concentration (CHCH), the sum of the chlordane-related species (CCHLOR),the sum of the DDT, DDD, and DDE isomers
0013-936X/92/0926-0276$03.00/0
0 1992 American Chemical Society
O r g o n o c h l o r , i n e s i n Air
