Environ. Sci. Technol. 1998, 32, 1793-1798
Nonlinearity in the Slopes of Clausius-Clapeyron Plots for SVOCs R. M. HOFF* Centre for Atmospheric Research Experiments, Atmospheric Environment Service, Rural Route #1, Egbert, Ontario L0L 1N0, Canada K. A. BRICE Atmospheric Environment Service, 4905 Dufferin Street, Downsview, Ontario M3H 5T4, Canada C. J. HALSALL Institute for Environmental and Biological Sciences, Lancaster University, Lancaster LA1 4YQ, U.K.
Plots of partial pressures of semivolatile organic compounds versus inverse temperature at the time of measurement (Clausius-Clapeyron or CC plots) have been used to infer local air exchange with surfaces. Recent studies have shown that the slopes of such plots are smaller than would be expected from the known heats of vaporization, of airwater exchange, and probably also of air-vegetation exchange. Using data from the Point Petre Master Station of the Integrated Atmospheric Deposition Network, CC plots for trans-chlordane are examined for various wind directions approaching the measurement site. The slopes of the CC plots are not linear over the entire range of temperatures measured. It is shown that such behavior can be expected from the combination of exchange and transport processes that affect the air concentration at a remote site. The positive curvature of the CC plots can be detected in the usually noisy CC plots by use of regressions, which include an increasing number of data from lower temperatures. The degree of such curvature is postulated to indicate the degree of importance of longrange transport (LRT) versus local exchange. A conceptual model is presented in which the variation in CC slopes can be ascribed to the relative amount of LRT versus local exchange.
Clausius-Clapeyron Plot It has been known for some time that the air concentrations of semivolatile organic compounds (SVOCs) have a strong relationship to atmospheric temperature. At elevated temperatures, the air concentrations increase indicating that volatilization from the Earth’s surface is an important factor in the movement of these chemicals. A number of workers have shown that the air concentrations increase with temperature over a number of types of surfaces: water (1-6), particulates in air (7-15), vegetation(16, 17), snow (18, 19), etc. This behavior is consistent with the theory of liquid-vapor equilibrium described by the Clapeyron equation: * To whom correspondence should be addressed. Phone: (705)458-3310; fax: (705)458-3301; e-mail:
[email protected]. S0013-936X(97)00974-7 CCC: $15.00 Published on Web 05/09/1998
Published 1998 by the Am. Chem. Soc.
∆H dP ) dT T(∆V)
(1)
When coupled with the ideal gas law and integrated, a form of the Clausius-Clapeyron equation arises:
ln P ) -
∆H + constant RT
(2)
Thus, a plot of the natural logarithm of the partial vapor pressure as a function of the inverse temperature will be a straight line with the negative slope equal to the heat of vaporization divided by the gas constant. Since the partial vapor pressure is related to the air concentration through a change of units and the ideal gas law, a plot of the air concentration versus inverse temperature will give a similar slope (slightly changed by the density change of air with temperature) and it will have a different constant (intercept). The temperature dependence of a phase transition (i.e., surface to air) can be expressed by the Clausius-Clapeyron equation when the system is at equilibrium. In the laboratory, these equilibrium slopes are determined from the pure compound over a range of temperatures. In environmental studies, these plots can also be generated from samples taken at different temperatures, although a true equilibrium between the atmosphere and the surface may not exist. However, by comparing the slopes of the empirically derived ambient data and laboratory data, the hope is that one can infer something about the processes of volatilization by reference to the literature slopes of the vapor pressure, Henry’s law, Koa, etc. In a study of air concentrations of a number of SVOCs at Egbert, ON, Canada in 1988 and 1989 (20), the first author here presented a number of Clausius-Clapeyron (hereafter called CC) plots (which we called Antoine-type plots in that paper) for compounds ranging from R-HCH to endosulfan. Given the previous discussion about the potential for nonequilibrium phenomena to influence the exchange, there may be concern about calling such diagrams “ClausiusClapeyron” plots. However, without inventing another name for such environmentally derived diagrams where nonequilibrium effects may be involved, we will use that convention here. The slope of the CC plots shown in Table 2 of ref 20 generally led to heats of vaporization that are similar to known literature values. This led to the conclusion that volatilization from surfaces relatively near the sampling location were important in controlling the air concentration. This conceptually fit with our understanding of the “grasshopper effect” (21) where chemicals migrate from warm climates to colder ones in a series of volatilization/adsorption hops. In that work though, we noticed some odd behavior for certain species that did not fit the pattern. R-endosulfan had a much greater slope than the vapor pressure slope, and this was attributed to volatilization of locally applied chemical to crops. The PCB congeners had an unusual behavior in that some of the lighter PCB congeners had a slope change near 0 °C, which we did not completely understand (PCB 52 from that work is shown in Figure 1). It was postulated that this behavior could be the result of emissions of additional PCB52 from combustion when low temperatures occurred. We also speculated on hydrometeors and the possibility of the effect arising from the change in temperature due to transport. Recently, several workers have used the same approach to examine the behavior of SVOCs measured at a number of VOL. 32, NO. 12, 1998 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 1. CC plot of PCB congener 52 from the Egbert study of 1988/1989 (20). The points to the right and below the arrows have experienced some freezing during the sampling period.
TABLE 1. Enthalpies for CC Slopes for All Data 1990-1994 for 11 Organochlorines Measured at the Burnt Island and Point Petre IADN Master Stations Burnt Island chemical
∆H (kJ)
( SD (kJ)
R-HCH γ-HCH o,p-DDT p,p′-DDT o,p-DDD p,p′-DDD p,p′-DDE γ-chlordane R-chlordane trans-nonachlor dieldrin
22.2 36.7 30.0 28.9 28.8 20.4 31.8 49.5 52.8 52.3 58.1
( 2.0 ( 3.9 ( 4.1 ( 4.9 ( 4.6 ( 3.7 ( 3.4 ( 3.7 ( 3.2 ( 3.3 ( 3.5
Point Petre
ref 24
r2
∆H (kJ)
( SD (kJ)
r2
∆Hv (kJ)
0.52 0.43 0.32 0.23 0.25 0.20 0.42 0.61 0.70 0.69 0.70
15.4 36.4 55.7 64.7 34.8 32.8 54.3 47.9 57.7 45.2 61.0
( 3.5 ( 3.5 ( 4.6 ( 5.2 ( 3.8 ( 3.2 ( 4.0 ( 3.8 ( 4.1 ( 3.5 ( 3.7
0.07 0.31 0.38 0.39 0.26 0.31 0.43 0.40 0.46 0.41 0.53
68.4 70.4 88.5 93.1 na 88.5 87.1 80.7 82.0 85.5 82.5
locations from mid-latitudes to the high Arctic (3, 4, 18, 20, 22, 23). In some of these studies (20, 22, 23), the slopes of the CC plots are similar to those expected from the subcooled liquid vapor pressure enthalpies. More recently, however, the slopes of the CC plots are small; too small to be understood by partitioning to nonbinding surfaces through the heat of vaporization, to water via Henry’s law partitioning, or to vegetation. It is the need to understand what these reduced and nonlinear slopes are telling us that has prompted this paper.
Evidence for Reduced Clausius-Clapeyron Slopes Since 1988, we have been measuring the air concentrations of SVOCs at the Canadian stations of the Integrated Atmospheric Deposition Network (IADN (5)). We have examined the slopes of the CC plots for 11 organochlorines from two stations in the IADN program, Point Petre and Burnt Island, ON (Table 1). The CC slopes derived from these data taken between 1990 and 1994 consistently are smaller (at the 95% confidence level) than the published vapor pressure slopes in the literature (24). We can use the air concentrations of one species, trans-chlordane, to illustrate the problem described above. The conclusions, however, are not limited to trans-chlordane, but this chemical does allow us to show that there are consistencies from site to site in North America. Figure 2 shows the logarithm of the partial pressure of trans-chlordane (we will use the natural logarithm of partial pressure here to be strictly consistent with the form of the Clausius-Clapeyron equation) for the Point Petre station of IADN. The best fit regression of the data shows a slope of -5950 ( 440 (r 2 ) 0.39) corresponding to an enthalpy of 1794
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FIGURE 2. CC plot of trans-chlordane at Point Petre, ON, from IADN sampling from 1990 to 1994. The regression slope is shown against the thin solid line. A description of the other lines is given in the text. 49.5 ( 3.7 kJ. Figure 2 shows a number of other slopes. The boldest line shows the -10600 vapor pressure slope taken from Hinckley’s data (24). The light colored solid line shows the expected Henry’s law slope that is taken from Tateya’s work (25). Unfortunately, a good Henry’s law measurement of the temperature slope of trans-chlordane does not exist. The slopes for the 1988/1989 Egbert, ON, data (converted by a factor of 2.303) are shown in the weaker dashed line, and these have been compared by Bidleman et al. (26) to slopes obtained in South Carolina for the years 1977-1995 (heavy dashed line). From those comparisons, Bidleman et al. confirmed that similar volatilization processes were occurring in the U.S. South as in the Great Lakes region, and the only difference was in the strength of the source that gave concentration differences of about a factor of 10. Unfortunately, the situation has changed remarkably since these earlier studies. Bidleman et al. also showed data from 1989 (fine line in Figure 2) and 1994 (medium solid line) where the CC slopes have flattened considerably. At the same time, current data from Point Petre shows a nearly identical slope to the 1994 SC data. However, both are too flat to be explained by either the vapor pressure slope or the Henry’s law slope. These results show that similar behavior for trans-chlordane occurs regardless of whether the site is in the U.S. South or the Canadian south. The reasons for such a flat CC slope will be discussed later.
Different Sources, Different Directions, Different Temperatures We know that the scatter in the CC plot is due to the varied exposure air parcels encounter before arriving at our sampling site. These air parcels arrive from different directions on days with widely varying temperatures and with varied potential sources. It is logical to try to see if some improvement in our understanding of the CC slopes can be gained by grouping the data according to different wind directions. We computed 5-day back trajectories for air parcels arriving at Pt. Petre at 1000, 925, and 850 mbar. These air trajectories were then separated into trajectories which spent the majority of the preceding 5 days, either north or south of Point Petre. This grouping is shown in Figure 3. There is a difference between these subsets of the data with flows from the south being of higher trans-chlordane partial pressures than those from the north. We know that chlordane usage in the United States was higher than in Canada, and this seems to indicate that we are seeing the results of that increased usage. However, the slopes of the two subsets of the data are not statistically different and certainly do not approach the Henry’s law or vapor pressure slopes.
FIGURE 3. CC plot of the trans-chlordane data from Pt. Petre stratified by trajectories with generally northerly (solid symbols) and southerly flow (open symbols). While there is no significant offset in the slopes of the lines, there is an obvious displacement in the magnitude of ln P for southerly over northerly flow.
FIGURE 4. Trajectories for cases of generally northerly flow for the last 6-12 h of transport to Pt. Petre. Solid lines, cases where the ln P > -35; dotted lines, cases where ln P < -37.5. This shows that local transport over Lake Ontario is important in contributing trans-chlordane from air/water revolatilization.
A recent paper by Honrath et al. (3) showed some success in improving the r 2 for the slopes of the CC plots when the data were separated between flow over water or over land. Honrath et al. considered very local flows to be indicators of where sources of the chemicals arise. This is part of the answer, since if we further stratify the northerly subset into two categories: ln P > -35 and ln P < -37.5 (a small set of the very highest and lowest observed pressures), we see that the surface air for the highest cases actually spent the last 6-12 h before arriving at the Point Petre monitoring site coming from the south (over Lake Ontario). This means the air that travelled from the north over Canada for the preceding 4 days turned over Lake Ontario to arrive at Point Petre from a southerly direction. Conversely, the lowest partial pressure cases all arose from flow that was predominantly from the land sector during the last 6-12 h of transport. The 1000 mbar trajectories shown in Figure 4 as solid lines are for ln P > -35 and the dotted lines are for ln P < -37.5. Thus, even
FIGURE 5. CC plot of all the cases of northerly flow where the last 6-12 h traveled over Lake Ontario (off water, open symbols) or over Ontario (off land; solid symbols). There is a statistically significant difference in the slopes of the plots; however, both slopes are too small for either Henry’s law or VP temperature dependence.
FIGURE 6. Comparison of air temperatures at 1 m at Pt. Petre (100 m from shore) versus over-water skin temperatures from AVHRR imagery. The estimate of the water temperature is taken from the average of the last skin temperature derived in December and the first seen in May in the imagery and is known to be biased high. The open water values will not go below 0 °C however.
when the general flow pattern is largely from the north, what occurs in the last 6-12 h is very important. The conclusion to be drawn is that an important additional amount of transchlordane is being delivered to the air at Point Petre by volatilization from Lake Ontario itself. Using the sectors of 330-60° N as an “off land” trajectory and 90-250° N as an “off water” trajectory for from the surface flows during the last 6-12 h of transport, there is a statistically different slope for the two cases, with the off land subset having a higher CC slope (Figure 5). We did not find in this data set, however, that using local water surface temperatures versus local air temperature changed the CC slopes (as did Honrath et al.). Using surface water temperatures derived from NOAA-11 AVHRR imagery provided a slope of -3300, which is not statistically different than the off water slope using air temperatures. In fact, mean air temperatures were well correlated with the NOAA AVHRR water skin temperatures above 0 °C (Figure 6). These results show that there is imbedded information within subsets of the CC plotted data, which can be used to infer volatilization sources for the SVOCs. In the next section, we further examine whether these sources are unambiguous. VOL. 32, NO. 12, 1998 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 7. CC slopes calculated from data obtained above a given minimum temperature (up to a maximum of 21.3 °C, at which point 10 samples become the smallest data set). The dotted lines show the RMS error in the slope computed from the data. Slopes using data obtained below 10 °C converge to the value shown in Figure 2.
Why Do the Slopes Appear Flatter than Indicated by the Known Enthalpies? There is a nonlinearity in the derived slopes of the ClausiusClapeyron equation from this trans-chlordane data. While it is not readily apparent in Figure 2 that there is such a change in slope with temperature, we chose to examine what would happen if sampling had been terminated at some minimum temperature (for example, if we only measured in the summertime). In Figure 7, we have calculated the CC slope using data only above a minimum average daily temperature and allowed that minimum temperature to range from -18 °C for the entire data set up to a final data
set of only 10 samples (at 21 °C and above). The dotted lines in the figure show the high and low estimates of the computed slope using a linear regression model. Once the sampling data set is limited to temperatures above 10 °C at this Lake Ontario shoreline location, the derived slope of the CC equation becomes increasingly negative. This provides an indication that different studies, even at the same location, might obtain difference CC slopes if the range of temperatures sampled do not overlap. Important to this discussion, it also shows that there is imbedded curvature in the data of Figure 2. To further understand why the CC slopes flatten at lower temperatures, we have used some knowledge of the CC slopes measured in the high Arctic during the Canadian Arctic Environmental Strategy measurements (18, 19, 27, 28). One of the more intriguing discoveries in regard to the CC slopes is shown in Figure 8. For PCB congeners measured at Alert, NWT, Canada (some 600 km from the North Pole), the lightest congeners show extremely flat CC slopes (
