Environ. Sci. Technol. 1996, 30, 1756-1763
OH Radical Reactions: The Major Removal Pathway for Polychlorinated Biphenyls from the Atmosphere PHILIP N. ANDERSON AND RONALD A. HITES* School of Public and Environmental Affairs and Department of Chemistry, Indiana University, Bloomington, Indiana 47405
Polychlorinated biphenyls (PCBs) are transported through the environment primarily in the atmosphere and may undergo chemical reactions, particularly with the OH radical, while in the vapor phase. Rate constants for the gas-phase reaction of 14 PCB congeners with the OH radical over the temperature range of 323363 K were measured. The calculated temperature dependences of the reactions were used to estimate OH-PCB reaction rate constants at 298 K; see Table 4. These 298 K rate constants agree well with literature rate constants for PCBs containing zero to two chlorines and with OH-PCB reaction rate constants estimated from a structure-activity method. Calculated atmospheric lifetimes of PCBs due to OH-initiated reactions varied from 2 days for biphenyl to 34 days for a pentachlorobiphenyl. A simple model for the vertical concentration gradient of PCBs in the troposphere was developed and used to calculate the total global loss rate (8300 t yr-1) of PCBs from the atmosphere due to removal by OH. This pathway is very large in comparison to other known permanent PCB loss processes from the environment, such as deep ocean sediment burial (240 t yr-1).
the atmosphere (9). Although the vapor pressures of PCBs are low (10-8 to 10-2 Torr at 298 K), these compounds are present in the atmosphere primarily in the vapor phase (10, 11), and thus, they may react with the OH radical. In fact, it has been experimentally shown by several groups that OH reactions occur at environmentally significant rates for PCBs with up to two chlorine substituents (12-19). From these results, the OH reaction rate constants of more highly chlorinated PCBs have been estimated by Atkinson (13), but no experimentally determined OH rate constants for PCBs with more than two chlorines have been reported. The determination of OH-SOC reaction rate constants at environmentally significant temperatures is experimentally difficult. The vapor pressures at 298 K of PCBs with more than two chlorines are less than 10-3 Torr (20). Furthermore, these compounds tend to partition to hydrophobic surfaces, such as the Teflon films frequently employed in laboratory reaction chambers. In our laboratory, we have constructed and validated a system to determine SOC reaction rate constants over an elevated temperature range (14). Our system features a small heated quartz chamber with on-line detection of reactants by mass spectrometry. From the experimentally determined Arrhenius parameters of OH-SOC reactions, we can extrapolate the rate constants to obtain OH-SOC rate constants at ambient, and lower, temperatures. In this paper, we report on the determination of OH reaction rate constants and their temperature dependences for 14 PCB congeners with zero to five chlorines. Some congeners were selected because they are present in large quantities in the atmosphere, and other congeners were selected for structural reasons. These kinetic experiments all used the relative rate method (13) with cyclohexane or isobutane as the reference compound. From the temperature dependences, we have calculated rate constants at 298 K for these 14 PCBs and compared these values to predictions based on Atkinson’s estimation method (13). Based on these data, we have constructed a simple model to assess the importance of the OH reaction as a PCB loss process on a global scale.
Experimental Section Introduction Polychlorinated biphenyls (PCBs) are stable, semivolatile, organic compounds (SOCs). Over 1.4 × 106 t of PCBs was produced from the late 1920s until the 1970s, and smaller scale production continued in some countries until recently (1, 2). PCBs are lipophilic and tend to bioaccumulate. Some congeners interfere with mammal and bird reproduction, and some may disturb development or cause cancer in humans (3-6). Because PCBs degrade very slowly, they are now ubiquitous in the natural environment. These compounds are atmospherically transported from source regions to remote locations through repeated deposition to and volatilization from soils, lakes, and oceans (7, 8). Extensive atmospheric chemistry research has established that chemical reactions with the OH radical are the dominant loss processes for most organic compounds in * Corresponding author e-mail address:
[email protected].
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The OH-PCB reaction rate constant determination experiments were carried out in the laboratory reaction system we have described previously (14). All experiments were performed at atmospheric pressure in a 195-mL quartz reaction chamber, at temperatures ranging from 323 to 364 K. The chamber, situated in a GC oven for temperature control, is continuously sampled by an on-line mass spectrometer operated in the selected ion monitoring (SIM) mode. Rate constant determinations for compounds with fewer than three chlorines were carried out in 10% oxygen in nitrogen. Rate constants for congeners with three or more chlorines were measured in 10% oxygen in helium to provide better mass spectrometric detection limits. Individual PCB congeners (Accustandard Inc., New Haven, CT; stated purities >99%) were weighed as the solid and dissolved in CCl4 to provide solutions of 0.5-2 mg mL-1. The PCB solutions were injected directly into the chamber through a septum fitted to a side-arm of the chamber. Cyclohexane (>99.99%, EM Science, Gibbstown,
0013-936X/96/0930-1756$12.00/0
1996 American Chemical Society
TABLE 1
PCB Congeners for Which OH Reaction Rate Constants and Temperature Dependences Are Reported in This Study congener IUPAC No.
subcooled liquid vapor pressure at 25 °C (Torr) (20)
ions monitored (m/z)
0 1 2 3 4 7 15 28 29 31 33 44 47 95 110 116
2.8 × 10-2 1.9 × 10-2 7.5 × 10-3 6.8 × 10-3 4.5 × 10-3 1.9 × 10-3 6.0 × 10-4 1.8 × 10-4 3.3 × 10-4 2.1 × 10-4 1.4 × 10-4 8.8 × 10-5 9.9 × 10-5 4.7 × 10-5 1.9 × 10-5 1.7 × 10-5
153, 154 152, 188 152, 188 152, 188 222, 224 222, 224 222, 224 256, 258 256, 258 256, 258 256, 258 220, 292 290, 292 256, 326 256, 326 326, 328
biphenyl 2-chlorobiphenyl 3-chlorobiphenyl 4-chlorobiphenyl 2,2′-dichlorobiphenyl 2,4-dichlorobiphenyl 4,4′-dichlorobiphenyl a 2,4,4′-trichlorobiphenyl 2,4,5-trichlorobiphenyl 2,4′,5-trichlorobiphenyl 2′,3,4-trichlorobiphenyl 2,2′,3,5′-tetrachlorobiphenyl 2,2′,4,4′-tetrachlorobiphenyl 2,2′,3,5′,6-pentachlorobiphenyl 2,3,3′,4′,6-pentachlorobiphenyl 2,3,4,5,6-pentachlorobiphenyl a
OH rate constant data for this congener previously reported in ref 14.
NJ), in CCl4 solution, was also introduced into the chamber by direct injection. Isobutane (99.99% instrument grade, Linde, Danbury, CT) was injected through the side-arm septum with a gas-tight syringe. Concentrations of PCBs in the cell varied from 3 × 1013 to 2 × 1014 cm-3. Cyclohexane or isobutane concentrations were approximately 2 × 1014 cm-3. OH radicals were generated by the photolysis of O3 in the presence of H2O:
O3 + hν(λ < 320 nm) f O(1D) + O2
(1)
O(1D) + H2O f 2 OH
(2)
The light source was a Pen-Ray low-pressure Hg lamp. It illuminated the cell through a quartz window and vacuum UV cutoff filter in the GC oven door. The UV cutoff filter, which completely absorbs the weak 185-nm line in the mercury spectrum, ensured that no Cl atoms were produced from photolysis of CCl4, the OH-inert solvent used for the introduction of organic compounds. This lack of photolysis was confirmed by monitoring m/z 117 (CCl3+) during photolysis control experiments; no change was observed during irradiation of the chamber. The concentrations of O3 in the cell ranged from 2 × 1013 to 2 × 1014 cm-3, and the concentration of H2O was about 7 × 1017 cm-3. With nitrogen as the diluent and with an O3 concentration of about 1014 cm-3, the OH concentration was about 5 × 108 cm-3; with helium as the diluent and with an O3 concentration of about 5 × 1013 cm-3, the OH concentration was about 3 × 109 cm-3. The electron impact mass spectrometer (a HewlettPackard 5995 system) was operated in the selected ion monitoring mode (SIM). It measured the two most abundant ions for each compound and other masses for blank corrections. The masses monitored for each PCB congener are listed in Table 1, along with their vapor pressures. We also monitored m/z 56 and 69 for cyclohexane and m/z 41 and 42 for isobutane. Although m/z 43 is the most abundant ion in the isobutane mass spectrum, initial experiments showed that m/z 43 did not decline when isobutane was subjected to OH radical attack. Presumably, this lack of change was the result of the m/z 43 fragment
ion from acetone, which is a known product of the OHisobutane reaction (21). Ions at m/z 41 and 42 did decay as expected, with no observed interference from product ions. OH rate constants were determined by the relative rate method in which the disappearance rates of the PCB congener and a reference compound are measured while both compounds are simultaneously subject to OH attack (22). Assuming that reaction with OH is the only significant loss process for both the PCB and the reference compound (cyclohexane or isobutane) while the UV lamp is on, the following equation applies:
ln
( ) [PCB]0 [PCB]t
)
(
)
k1 [reference]0 ln k2 [reference]t
(3)
where the the concentrations of the organic compounds are measured at time t ) 0 and at succeeding times, t. A plot of ln([PCB]0/[PCB]t) versus ln([reference]0/[reference]t) has a slope with the ratio of rate constants (k1/k2) and a zero intercept. Since the reference rate constant (k2) is known, the PCB rate constants (k1) can be easily calculated. Data analysis was performed as described previously (14). Raw data extracted from each experiment were imported into a spreadsheet program and background corrected, and values were plotted according to eq 3. A linear regression was performed to obtain the rate constant ratio from the plot’s slope. The rate constant ratio was converted to a PCB rate constant by multiplying by the known cyclohexane or isobutane rate constant. In our study, we calculated the reference rate constant (k2) for each experimental temperature from the temperaturedependent rate expressions recommended by Atkinson for cyclohexane (12) and isobutane (23): +0.85 k(cyclohexane) ) (2.66-0.65 )×
10-17T 2e(344(95)/T cm3 s-1 (4) +0.14 k(isobutane) ) (1.11-0.13 ) × 10-17 T 2e(256(47)/T cm3 s-1 (5)
Results and Discussion PCB-OH Reaction Rate Constants. PCBs absorb radiation in the mid-UV spectral range (24), but the lamp flux was
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sufficiently low so that direct photolysis was negligible. To confirm this expectation, we performed control experiments for each congener in which all reagents except ozone were present in the chamber. In the control experiments, no change in the level of any of the congeners was detected when the UV lamp was turned on. In experiments in which OH was generated, concentrations of other reagents in the reaction chamber were similar to those in our previously reported work (14). Therefore, we expected interfering reactions of the organic compounds with other species such as O(1D), O(3P), or O(1D)-CCl4 reaction products to be negligible compared to the OH-organic reactions. Rate constants were measured between approximately 323 and 363 K for each of the PCB congeners listed in Table 1, except for the three pentachlorobiphenyls (congener nos. 95, 110, and 116). At 323 K, we could not detect any pentachlorobiphenyls in our system; thus, for these compounds, the OH reaction rate constant experiments cover a temperature range of 343-363 K. Table 2 lists the PCBreference compound rate constant ratios and the absolute PCB rate constants derived from the ratios using the reference rate constant expressions given above for cyclohexane or isobutane. Rate constants at 298 K have previously been reported for congeners 0-4 (12-19); no rate constants have been reported for any PCBs with more than two chlorines. Most of the rate constants reported here were determined using isobutane as the reference compound. In the case of biphenyl and 4-chlorobiphenyl, some experiments were also done with cyclohexane to compare rate constants derived with each reference compound. Cyclohexane was also the reference compound used in earlier reaction rate constant studies of toluene and 4,4′-dichlorobiphenyl in our system (14). There was no statistically significant difference between the rate constants determined using either reference hydrocarbon with 95% confidence. Previous experiments showed that OH-toluene reaction rate constants were not statistically different when measured in either nitrogen or helium as the diluent (14). Since OH-aromatic ring addition is likely to be the predominant reaction of the biphenyls (by analogy to toluene), we anticipated that PCB-OH reaction rate constants would be comparable in either diluent. Rate constant experiments using either nitrogen or helium as the diluent were performed with 2,2′,4,4′-tetrachlorobiphenyl (IUPAC No. 47) to verify this assumption. Comparison of the rate constants measured in each diluent showed no statistically significant difference with 95% confidence. In order to compare our OH-PCB reaction rate constants to other values in the literature and to put these rate constants in perspective with the reaction rate constants of other organic compounds, we determined the Arrhenius temperature dependence of the OH reaction of each individual PCB congener. Linear regression was used to fit a straight line to the natural logarithm of the experimental rate constants for a given congener versus the reciprocal temperature of the experiment. Table 3 lists the Arrhenius parameters (pre-exponential factor and activation energy, given as Ea/R) for each OH-PCB reaction that was investigated. The table also indicates the number of data points upon which each linear regression is based. Except for biphenyl, all the reactions show positive, but relatively weak, temperature dependences. From the Arrhenius parameters for each individual congener, the rate constant at 298 K for each congener was
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calculated; these are given in Table 4, along with the 95% confidence interval of the estimate. In extrapolating to temperatures outside the range we studied, we implicitly assume that the temperature dependence is well-behaved and follows the Arrhenius form. The excellent agreement of the extrapolated rate constants for biphenyl, the three monochlorobiphenyls, and 2,2′-dichlorobiphenyl (congeners 0-4) with previously reported rate constants measured at 298 K (12, 13, 15-19) gives us confidence that our extrapolated rate constants are reasonable. Figure 1 groups our 298 K OH-PCB reaction rate constants by homologue class. The general trend in rate constants was expected: The OH-PCB reaction slows down with each electron-withdrawing chlorine substituent added. Note that within homologue classes, the congener with all chlorines on the same ring reacts most rapidly (congeners 7, 29, and 116). Comparison of our 298 K rate constants, grouped by homologue class, to rate constants calculated by Atkinson’s structure-activity estimation method is also interesting. Figure 2 plots the logarithm of the OH-PCB reaction rate constant versus the number of chlorine substituents. The solid line is a linear regression of our 298 K rate constants, while the dashed line plots the average of the rate constants of all possible congeners with zero to six chlorines, as calculated by Atkinson’s method (25). Although our rate constants are systematically lower than the predicted rate constants by 15-20%, the slope of both data sets is in excellent agreement, supporting the validity of Atkinson’s estimates (13). Combining our PCB-OH rate constants with a global 24-h averaged OH concentration of 9.7 × 105 cm-3 (26) yields atmospheric lifetimes for each congener, which are tabulated in Table 4, last column. These values range from 2 to 34 days. Our laboratory recently reported atmospheric lifetimes for PCB congeners determined in a long-term air sampling study. Lifetimes for trichlorobiphenyl congeners 28, 31, and 33 (the most abundant PCBs in ambient air samples) were 75-80 days (11). These latter values are considerably higher than our current estimates of 10-12 days for the same congeners. The higher values (75-80 days) are total atmospheric lifetimes estimated by the Junge method (27). This method assumes that the tropospheric lifetime of a gas is inversely proportional to the relative standard deviation (RSD) of the concentration measurements of that gas. Junge made several crucial assumptions: First, the RSD of the concentration measurements must be primarily due to the time and space variability of the gas not to the variability of the analytical method. The measurements should also be made at numerous locations around the earth over a span of at least 1 year. Second, the tropospheric mixing time of the gas must be short compared to its tropospheric lifetime. Third, sinks of the gas are assumed to be distributed uniformly throughout the troposphere; for instance, homogenous vapor phase reactions qualify, but removal by the earth’s surface does not. The uncertainties associated with the application of the Junge method to PCB lifetime calculations are significant. It is unlikely that PCBs are evenly distributed throughout the troposphere, and methods to measure PCBs in the atmosphere are imprecise. PCB vapor pressures are low, as opposed to the substances on which the Junge relationship is based (H2O, Rn, O3, CO, CH4, H2, N2O, CO2, and O2). Unlike these volatile gases, PCBs can partition into other environmental compartments such as bodies of water and
FIGURE 1. Rate constants at 298 K for the vapor-phase reaction between OH and various PCB congeners. The indicated error bars are the 95% confidence limits.
In order to calculate a flow of PCBs from the atmosphere due to OH reactions, we must estimate the atmospheric PCB burden. Based on the presumption that PCBs are photochemically stable, it has been assumed that PCBs are well-mixed and evenly distributed throughout the atmosphere. In that case, the atmospheric burden would be simply the average PCB concentration multiplied by the atmospheric volume. However, our OH-PCB reaction rate constants show that PCBs are, in fact, photochemically reactive, and PCBs will not necessarily be evenly distributed in the atmosphere.
FIGURE 2. Plot of experimental and calculated OH-PCB reaction rate constants at 298 K versus the number of chlorine atoms. Solid circles are the rate constants reported in this study; open squares are the experimental rate constants in the literature (12, 13). The solid line is a linear regression fitted to our experimental rate constants. The dashed line represents the average PCB-OH rate constant of all possible congeners in the homologue classes with zero to six chlorines, as estimated by Atkinson (13).
soils, which are not distributed throughout the troposphere but are at the earth’s surface. Given all of these uncertainties and given that the estimated lifetimes in Table 4 are also uncertain, we feel that the agreement between the two estimates is adequate. Atmospheric Removal of PCBs by OH Reactions. How important are OH reactions in removing PCBs from the environment compared with other sinks? We have developed a simple model to estimate the global flow of PCBs out of the atmosphere due to OH reactions. Using that flow, we can compare estimated PCB losses due to OH reactions versus estimated PCB losses due to other permanent sinks.
Because almost all studies that have measured PCBs have collected samples at or near the earth’s surface, little experimental information is available on the vertical distribution of PCBs in the atmosphere. The only evidence to support the uniform atmospheric distribution hypothesis is one study in which SOCs were collected with an airborne sampler at varying altitudes over the Adirondack Mountains and over the North Atlantic near Norfolk, VA, and near Bermuda (28). The data set for PCBs reported in this study is small (24 total samples) and highly variable: The overall average PCB concentration is 0.45 ng m-3 with a standard deviation of 0.38 ng m-3. Four samples over the Adirondacks (three at high altitude, one at low altitude) and nine samples near Bermuda (five at high altitude, four at low altitude) are classified as coming from different levels of the atmosphere; the other 11 samples are integrated over altitudes ranging up to 3 km. For the larger “high-low” data set collected near Bermuda, the average PCB concentration for high altitude samples was 0.16 ( 0.09 ng m-3; the average PCB concentration at low altitudes was 0.33 ( 0.25 ng m-3. The difference between these two averages may indicate a vertical concentration gradient for PCBs in the atmosphere, but the variability and limited number of the samples preclude a definite judgment on this issue. We believe that, given the variability of this data set, there is no conclusive experimental evidence that PCBs
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TABLE 2
Experimental Conditions, Rate Constant Ratios, and Rate Constants for Reaction of PCB Congeners with OH Radicals diluent gasa
T (K)
ref compdb
k (PCB)/k (reference)c
rate constant (×10-12 cm3 s-12)d
T (K)
diluent gasa
ref compdb
k (PCB)/k (reference)c
rate constant (×10-12 cm3 s-12)d
C C I I I I I I I
0.85 ( 0.26 0.82 ( 0.04 2.43 ( 0.38 2.45 ( 0.16 2.32 ( 0.15 2.39 ( 0.28 2.37 ( 0.24 2.56 ( 0.10 2.84 ( 0.14
6.9 ( 1.7 6.6 ( 1.6 6.7 ( 1.3 6.8 ( 1.4 6.4 ( 1.3 6.6 ( 1.3 6.5 ( 1.3 6.6 ( 1.3 7.3 ( 1.5
356 356 356 356 348 338 338 338 324
N2 N2 N2 N2 N2 N2 N2 N2 N2
C C C C C C C C C
0.86 ( 0.06 0.74 ( 0.04 0.72 ( 0.02 0.77 ( 0.04 0.85 ( 0.04 0.92 ( 0.04 0.83 ( 0.06 0.71 ( 0.10 0.83 ( 0.04
Biphenyl (IUPAC No. 0) 7.6 ( 1.9 324 N2 6.6 ( 1.6 323 N2 6.4 ( 1.6 343 N2 6.8 ( 1.7 343 N2 7.4 ( 1.8 343 N2 7.8 ( 2.0 343 N2 7.0 ( 1.8 343 N2 6.0 ( 1.5 323 N2 6.7 ( 1.7 323 N2
363 363 363 343 343
N2 N2 N2 N2 N2
I I I I I
0.94 ( 0.10 1.21 ( 0.28 1.08 ( 0.24 1.12 ( 0.20 1.14 ( 0.16
2-Chlorobiphenyl (IUPAC No. 1) 2.8 ( 0.6 343 N2 3.6 ( 0.7 326 N2 3.2 ( 0.6 323 N2 3.1 ( 0.6 323 N2 3.1 ( 0.6
I I I I
1.07 ( 0.24 1.26 ( 0.06 1.10 ( 0.26 1.03 ( 0.32
3.0 ( 0.6 3.3 ( 0.7 2.8 ( 0.6 2.6 ( 0.5
363 363 363 343 343
N2 N2 N2 N2 N2
I I I I I
1.94 ( 0.44 1.90 ( 0.28 1.83 ( 0.20 1.84 ( 0.42 2.01 ( 0.18
3-Chlorobiphenyl (IUPAC No. 2) 5.8 ( 1.2 343 N2 5.6 ( 1.1 323 N2 5.4 ( 1.1 323 N2 5.1 ( 1.0 323 N2 5.6 ( 1.1
I I I I
2.14 ( 0.10 1.96 ( 0.34 2.15 ( 0.12 2.04 ( 0.26
5.9 ( 1.2 5.0 ( 1.0 5.5 ( 1.1 5.2 ( 1.0
356 355 355 355 339 337 327 364 363
N2 N2 N2 N2 N2 N2 N2 N2 N2
C C C C C C C I I
0.47 ( 0.02 0.48 ( 0.04 0.59 ( 0.04 0.56 ( 0.06 0.50 ( 0.04 0.49 ( 0.02 0.47 ( 0.04 1.60 ( 0.14 1.74 ( 0.06
4-Chlorobiphenyl (IUPAC No. 3) 4.2 ( 1.0 363 N2 4.3 ( 1.1 363 N2 5.2 ( 1.3 363 N2 5.0 ( 1.2 343 N2 4.3 ( 1.1 343 N2 4.1 ( 1.0 323 N2 3.9 ( 1.0 323 N2 4.8 ( 1.0 323 N2 5.2 ( 1.0
I I I I I I I I
1.67 ( 0.22 1.82 ( 0.32 1.77 ( 0.28 1.77 ( 0.38 1.80 ( 0.12 1.69 ( 0.22 1.42 ( 0.14 1.75 ( 0.10
5.0 ( 1.0 5.4 ( 1.1 5.3 ( 1.1 4.9 ( 1.0 5.0 ( 1.0 4.3 ( 0.9 3.6 ( 0.7 4.5 ( 0.9
364 364 363 363 343
N2 N2 N2 N2 N2
I I I I I
1.38 ( 0.04 1.28 ( 0.04 1.35 ( 0.10 1.26 ( 0.06 1.30 ( 0.04
2,2′-Dichlorobiphenyl (IUPAC No. 4) 4.1 ( 0.8 343 N2 3.8 ( 0.8 343 N2 4.0 ( 0.8 323 N2 3.7 ( 0.7 323 N2 3.6 ( 0.7 323 N2
I I I I I
1.05 ( 0.04 1.25 ( 0.04 0.98 ( 0.02 1.25 ( 0.12 1.06 ( 0.06
2.9 ( 0.6 3.4 ( 0.7 2.5 ( 0.5 3.2 ( 0.6 2.7 ( 0.5
363 363 363 343 343
N2 N2 N2 N2 N2
I I I I I
1.15 ( 0.14 1.18 ( 0.10 1.35 ( 0.30 1.14 ( 0.18 1.25 ( 0.42
2,4-Dichlorobiphenyl (IUPAC No. 7) 3.4 ( 0.7 343 N2 3.5 ( 0.7 323 N2 4.0 ( 0.8 323 N2 3.1 ( 0.6 323 N2 3.5 ( 0.7
I I I I
1.21 ( 0.26 1.08 ( 0.40 1.37 ( 0.28 1.13 ( 0.12
3.3 ( 0.7 2.8 ( 0.6 3.5 ( 0.7 2.9 ( 0.6
363 363 362 343 343
He He He He He
I I I I I
1.00 ( 0.06 0.98 ( 0.06 0.92 ( 0.08 0.97 ( 0.06 0.77 ( 0.04
2,4,4′-Trichlorobiphenyl (IUPAC No. 28) 3.0 ( 0.6 343 He 2.9 ( 0.6 323 He 2.7 ( 0.5 323 He 2.7 ( 0.5 322 He 2.1 ( 0.4
I I I I
0.72 ( 0.04 0.65 ( 0.06 0.67 ( 0.06 0.60 ( 0.04
2.0 ( 0.4 1.7 ( 0.3 1.7 ( 0.3 1.5 ( 0.3
364 363 363 344 343
He He He He He
I I I I I
0.67 ( 0.18 0.64 ( 0.10 0.70 ( 0.10 0.71 ( 0.04 0.58 ( 0.08
2,4,5-Trichlorobiphenyl (IUPAC No. 29) 2.0 ( 0.4 343 He 1.9 ( 0.4 342 He 2.1 ( 0.4 323 He 2.0 ( 0.4 323 He 1.6 ( 0.3 323 He
I I I I I
0.63 ( 0.02 0.65 ( 0.20 0.60 ( 0.08 0.59 ( 0.06 0.65 ( 0.04
1.7 ( 0.3 1.8 ( 0.4 1.5 ( 0.3 1.5 ( 0.3 1.7 ( 0.3
363 363 362 343 343
He He He He He
I I I I I
0.63 ( 0.04 0.65 ( 0.06 0.60 ( 0.04 0.63 ( 0.04 0.53 ( 0.04
2,4′,5-Trichlorobiphenyl (IUPAC No. 31) 1.9 ( 0.4 343 He 1.9 ( 0.4 323 He 1.8 ( 0.4 323 He 1.7 ( 0.3 322 He 1.5 ( 0.3
I I I I
0.68 ( 0.10 0.54 ( 0.04 0.58 ( 0.08 0.58 ( 0.08
1.9 ( 0.4 1.4 ( 0.3 1.5 ( 0.3 1.5 ( 0.3
363 363 363 343 343
He He He He He
I I I I I
0.61 ( 0.04 0.58 ( 0.04 0.56 ( 0.04 0.48 ( 0.04 0.69 ( 0.06
2′,3,4-Trichlorobiphenyl (IUPAC No. 33) 1.8 ( 0.4 343 He 1.7 ( 0.3 324 He 1.7 ( 0.3 323 He 1.3 ( 0.3 323 He 1.9 ( 0.4
I I I I
0.53 ( 0.08 0.47 ( 0.14 0.58 ( 0.10 0.46 ( 0.04
1.5 ( 0.3 1.2 ( 0.2 1.5 ( 0.3 1.2 ( 0.2
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Table 2 (Continued)
T (K)
diluent gasa
ref compdb
k (PCB)/k (reference)c
363 363 363 343 343
He He He He He
I I I I I
0.85 ( 0.08 0.80 ( 0.04 0.75 ( 0.06 0.56 ( 0.04 0.70 ( 0.04
2,2′,3,5′-Tetrachlorobiphenyl (IUPAC No. 44) 2.5 ( 0.5 343 He 2.4 ( 0.5 324 He 2.2 ( 0.4 323 He 1.6 ( 0.3 323 He 1.9 ( 0.4
I I I I
0.80 ( 0.04 0.60 ( 0.02 0.51 ( 0.02 0.45 ( 0.02
2.2 ( 0.4 1.5 ( 0.3 1.3 ( 0.2 1.2 ( 0.2
363 363 363 363 364 363 363
He He He He N2 N2 N2
I I I I I I I
0.68 ( 0.16 0.64 ( 0.10 0.49 ( 0.08 0.60 ( 0.08 0.59 ( 0.06 0.53 ( 0.10 0.55 ( 0.02
2,2′,4,4′-Tetrachlorobiphenyl (IUPAC No. 47) 2.0 ( 0.4 362 N2 1.9 ( 0.4 343 He 1.5 ( 0.3 343 N2 1.8 ( 0.4 324 N2 1.8 ( 0.4 323 He 1.6 ( 0.3 323 N2 1.6 ( 0.3
I I I I I I
0.72 ( 0.30 0.45 ( 0.04 0.40 ( 0.12 0.48 ( 0.14 0.50 ( 0.10 0.64 ( 0.20
2.1 ( 0.4 1.2 ( 0.2 1.1 ( 0.2 1.2 ( 0.2 1.3 ( 0.3 1.6 ( 0.3
363 363 363
He He He
I I I
0.45 ( 0.02 0.37 ( 0.04 0.35 ( 0.04
2,2′,3,5′,6-Pentachlorobiphenyl (IUPAC No. 95) 1.3 ( 0.3 343 He I 1.1 ( 0.2 343 He I 1.0 ( 0.2 343 He I
0.31 ( 0.02 0.31 ( 0.02 0.30 ( 0.02
0.85 ( 0.17 0.86 ( 0.17 0.82 ( 0.16
364 363 363 363
He He He He
I I I I
0.43 ( 0.04 0.36 ( 0.12 0.43 ( 0.10 0.45 ( 0.06
2,3,3′,4′,6-Pentachlorobiphenyl (IUPAC No. 110) 1.3 ( 0.3 343 He I 1.1 ( 0.2 343 He I 1.3 ( 0.3 343 He I 1.3 ( 0.3
0.33 ( 0.16 0.40 ( 0.12 0.38 ( 0.10
0.92 ( 0.18 1.1 ( 0.2 1.0 ( 0.2
363 363 363
He He He
I I I
0.58 ( 0.06 0.76 ( 0.21 0.74 ( 0.17
2,3,4,5,6-Pentachlorobiphenyl (IUPAC No. 116) 1.7 ( 0.3 343 He I 2.2 ( 0.4 343 He I 2.2 ( 0.4 343 He I
0.61 ( 0.22 0.60 ( 0.10 0.55 ( 0.08
1.7 ( 0.3 1.7 ( 0.3 1.5 ( 0.3
rate constant (×10-12 cm3 s-12)d
T (K)
diluent gasa
ref compdb
k (PCB)/k (reference)c
rate constant (×10-12 cm3 s-12)d
a 10% oxygen in either nitrogen or helium. b C, cyclohexane; I, isobutane. c Stated uncertainties of the rate constant ratio are two standard deviations. d Stated uncertainty of the OH-PCB rate constants reflect the estimated overall uncertainty in the recommended reference rate constants: isobutane ( 20%, cyclohexane ( 25%.
TABLE 3
TABLE 4
Experimentally Determined Temperature Dependence of OH-PCB Reaction Rate Constants
Estimated Rate Constants of PCB-OH Reaction at 25 °C Compared with Available Literature Values and Atmospheric Lifetimes of PCBs due to Reaction with OH
congener no.
N
pre-exponential factora
Ea/R (K)b
0 1 2 3 4 7 15 28 29 31 33 44 47 95 110 116
18 9 9 17 10 9 14 9 10 9 9 9 13 6 7 6
7.2 7.3 9.7 28 58 15 13 270 13 14 19 290 19 250 28 110
-20 ( 150 300 ( 220 200 ( 120 630 ( 160 980 ( 200 520 ( 200 560 ( 140 1650 ( 220 670 ( 150 730 ( 170 870 ( 290 1740 ( 300 870 ( 340 1950 ( 470 1130 ( 460 1440 ( 580
a Units are 10-12 cm3 s-1. b Uncertainty in E /R is based on one a standard error of the slope of the temperature dependence regression.
are evenly distributed in the atmosphere, and we have used a different approach to calculate the atmospheric burden of PCB. Although the information on the concentration gradient of PCBs in the troposphere is ambiguous, vertical profile measurements of hydrocarbons in both continental and marine environments show that the concentration gradient generally increases with a compound’s reactivity in the atmosphere (29-31). Organic compound emissions primarily occur at the earth’s surface, and the compounds are then transported upward by eddy diffusion. This bulk
literature atmospheric congener estd k at 25 °Ca (95% confidence limits) rate constants lifetime (days)b no. 0 1 2 3 4 7 15f 28 29 31 33 44 47 95 110 116
6.7 (5.9-7.7) 2.7 (2.1-3.4) 5.0 (4.4-5.7) 3.4 (2.9-4.0) 2.2 (1.7-2.7) 2.6 (2.1-3.3) 2.0 (1.7-2.3) 1.1 (0.8-1.4) 1.3 (1.1-1.5) 1.2 (1.0-1.5) 1.0 (0.8-1.4) 0.8 (0.6-1.2) 1.0 (0.7-1.5) 0.4 (0.2-0.7) 0.6 (0.3-1.1) 0.9 (0.4-1.8)
7.2c 2.8d 5.3d 3.9d 2.0e
2 4 2 3 6 5 6 11 9 10 12 14 12 34 19 14
a Units are 10-12 cm3 s-1. b Lifetimes calculated based on 24-h global averaged OH concentration of 9.7 × 105 cm-3 (26). c Ref 12. d Ref 17. e Ref 13. f Data previously reported in ref 14.
transfer process does not differentiate on the basis of molecular weight (32); thus, all compounds in the gas phase will be transported at the same rate in the troposphere. The development of a vertical concentration gradient is, therefore, a function of the rate of transport relative to the rate of removal by OH reactions. In the case of compounds that react rapidly in the atmosphere, a significant vertical gradient will develop (33).
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TABLE 5
Model Input Parameters and Calculated PCB Removal Flow from Vertically Layered Atmospheric Model altitude (km)a
temp (K)
pressure (Torr)
0.25 0.75 1.25 1.75 2.25 2.75 3.25 3.75 4.25 4.75 5.25 5.75 6.25 6.75 7.25 7.75 8.25 8.75 9.25 9.75
286.4 283.1 279.9 276.6 273.4 270.1 266.9 263.6 260.4 257.1 253.9 250.6 247.4 244.1 240.9 237.6 234.4 231.1 227.9 224.6
737.7 694.8 653.9 614.9 577.9 542.7 509.2 477.5 447.3 418.8 391.7 366.0 341.8 318.8 297.1 276.7 257.3 239.1 222.0 205.8
a
PCB concn (ppqv)
av PCB-OH rate constantb
flow (t yr-1)
cumulative % of total
14.97 13.36 12.04 10.95 10.06 9.32 8.72 8.22 7.81 7.47 7.19 6.96 6.77 6.62 6.49 6.39 6.30 6.23 6.17 6.12
0.94 0.90 0.87 0.83 0.79 0.76 0.73 0.69 0.66 0.63 0.60 0.57 0.55 0.52 0.49 0.47 0.44 0.42 0.40 0.37
1362 1109 920 759 631 535 456 386 333 288 250 218 193 169 147 131 114 102 90 78
16.5 29.9 41.0 50.2 57.8 64.3 69.8 74.5 78.5 82.0 85.0 87.6 90.0 92.0 93.8 95.4 96.7 98.0 99.1 100.0
total flow of PCBs (t yr-1)
8271
The altitude given in the table is the midpoint of each 0.5 km thick atmospheric layer.
Using this information, we have estimated the vertical concentration gradient of PCBs in the atmosphere by selecting a surrogate organic compound with a similar OH reaction rate constant. We assumed an “average” PCB has 3.5 chlorines, based on the typical distribution of PCBs in ambient air samples. Thus, the OH-PCB rate constant would be the average of rate constants for those PCBs with 3 and 4 chlorines. At 298 K, this average rate constant is 1.1 × 10-12 cm3 s-1, which is the same as the recommended 298 K OH reaction rate constant of propane (1.15 × 10-12 cm3 s-1) (23). Based on this similarity, we expected that PCB concentrations would exhibit a vertical gradient similar to that of propane. We have used modeled winter and summer propane vertical gradients (33), which are in general agreement with vertical measurements (31), to approximate the PCB concentration gradient in the atmosphere. These gradients were generated from a seasonally varying, one-dimensional photochemical model of both the continental and marine tropospheres at 45° N. For simplicity, we averaged the winter and summer gradients and scaled the predicted mixing ratios to a PCB surface concentration of 16 ppqv (parts per quadrillion by volume). The latter was derived by assuming an average PCB molecular weight of 300 g mol-1 and an average PCB concentration at the earth’s surface of 0.2 ng m-3. We developed an expression to fit this scaled gradient
[PCB] ) 5.9 + 10 exp(-0.39A)
(6)
where the PCB concentration is in ppqv and altitude (A) is in km. This model profile extends from 0 to 10 km altitude and predicts an exponential decrease of organic compound mixing ratios to 37% (6 ppqv) of the surface value at 10 km. To calculate the loss of PCB due to OH reactions, we divided the atmosphere from 0-10 km into 20 layers, each 0.5 km in height. Using the midpoint altitude of each layer, we determined an average temperature, pressure, and PCB concentration for each layer. The average temperature was calculated from a surface temperature of 288 K and a lapse
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b
Units of the OH-PCB rate constant are 10-12 cm3 s-1.
rate of -6.5 K km-1, and the average pressure was calculated using the expression
P ) 760(288/T)-5.256
(7)
where P is the average pressure (in Torr), and T is the average temperature of the layer (34). The PCB concentration was calculated at the midpoint altitude of each layer from the model gradient described above. We assumed a constant OH concentration of 9.7 × 105 cm-3 from 0 to 10 km (26). We also calculated the average PCB-OH rate constant for each layer, averaging the rate constants for the tri- and tetrachlorinated biphenyls extrapolated individually to the layer’s average temperature. In each layer, we calculated a PCB removal flow by multiplying the respective PCBOH rate constant by the OH and PCB concentrations and by the volume of the 0.5 km high atmospheric layer. The model parameters and the resulting PCB flow rate for each layer are presented in Table 5. Based on this simple atmospheric model, 50% of the PCB loss occurs in the lower 2 km of the atmosphere and 90% in the lower 6 km; see the last column of Table 5. We summed the PCB removal flows from each layer to get an overall OH reaction-initiated PCB flow from the troposphere of 8300 t yr-1 for the entire 10 km height. By dividing this flow by the earth’s area, we obtained a flux from the atmosphere of 16 µg m-2 yr-1. Estimating cumulative worldwide PCB production to be 1.4 million t (1, 2), approximately 0.6% of the world’s supply of PCBs is eliminated by OH reactions each year. How do these flow estimates compare with other permanent PCB loss processes? In our comparison, we will focus on “permanent” sinks; that is, sinks that result in the complete removal of the PCB molecule from atmospheric cycling. Examples of permanent sinks are freshwater or marine sediment burial (below the resuspension layer) and chemical reactions of PCBs to other compound classes, such as the transformations that are initiated by OH attack. Although total flows to terrestrial, freshwater, and ocean surfaces are significant, recent studies
have indicated that freshwater bodies are likely to return the majority of their PCB burden to the atmosphere (7). There is no estimate of PCB removal due to global lacustrine sediment burial; however, a recent estimate of that deposition process in Lake Superior indicated a loss of 110 kg of PCB yr-1 (35). Multiplying this value by the ratio of the world’s total lake surface area (1.6 × 106 km2) to the area of Lake Superior (8.2 × 104 km2) (35, 36) gives a resulting PCB loss due to sediment burial of about 2 t yr-1. Like freshwater bodies, most of the PCBs that are deposited to the ocean are volatilized back into the air (37). Duce et al. estimated a global average atmospheric PCB flux to deep ocean sediments of 0.64 µg m-2 yr-1, resulting in a total PCB deposition of 240 t yr-1 to marine sediments (37). Biotransformation of PCBs, especially in sediments, has not been shown to result in complete dechlorination of the biphenyl backbone or transformation of the PCB into another compound class. Thus, we are not considering biotransformation as a permanent PCB sink. We know of no other permanent PCB sink that is significant in comparison to those described above. A global flow due to the OH-PCB reaction of 8300 t yr-1 is substantially larger than the estimates of 240 and 2 t yr-1 for marine and freshwater sediment burial, respectively. OH reactions appear to be the major permanent loss process of PCBs from the atmosphere. There are substantial uncertainties associated with our flow estimate: Experimental uncertainties in our rate constants and the Arrhenius parameters derived from them may be as high as 25-50%; extrapolation of Arrhenius parameters to regions outside the experimental temperature domain is uncertain; our atmospheric model is highly simplified. However, the sheer magnitude of the loss due to OH relative to other loss processes is startling. Estimates of global PCB inputs to deep ocean sediments are small, only 3% of our calculated OH process, and other permanent sinks are even smaller. Even if our value were overestimated by an order of magnitude, removal of PCBs by OH reactions would still be an important loss mechanism from the atmosphere, and this pathway must be considered when evaluating the fate of PCBs in the environment.
Acknowledgments The authors would like to thank Sue Grimmond and Sara Pryor (Indiana University, Department of Geography) and Phillip Stevens (Indiana University, School of Public and Environmental Affairs) for helpful discussions on concentration gradient models and global average OH concentrations.
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Received for review October 17, 1995. Revised manuscript received January 16, 1996. Accepted January 17, 1996.X ES950765K X
Abstract published in Advance ACS Abstracts, March 15, 1996.
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