Environ. Sci. Techno/. 1995, 29, 1666-1673
Introduction
RENEE L. FALCONER* AND
TERRY F . BIDLEMAN Atmospheric Environment Service, 4905 Dufferin Street, Downsview, Ontario M3H 5T4, Canada
WILLIAM E . C O T H A M Department of Chemistry and Biochemistry, University of South Carolina, Columbia, South Carolina 29280
Two experimental techniques were used to test the hypothesis that mono- and non-ortho-substituted polychlorinated biphenyls (PCBs) are sorbed to urban aerosols to a greater extent than multi-ortho congeners of the same homolog. Particulate and gaseous PCBs in Chicago air were operationally defined by sampling with a glass fiber filter followed by a polyurethane foam trap. After separation from organochlorine pesticides, PCBs were fractionated into non-ortho, mono-ortho, and multi-ortho groups on an activated carbon-silicic acid column and determined by capillary gas chromatography (GC) with electron capture detection or GC/mass spectrometry. Within a homolog group (e.!., pentachlorobiphenyls), the percentage of particulate PCBs increased in the order: multi-ortho < mono-ortho < non-ortho. This order was explained for the most part by the lower liquid-phase vapor pressures (POL) of mono- and nonortho-PCBs, although the non-ortho congeners 77 and 126 showed slightly enhanced sorption relative to expectations based on vapor pressure. To gauge the relative extent of sorption for PCBs having different numbers of ortho-chlorines,filters loaded with Chicago aerosols were exposed to gaseous PCBs a t a constant temperature. Particle/gas partition coefficients ( Kp) were inversely related to POL, but distinct differences were seen among the ortho-chlorine classes. For a given value of log POL, values of log Kp increased in the order: multi-ortho < mono-ortho < non-ortho. A multiple linear regression model using log POL and the dihedral angle between the biphenyl rings explained 98% of the variance in sorption.
Several polychlorinated biphenyl (PCB) congeners elicit toxic responses similar to those of polychlorinated dibenzop-dioxins and dibenzofurans (PCDDs and PCDFs). Most of the “dioxin-like” PCBs are the non-ortho-substituted congeners 77, 81, 126, and 169 and the mono-ortho congeners 105, 114,118, 123,156, 157, 167,and 189 (1,2). A number of these are contaminants of Great Lakes fish (2-5). When expressed as 2,3,7,8-tetrachlorodibenzo-pdioxin toxic equivalents TTEQs), the sum of non-ortho and mono-ortho-PCBs in fish and birds often outweighs total PCDDs and PCDFs (6, 7). Atmospheric deposition is a major contributor of PCBs to the Great Lakes and other large water bodies. Atmospheric processes are estimated to contribute 76-89% of total PCB loadings to Lake Superior and 58-63% to Lakes Michigan and Huron (8). PCBs and other semivolatile organic compounds are present in air as vapors and adsorbed to suspended particles, and partitioning between these phases in the atmosphere influences their removal mechanisms and lifetimes. Because their Henry’s law constants make gas scavenging unfavorable,wet deposition occurs mainly by washout of particles (9-1 1). Dry particle fluxes of PCBs in Chicago were up to 3 orders of magnitude greater than those found in nonurban areas, indicatingthat cities near the Great Lakes are likely to be major sources for the deposition of PCBs to the lakes (12). Over 75% of the flux in Chicago was due to PCBs associatedwith particles > 10 pm diameter, which have high deposition velocities (13).
The extent of associationwith particulate matter depends on the compound’s vapor pressure, the amount and type of particulate matter present, and the ambient temperature. We estimated vapor pressures for 180 PCB congeners as functions of temperature and ortho-chlorine substitution and predicted the fractions associated with particles (4) using the Junge-Pankow adsorption model (14,15):
4 = ce/(po, + ce)
(1)
In eq 1, POL is the liquid-phase saturation vapor pressure of the pure compound (Pa), e is the particle surface area concentration (cm2 of aerosol/cm3 of air), and c is a parameter that depends on the thermodynamics of the adsorption process and surface properties of the aerosol. Within each homolog group, congeners with fewer orthochlorines have lower liquid-phase vapor pressures and, consequently, higher predicted values of 6. The significance of this result is that mono- and non-ortho-PCBs will be associated with aerosols to a greater degree than multiortho-PCBs of the same homolog group, thereby increasing their chances for wet and dry deposition. The field and laboratory work reported here confirm the hypothesis of enhanced sorption of coplanar PCBs to urban air particles. * Address correspondence to this author at the Department of Chemistry, Youngstown State University, Youngstown, OH 44555.
1666 ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 29, NO. 6,1995
0013-936x/96/0929-1666$09.00/0
0
1995 American Chemical Society
Filter-Sorbent Measurements of ParticlelGas Distribution
Air volumes of 260-370 m3were pulled through the sampler
The particulate fraction can be defined by q!~ = C,(TSP)/[C,
+ Cp(TSP)I
(2)
where C, is the PCB concentration associatedwith aerosols (nglpg of particles), C, is the gas-phase PCB concentration (ng/m3),and TSP is the total suspended particle concentration @g/m3). Equation 2 is a general relationship that applies to any experimental or theoretical estimate of C, and C,. Experimental q/C,distributions are often obtained from filter-sorbent air samples using double glass fiber filters (GFFs)and a polyurethane foam (PUF)trap. Analytes on the front filter (FF) are assumed to be particulate species and gases adsorbed to the GFF while the back filter (BF) is assumed to adsorb the same amount of gases as the front (16,17). C, and C, are calculated from
C, = (FF - BF)/,ugof particles Cg = (PUF
+ 2BF)/m3of air
(3)
(4)
Combining eqs 1 and 2 yields
log Cp/Cg= log cO/TSP - log poL
(5)
Here and in other work (16,181, C,/Cg is referred to as the particle/gas partition coefficient K,. Its inverse, C,/C, = l/Kp, has also been used for these correlations (17). Experimental estimates of Kp are correlated to P O L through eq 6 to yield the regression parameters b, and m, (18):
log I$ = b, - m,log poL
(6)
Field investigationsof particle/gasdistributions are often complicated by artifacts caused by changing variables during sample collection. These include temperature, contaminant and TSP concentrations, particle properties (surface area, carbon content), and humidity (18, 19). In the absence of sampling artifacts and when c is constant for a class of compounds, the expected value for the slope m, = -1 and the intercept (br)is related to the specific surface area of the aerosol (eq 5). Factors leading to values of m, t -1 include variability in c among compounds, changes in temperature, atmospheric concentrations of contaminants, TSP during a sampling period, and filtration artifacts (18). In these cases, b, depends not only on the specific surface area of the aerosol but also on m,. An absorption model has been proposed in which organic compounds are assumed to partition into a liquid film on the aerosol (20). Factors affecting absorption are the sorbate P O L , the weight fraction of the aerosol that consists of absorbingliquid film, and the activity coefficient of the partitioning compound in the liquid film. The form of the relationship between Kp and P O L is similar to eq 6, with an expected slope of -1 and an intercept related to these aerosol properties.
Experimental Section Field Studies. Sample Collection. Air samples were taken February 17-21, 1988, and June 13-16, 1989, in south Chicago with a high volume train consistingof double GFFs (20 x 25 cm, Gelman A/E) followed by two 7.8 cm diameter x 7.5 cm thick PUF plugs (17,211. Preparation procedures for sampling media have been reported previously (22,23).
for 11-16 h at 0.5 m3/min with a Rotron DR-313 pump. Average temperatures over the 11- 16-h collection periods ranged from -10 to 4 "C in February and from 12 to 18 "C in June. After sampling, plugs and filters were kept on ice for transport to the laboratory and then frozen at -10 O C . A separate train containing a single GFF and two PUF plugs was used to determine TSP. Filters were held for -8 h over a desiccant (Drieritel before and after sample collection and then weighed. The purpose of the PUF plugs was to maintain the same flow rate as for the PCB sampler. TSP values were 30-169 pg/m3 in February and 74-147 pg/m3 in June. To check for recoveries, clean PUF plugs and filters were spiked with 10 ng of each multi-ortho congener and 2 ng of each mono- and non-ortho congener. Back PUFs were used for blanks since they were treated in exactly the same manner as front PUFs, and breakthrough was expected to be low for the Cl3-Cl7 PCBs under these sampling conditions (23-25). Three unused GFFs, taken to Chicago and frozen for the same time period as sample filters,were used to determine filter blanks. Extraction and Cleanup. PUF plugs were Soxhlet extracted overnight with petroleum ether (PE),and filters were refluxed for the same time with dichloromethane (DCM). After volume reduction and transfer to isooctane, elemental sulfur was removed by shaking the extracts with metallic mercury. Further cleanup was done by shaking extracts with 18 M sulfuric acid. PCBs were separated from most organochlorine pesticides using a column of 3 g of Mallinckrodt 100-mesh silicic acid (SA)containing 50pL of added water overlaid with 2 g of neutral alumina (6%added water). The columns were prewashed with 25 mL of DCM followed by 25 mL of PE. The sample was applied in 5 1 mL of isooctane, and PCBs were eluted with 25 mL of PE. PCBs were further fractionated into ortho-chlorine groups using columns containing AX-21 activated carbon (Anderson Development Co., Adrian, MI) mixed 1:20 with SA. The carbon was precleaned by Soxhlet extraction in cellulose thimbles with toluene for 24 h and then dried at 40 "C under vacuum. Carbon-SA (100 m@was sandwiched between 50-mg layers of SA in a disposable Pasteur pipet containing a small plug of glass wool in the lower end. The column was prewashed with 5 mL of toluene followed by 5 mL of 10% DCM-cyclohexane pushed through the column with nitrogen. The sample was applied in < 1 mL of isooctane, and PCBs were eluted in three fractions: F1 (multi-orthos): 6 mL of 10%DCM-cyclohexane; F2 (monoorthos): 3 mL of 2% toluene-DCM; F3 (non-orthos): 5 mL of toluene. PCBs divided among the fractions as follows: multi-ortho congeners in F1, mono-ortho-penta- and hexachlorobiphenyls in F2, and non-ortho-tetra-, penta-, and hexachlorobiphenyls in F3. Some congeners split between F1 and F2 and were quantified from both fractions. These were di-ortho-PCB 110 and mono-ortho congeners containing three to four chlorines (PCBs 28, 31, 74, 70, 56, and 60). Fractions were reduced into toluene by nitrogen blowdown. Analysis. PCBs were determined by capillary gas chromatography/"Ni electron capture detection (GC/ECD)or GClelectron impact mass spectrometry (GC/MS) on a30-m DB-5 column (0.25 id., 0.25 pm film thickness, J&W Scientific). GC/ECD results for multi- and mono-orthoPCBs were obtained on a Hewlett Packard 5890 with Hz carrier gas at 60 cmls, injector temperature 250 "C, and VOL. 29, NO. 6,1995 / ENVIRONMENTAL SCIENCE &TECHNOLOGY a 1867
TABLE 1
PCB Congeners for Particle/Gas Equilibration Experiments vapor pressureb GASEOUS PCBs
PARTICLE COATED
CLEAN
PUF
GFF
GFF
FIGURE 1. Experimentaldesignfor partielelgas equilibration system. Gaseous PCBs are introduced into a mixing chamber by generator columns (17)
detector temperature 300 “C. Samples (1pL)were injected in the splitless mode (split opened after 0.5 minl with the oven at 90 “C, held 1 min, ramped 10 “Clmin to 160 “C, ramped 4 “Clmin to 250 “C, and held for 10 min. Due to the low levels of the non-ortho 77, 126, and 169, samples were combined in some cases for analysis by GCI MS. These determinationswere done on adouble-focusing VG70SQ (VG Instruments) with a Hewlett Packard 5890 GC. Injector and transfer line temperatures were 250 “C. Samples (1 pL1 were injected in the splitless mode (split opened after 1 minl at 90 “C,ramped 10 “ C h i n to 200 “C, ramped 4 “Clmin to 260 “C, ramped 10 “ C h i n to 300 “C, and held for 20 min. Ions 292, 298, 326, and 360 were monitored at a resolution of 3000. Mixed individual standards (Accustandard, New Haven, CT) were used to quantify30 congeners, whereas a standard of equal mixtures of Aroclors 1242, 1254, and 1260 (Accustandard) was used for another 15 congeners. Percentages of individual congeners in the Aroclor mixture were obtained from ref 26. Internal standards for GCIECD and GCIMS were 2,4,5-tribromobiphenyl and 3,3’,4,4’-tetrachlorohiphenyl-d6 (77-d6),respectively. Laboratory Studies. Collection of Aerosols. Particles for laboratory equilibration experiments were collected in Chicago on 20 x 25 cm GFFs using multiple samplers operated at 1.4 m3 m i d (17). Two sets of particles were collected from June 12-14, 1989 (setA1, and June 14-17, 1989 (set B). Sections fromparticle-loadedandblankfilters were ground to auniformcolorandconsistencyinanagate mortar and analyzed for total carbon by dry combustion (Desert Analytics, Tucson, AZ). ParticlelGas Equilibration Experiments. The particle1 gasequilibrationsystemhasheenpreviouslydescribed ( I 7), and the experimental design is outlined in Figure 1. In the present study, the 47-mm diameter circles cut from 20 x 25 cm GFFs contained 7.0- 13.7 mg particles. The particleloaded GFF was hacked up by a clean GFF disk to correct for adsorption to the filter matrix by eq 3. Air was pulled through the system at 15-16 Llmin. PCB vapor concentrations downstream of the GFFs were monitored by 4.5 cm long x 4.5 cm diameter PUF plugs. An initial preequilibration time of 30-40 h was allowed, and then PUF plugs were changed periodically (3-7 h) and analyzed to obtain average C, for that time intewal. The experiments were carried out with IO PCB congeners (Table 1) representing different homologs and orthochlorine classes. The PCBs (Ultra Scientific, North Kingstown, RI) were coated onto quartz sand in generator columns fitted with 0.2-pm Teflon Acrodisc filters on the outlet end, as described in ref 17. Extraction and Analysis. PUF plugs were extracted for 3 h in a Soxhlet apparatus with PE. GFFs were refluxed 1668 m ENVIRONMENTAL SCIENCE & TECHNOLOGY i VOL. 29, NO. 6.1995
congener
31 37 49 77 101 105 118 126 138 171
total dihedral CI ortho-Cl angle.
3 3 4 4 5 5
5 5 6 7
1 0 2 0 2 1 1 0 2 3
57.82 41.42 77.29 40.35 79.08 58.79 58.79 42.19 77.08 86.27
POL
m
b
IPa.20”CI
-4058 -4242 -4229 -4552 -4514 -4758 -4664 -4956 -4800 -5008
12.15 12.33 12.41 12.61 12.67 12.90 12.72 13.31 12.81 13.07
2.03 x 7.24 x 9.57 x 1.21 x lo-’ 1.87 x 4.68 x 6.45 x 2.54 x 2.73 x l O V 9.71 x
*Ref 34 and ~ e r s o n a lcommunication. Loq PL= m l T f b 1141.
withDCMfor3h. ReextractingPUFandfiltersanadditional 3 h gave no detectable concentrations of the PCBs. The extracts were concentrated and transferred into isooctane usinga rotaryevaporator and nitrogen blowdown. Extracts were cleaned by shaking with 0.5 mL of 18 M sulfuric acid. Analysis was done by GCIECD under the same conditions as for field samples, with 2,4,5-tribromobiphenyl as an internal standard.
Results and Discussion QuaIity Control. PUF and filter recoveries (Table 2) ranged from 64 to 117% and from 59 to SI%, respectively. Recoveries of non-ortho-PCBs 77 and 126 were higher by GClECD than the GUMS, whereas no difference was seen for 169. This could be due to loss during nitrogen blowdown, as GCIMS analysis required more concentrated samples. GCIMS recoverieswere used for correctingresults for non-orthos. Sample quantities exceeding the limit of detection (LOD, Table 2), defined as the mean blank plus 3 standard deviations, were corrected for the mean blank, adjusted for recovery, and then divided by the volume of air to yield concentrations. A common problem encountered with carbon column fractionation is incomplete separation of PCB 77 (nonortho) from PCB 110 (di-ortho), which coelute on a DB-5 column. PCB 110 is found at much higher concentrations in PCB fluids and the environment and sometimes bleeds into the non-ortho fraction (27). With the carbon column method employed here, 51% of 110 was found in F3 for spiking experiments and in field samples. Even this small percentage of 110 in F3 may interfere with 77 in GC/ECD analysis, since the percentage of 77 is only 0.2-0.35% in Aroclors 1242 and 1248 and 0.01-0.02% in Aroclor 1254 (27,28). For this reason, congeners 77,126, and 169 were determined by GCIMS rather than GCIECD. Field Samples. In June 1989, the IPCBs for 45 congeners (listedinTable21rangedfrom 1.9 to3.9ng/m3(mean = 2.6 0.8 ng/m3, n = 61. The homolog distribution was dominated by tri- and tetrachlorobiphenyls, with lower proportions of the more chlorinated homologs. This is typical of the profiles seen in other urban and rural air investigations (12, 24, 25, 27, 28). The range of February IPCB concentrations was greater, 0.3-10 ng/m3 (mean = 2.3~2.8ng/m3,n=ll),andthissetcontainedtwosamples displaying a heavy PCB pattern similar to Aroclor 1260 (21). Holsen et al. (121 found atmospheric concentrations of
+
TABLE 2
Limits of Detection (LOD, ng) and Percent Recoveries (Mean f SD) of PCBs from Polyurethane Foam (PUF) Plugs and Glass Fiber Filtersa congener
PUF LOD
filter LOD
18 17 16,32 28,31
1.07 7.30 1.31 2.97
1.81 117h32 6 7 f 12 0.03 8 8 f 1 9 59% 1 1 0.83 102f25 6 4 f 12 -b 0.94
52,69
8.79 0.04
9 6 f 15 6 8 f 14
1
49
4.30 0.39
8 9 f 14 6 7 f 1 4
1
41,64
1.83 0.01
-
-
1
74
0.80 0.16
-
-
1 +2
56,60
0.68 0.16
7 6 f 12 6 8 f 9
1+2
90,101
2.29
97% 15 7 7 f 1 4
1
99
0.34 0.58
-
-
1
97
1.01
1.00
-
-
1
87,115
0.87 0.97
-
-
1
1.80
PUF
filter
YO rec
YO rec
77
(MS) 0.01 0.03 (ECD) -
110
0.60 0.38
149
1.31
118
0.51 0.36
69f7
153
2.73 0.99
9 8 f 1 5 80% 14
a
70f1 85f9
PUF LOD
fraction quantified congener
59f9 75f10
88f 17 6 7 f 12
1.40 101 k 1 6 81 f 14 65f6
105 141,179 138 126 (MS)
0.26 1.25 0.58 0.01 (ECD) -
1 1 1 1+2
filter LOD
PUF
filter
YO rec
YO rec
fraction quantified
72 f 3 8 6 f 16 64 f 8 70 f 7
2 1 1 3 3
0.30 7 2 f 10 0.81 0.29 102 k 18 0.02 72 f 6 73 f 8
187
0.13 0.19
-
-
1
183
0.10 0.06
-
-
1
128
0.30 0.12
174
0.41
0.08
177
0.51
0.08
97 f 16 81 f 14
1
171
0.43 0.63
9 8 f 16 82 f 14
1
156
0.25 0.13
75 f 6
180
1.02 0.79
9 6 k 17 80 f 14
1
169
0.01 0.01 (MS) (ECD) -
72 f 6 6 3 f 10 6 4 f 13 62 k 6
3 3
3 3
9 5 i 17 77 f 14 -
1
-
1
69 f 6
2
199
0.61
0.05
-
-
1
195,208
0.03 0.09
-
-
1
196,203
0.03 0.03
-
-
1
194
0.11
-
-
1
1+2 1
2 0.17
1
n = 3 for PUFs and filters. Dash ( - ) indicates that spike experiments were not done for these congeners.
TABLE 3
Concentrations of Mono= and Non-ortho-PCBs in Ambient Air 105
Chicago, June 1989 mean f SDa positive/total samples Chicago, February 1988 mean f SDa positive/total samples Lake Oreno, Minnesota Egbert, Ontario, 1988-1989 Kenora, Ontario Lake Superior, 1986 North Lake Michigan Green Bay Green Bay
118
(pUm3)
156
77
126
169
ref
2 2 f 14 414
39 f 31 414
3.4f 3.4 314
5.0 f 1.3 2/26
1.4f 0.1 2/2b
0.47 f 0.34 2/26
this work
8.1 f6.2 616 0-1.6 0.16
