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Phase distribution and artifact formation in ambient air sampling for polynuclear aromatic hydrocarbons
PHASE DISTRIBUTION AND ARTIFACT FORMATION IN AMBIENT AIR SAMPLING FOR POLYNUCLEAR AROMATIC HYDROCARBONS ROBERT W. COUTANT,LORNABROWNand JANEC. CHUANG Battelle Columbus Division, 505 King Avenue, Columbus, OH 43201-2693, U.S.A. RALPH
M.
RIGGIN
Eli Lilly and Co., Indianapolis, IN 46285, U.S.A.
and ROBERTG. LEWIS U.S. Environmen~l
Protection Agency, Research Triangle Park, NC 27711, U.S.A.
(First received 23 January 1987 and in final form 25 June 1987)
Abstract-Laboratory and field sampling experiments were conducted to determine the phase.distribution of polynuclear aromatic hydrocarbons (PAH) in the ambient atmosphere, and to determine the potential for artifact formation due to volatilization and ozone (0,) reaction during normal sampling conditions. The study was conducted in two segments to investigate both summer and winter ambient temperature effects. The winter measurements reflect stronger association of PAH with the particulate phase than the summer data, but data from both seasons show appreciable filter losses due to volatilization of phenanthrene, anthracene, Buoranthene, benz(o)antbracene and chrysene. No evidence was found for volatilization of the heavier PAH, including benzo(e)pyrene, benzo(a)pyrene, indeno( 1,2,3-c,d)pyrene, ~nzo(g,k,i)~~lene and coronene. Although O3 reacted readily with particulate matter that was freshly spiked with PAH in the laboratory experiments, no evidence was found for reaction of 0s with particulate matter during the field sampling experiments. Key word index: Ambient air, PAH, phase distribution, isotherms.
1.
INTRODUCTION
The determination of concentrations of PAH in ambient air is of considerable importance to the ch~cte~~tion of air quahty. The task of sampling PAH is complicated by the fact that many PAH have equilibrium vapor concentrations that are considerably higher than their normal ambient air concentrations. This implies a temperature and concentration dependent distribution of such PAH between particulate and vapor phases, and also suggests the possibility for artifact occurrence due to volatilization during the sampling process (Junge, 1977). For consideration of human health risk assessment, it may be important to distinguish between the vapor and ~rticie-Lund PAH. Traditional sampling methods have used only filtration to collect ambient aerosol. More recently, the use of backup traps containing polyurethane foam (PUF) or other solid sorbents to collect vapor passing through the filter has become more widespread (e.g. Yamasaki et al., 1982; Van Vaeck et al., 1984). While this approach may permit total collection of PAH, it does not take into account the possibility of artifact formation as a result of either vaporization or condensation during the
sampling process. Furthermore, there is the possibility that the integrity of the collected sample may be altered by the reaction of PAH with reactive species, such as OS, during sampling. Other researchers have used vapor denuder systems to examine the questions of carbonaceous particle integrity (Appel et al., 1983) and s~plingofchlorj~ted organics (Johnson et al., 1985). This paper describes the use of a research level air sampler capable of operating at a flow rate of 15 l min-’ to investigate the phase distributions of PAH in the ambient atmosphere and to determine the extent of O3 reaction with ambient particulate matter under field sampling conditions. The field work was conducted outdoors in Columbus, Ohio, during two seasons to encompass typical sampling conditions of both summer and winter months.
2. EXPERIMENTS
2.1. Technical approach Preliminary laboratory experiments were conducted to evaluate the potential for both volatilization of PAH from tiher samples and reaction of PAH filter samples with 0s. In these experiments, National Bureau of Standards SRM-1649 urban particulate matter was loaded with selected PAH by a
404
ROBERT W. CWTANT
vapor spiking process. The particulate matter was dispersed in a nitrogen stream and was collected on quartz fiber filters. Sections of these filters were then exposed to various challenge conditions which included tests for vaporization and exposure to 0s as a function of time. For the field sampling experiments, two samplers were operated in parallel. The first sampler was a sampling train developed for semivolatile compounds (Lewis and Jackson, 1982). The commercial equivalent of this sampler is the General Metal Works PS-1 sampler. The train consists of a quartz-fiber filter and ~lyurethane foam (PUF) cartridge for series collection of particle-bound and vapor phase PAH. The second sampler was similar, but incorporated a multiple tube denuder (described below) to remove vapor phase PAH ahead of the filter. Filter and PUF samples from each of these samplers were analyzed by gas chromatography/mass spectrometry (CC/MS). Basically, the same sampler configuration was used for the field tests for 0s reactivity. However, in these experiments, the input air to the non-denuder sampler was spiked with sufficient 0s to increase the background level (ca 10-20 ppbv) to 180 ppbv. Sampling periods were nominally 24 h, with analyses being conducted for phenanthrene, anthra~ne, pyrene, guoranthene, benz(a)anthracene, chrysene, benzo(n)pyrene, benzo(e)pyrene, cyclopenta(c,d)pyrene, benzo(g,h,i)perylene, indeno( 1.2,3-c,d)pyrene and coronene. 2.2. Denuder assembly The denuder consisted of a parallel bank of seven stainless steel tubes, 61 cm long with the internal diameter of each tube being 1.5 cm. The assembly was interfaced with the sampling train using a 1S-cm transition section (see Fig. 1). The inner surface of each tube was cleaned and recoated with 5 g of either Dow Corning high vacuum silicone grease (for PAH vapor removal) or Sanford rubber cement (for 0s removal) before each sampling run. Initial tests of the denuder system were made using naphthalene as the challenge PAH. Air laden with naphthalene vapor was pumped through a single denuder tube at a rate of 2 / min- ‘, while the input and output concentrations were monitored by gas chromatography. Results of these tests showed an initial removal efficiency of 897; (theoretical efficiency = 92 %), and a capacity to remove several tens of pg without significant decline in removal efficiency. Tests of the OI removal efficiency were conducted using a challenge 03 concentration of 175 ppbv and a chemiluminescence monitor
er ui.
to determine input and output 0s concentrations. Results indicated that the initial removal efficiency of 9 1 y0declined to 85 ‘Y’after 24 h of continuous operation. 2.3.
Sample collection
and analysis
Filters were 104 mm QAST (quartz fiber) filters obtained from Pallffex. These were baked at 350°C for 6 h and were stored individ~lly between similarly treated watchglasses that were sealed with Teflon tape. PUF cartridges were cleaned by compression rinsing using 50 cycles in 8OOml toluene followed by 50 cycles in 10% diethyl ether in a-hexane and 50 cycles in acetone. The PUF plugs were then extracted in a Soxhlet-extractor with acetone ibr i 6 hand were dried under vacuum at room temperature in the dark. After drvina. each PUF plug was individually wrapped in hexane-washed aluminum oil&d was stored in a glass jar closed with a Teflon-lined cap. Typically, PUF plugs were used within 24 h after cleanup. All PUF and filter samples were extracted immediately after removal from the samplers. The filters were Soxhletextracted using 400 ml dichloromethane. The PUF cartridges were Soxhlet extracted with 800 ml of 10% ether in hexane for at least 16 h, or until the solvent in the top of the extractor was clear. After extraction, the solutions were reduced to 1 ml using a Kuderna-Danish apparatus. The extracts were then transferred to Chromflex tubes and were further concentrated to 100 ~1for filter samples and 200 ~1for PUF samples. Prior to analysis, the internal standard, 9-phenylanthracene, was added to yield a concentration of 1 pg ml-‘. A Finnigan 4500 GC/MS operated in an electron impact ionization-mode was used for analysis. A bonded-phase fused-silica canillarv column (Ultra No. 2, cross-linked 5 7’ phenylmethyl~ili~~e) was used in both the GC oven and the ionization source. Data acquisition and processing were performed using a Finni~n/~NCOS Model 2100 data system. Standard solutions containing the target compounds and the internal standard were prepared at four concentration levels (0.1, 0.5. 2.0 and 5.0 fig ml-‘) for generation of calibration curves, The MS was used to monitor the molecular ions of the target compounds. Identification of target compounds was based on detection of the molecular ion coupled with comparison of retention time relative to that of the internal standard. Detection limits were of the order of 0.05-0.1 ng mm3 depending on the particular compound and sample type
holder
Fig. 1. Sampler assembly.
405
Phase distribution and artifact formation in ambient air sampling 2.4. Data reduction The apparent vapor and artifact levels for each compound were calculated using the following equations: F,,=P-A” F,= PUF,
(1)
P-A,-A,
(2)
= V+ A,
(3)
PUFd=O.llY+A,+A,.
(4)
Here, F, PUF, P, Vand A refer to the amounts of each PAH associated with the filter catch, the PUF sample, the atmospheric particulate matter, the atmospheric vapor and the artifact, respectively. The subscripts d and nd denote denuder and non-denuder samplers, and subscripts n and e distinguish between the normal and excess artifact (that is caused by the use of the denuder). The vapor term (0.11 V) in the fourth equation is used to correct for the fact that the denuder efficiency was 89 %. (Based on calculated diffusion coefficients for the more volatile PAH, we estimate that the assumption of constant denuder efficiency introduces an error of no more than 3-5 % in the calculated vapor and artifact levels.) With several of the compounds that were found only at very low levels, these calculations are critically influenced by un-
certainties in the analytical results. Such cases are appropriately noted in the ‘Results’ section, and the total of vapor plus artifact data that were obtained from the nondenuder results alone were probably the more meaningful dam for those cases. 3. RESULTS AND DISCUSSION
3.1. Laboratory
experiments
Results of the laboratory experiments on spiked samples of SRM 1649 showed extensive volatilization (3&90x) of the more volatile PAH, including phenanthrene, anthracene and 1-methylpyrene at particle loadings of 10-20 pg g-’ when exposed to normal high-volume sampling flows (ca 2 e cm-’ min-’ for 24 h). On the other hand, BaP concentrations on the exposed particulate matter declined by no more than 10%. Results of the laboratory OJ exposures are illustrated by Fig. 2. In this experiment, all three of the PAH concentrations on the particulate matter declined rapidly over the first 8 h, but further exposure to 0s did not significantly reduce their concentrations. This implies ready reaction of O3 with PAH that is easily accessible at the surfaces of the particles but
physical protection particles.
of PAH
trapped
within
the
3.2. Field experiments 3.2.1. Ozone reactioity. Results of three field experiments to evaluate O3 reactivity with BaP during the sampling process are shown in Table 1. Although the data suggest the possibility of a small extent of reaction occurring in the non-denuder system, the agreement between pairs of BaP analyses is within analytical error, and no reaction is detected. This is in agreement with the findings of Grosjean et al. (1983). 3.2.2. PAH volatilization. In general, the lower molecular weight PAH including phenanthrene, anbenz(a)anthracene and fluoranthene, thracene, chrysene all displayed some tendency for volatilization and artifact formation during both the winter and summer sampling periods. However, no evidence was found for volatilization of the heavier PAH (5-ring and greater) during either sampling period. Tables 2 and 3 illustrate the measured and derived data obtained for one of the more volatile compounds, phenanthrene. Detailed data tabulations for the other PAH are available on request from the authors. Table 4 summarizes the results obtained for all PAH that were measured on a consistent basis for both the summer and winter runs. A cursory examination of the percentage vapor and artifact levels in Table 4 shows what appears to be considerable scatter in the fraction of any given PAH in the vapor phase. The overall vapor plus artifact values are, however, similar to numbers recently reported (Van Vaeck et al., 1984), and, as discussed further below, the observed variability can be related largely to temperature effects.
3.3. Artifact levels The variability of the apparent artifact levels is largely due to variations in temperature and the timing of these variations with respect to the sampling cycle. For example, samples collected during the winter tend to display lower artifact levels because the temperatures for those experiments were more nearly constant. Also, at the lower winter temperatures, changes in
Table 1. Ozone reactivity with ambient BaP*
8
a
Run No.
0.0-
la lb 2a 2b 3a 3b
0.6 -
0
I 4
I 6
I 12
I 16
I 20
Sampler
Sample volume (m?
BaP cont. (ng m-‘)
Denudert No denuder Denuder No denuder Denuder No denuder
20.7 20.6 22.2 21.6 18.9 19.6
0.39 0.38 0.81 0.77 0.86 0.83
24
Time (h)
Fig. 2. Effect of OJ on enriched PAH concentrations.
* 03 input to denuder sampler = 10-20 ppbv; OJ input to nondenuder sampler = 180 ppbv. t Denuder coating = rubber cement.
406
ROBERT W. COU.TANTet al. Table 2. Summary
of phenanthrene
phase distribution
Filter Run No. 1 2 3 4 5 6 I 8 9 10 I1 12 13
data measured
Q”C)
NDt
Dt
ND+
Dt
Total:
23.8 21.0 25.0 24.5 26.1 23.0 29.0 - 5.5 12.8 -1.0 3.0 - 2.0 4.8
69.5 13.5 66.0 59.5 138.0 87.0 91.5 94.5 118.0 114.0 74.5 81.5 106.0
4.20 0.75 0.72 1.30 1.40 0.89 0.60 4.50 2.30 10.1 5.70 4.70 6.70
3.10 0.75 0.56 0.89 1.00 2.10 0.65 2.70 3.20 6.10 2.00 3.60 2.80
50.7 80.6 146.0 41.2 138.0 68.2 83.2 18.1 95.8 73.4 84.8 59.3 94.9
39.6 250.0 31.9 0 84.2 46.0 48.4 9.9 63.8 24.5 32.8 30.6 42.2
54.9 81.4 147.0 48.5 139.0 69.1 83.8 22.6 98.1 83.5 90.5 64.0 102.0
* Total particulate loading. tphenanthrene concentrations (ng m ‘) associated with denuder $ Total phenanthrene concentration (ng m 3). 9:Vapor and volatilization artifact concentrations (ng m “). IIValues in error because of contamination of PUF sample.
Run No.
2 3 4 5 6 I 8 9 10 11 12 13
Capacity kvW* 60.4 10.2 10.9 21.9 10.1 10.2 6.56 47.6 19.6 88.6 76.5 57.7 63.2
air
PUF
Mass* (flgm-“)
Table 3. Phase distribution
in ambient
Art.?
V+A:
25 (-231) 87 109 43 34 46 49 35 70 68 52 62
68 (330) 13 - I2 56 65 53 31 62 18 25 41 32
92 99 100 97 99 99 99 80 98 88 94 93 93
* Particulate loading (W) of phenanthrene. t Vapor and artifact levels expressed as a percentage content of the sample. $ Sum of the vapor and artifact percentages. $Equilibrium vapor pressure of pure phenanthrene temperature. 11 Apparentisosteric heat of adsorption [see Equation
vapor pressure induced by temperature fluctuations are smaller. The timing of the sampling cycle also is important in determining the artifact level. For example, consider the hypothetical case where the total PAH content of the inlet air is constant, but the temperature rises continuously throughout the sampling experiment. In this case, particle-bound PAH collected at any instant during the sampling process would experience volatilization throughout the remainder of the run, thus increasing the artifact. For the opposite situation where temperatures continuously decline during the
13.6
(- 188.0) 127.0 52.9 60.2 23.3 38.6 11.1 34.6 58.8 61.9 33.1 62.9
(D) and non-denuder
data derived from ambient ments
W.t
Vapor 9:
air phenanthrene
Peg: (mg m-‘)
37.1 (268.O)ll 19.1 - 5.7 77.8 44.9 44.6 7.0 61.2 14.6 22.9 26.2 32.0
(ND) samplers.
measure-
4st” (kcal mole-‘) 6.81
1.40 1.01 1.60 1.51 1.94 1.21 2.51 0.033 0.38 0.062 0.11 0.054 0.14
of the total
Artifacta
s”ps 6.07 6.18 6.42 6.65 4.26 5.28 3.77 4.09 3.99 4.24
phenanthrene
at the average
sampling
(9)].
period, the volatilization artifact would be minimized. Obviously, real 24-h sampling cycles encompass a blend of these scenarios where volatilization may be appreciable during some periods and essentially nil at other times. Continuing this reasoning, the most severe artifact formation scenario would be associated with rising temperatures near the end of the sampling period. This suggests that a 24-h sampling cycle for PAH should normally be started and finished during the early morning hours so that only samples collected during the first 8-10 h would be subjected to rising temperatures within the sampler. sampling
Phase distribution and artifact for~tion
in ambient air sampling
Table 4. Summary of PAH vapor phase and artifact levels* Ranges Chemical Phenanthrene Anthracene Fluoranthene Pyrene ~~(=~nthra~ne Chrvsene Be&o(e)pyrene Benzo(a)pyrene Indeno( 1,2,3-c,d)pyrene
Benzo(g,h,i)perylene Coronene
Median
Vapor
Artifact
Vapor
Artifact
25-W $71 27-64 5-80 19-67 5-65 NAt NA NA NA NA
13-68 14-92 7-62 16-83 8-45 17-50 NA NA NA NA NA
39 26 43 43 26 28 NA NA NA NA NA
60 71 47 47 13 38 NA NA NA NA NA
*Ranges and median values expressed as percentages of the total amounts of each compound found. t NA = not applicable, no evidence of vapor or artifact found.
A complete description of the phase distribution of PAH requires the use of an adsorption isotherm that specifically relates theamount of the adsorbed phase to the vapor phase concentration. A variety of general isotherm forms have been developed for different applications. Some of these are based on sound, fundamental considerations, while others are purely empirical. The simplest theoretical relationship is the Langmuir isotherm, by which the fractional surface coverage is given as:
where P is the sorbate pressure and b is a constant. A number of authors have extended the utility of the Langmuir isotherm by expressing b as function of temperature, viz. Inb=a+E/RT
(6)
where E is taken as an adsorption energy. A more complex isotherm that specifically relates to both sorbate and sorbent properties was developed by Dubinin based on the Polanyi adsorption potential theory. This isotherm, referred to as either the Dubinin-Polanyi isotherm or the Dubinin-Radushkevich (DR) isotherm, is given by: W/W0 = exp-
(&‘f12)[RTln (P/Po)12
(7)
where W is the capacity for the sorbate at partial pressure P, Wo is the saturation capacity at the equilibrium pressure PO; B is a structural constant for the sorbent; and fi is the affinity coefficient for the sorbate. B is usually determined for a given sorbent by measuring the isotherm using a reference sorbate, and values of #I for different sorbates are found to be well correlated by their molar refractivities. Because of this ability to represent the sorption behavior of many sorbates in terms of a single set of reference parameters, the DR isotherm has also proven useful in
describing the simultaneous adsorption of multiple components (Jonas et al., 1979) (for an excellent
review, see Werner and Winters, 1986). Each of these isotherms has been used by various authors in describing adsorption of VOC by carbonaceous sorbents. All three yield similarly shaped curves, and, unless detailed measurements of the isotherm are made, choice between them is often arbitrary. The principal attraction of the DR isotherm is that it offers the possibility forextra~lation to other compounds once the m~surements have been made for one or two reference sorbates on a given sorbent. The Langmuir isotherm has been applied to volatilization of PAH (Junge, 1977; Yamasaki et al., 1982; Bidleman et al., 1986). The DR isotherm has been used extensively by Jonas and co-workers (op. cit.) for prediction of sorbent bed behavior. As noted above, the DR isotherm can represent adsorption of multiple sorbates in terms of a single set of sorbent parameters and the known molar refractivities of the sorbates. It is useful to note at this point that the bracketed term on the right-hand side of Equation (7) has units of (energy/mole)‘. The square root of this term is identical with the isosteric heat of adsorption, qs, (Pace, 1967). The total enthalpy of vaporization from the adsorbed state is given by: AH, = AH,+q,,
(8)
where AH, is the enthalpy of vaporization of the pure compound, and qst = RT In P,JPo.
(9)
Thus, the excess enthalpy of adsorption (or isosteric heat of adsorption) can be estimated from the ratio of the observed vapor concentrations to the equilibrium vapor concentrations of the pure PAH, and this in turn is directly related through the DR isotherm to the amount of PAH adsorbed on the particulate matter. It is important to note that this approach does not constrain the adsorption energy to be independent of
408
ROBERTW. COUTANTet al.
surface coverage and temperature, as is implied by the use of the modified Langmuir equation. Rather, the greater the amount of adsorbed PAH, the smaller qst is expected to be. Equation (7) is strictly intended for adsorption of single sorbates, but Grant and Manes (1966) have shown that it is equally applicable to mixtures of sorbates. provided that correction is made for the relative composition of the adsorbed phase. For the current samples, the relative compositions of the samples were quite constant, as can be seen in Table 5. Therefore, the applicability of the DR to the current data can be tested without correcting for compositional changes on a sample by sample basis. Figure 3 shows such a test for the phenanthrene data given in Tables 2 and 3. Here we have plotted In W (W expressed in ppmw) vs the square of the apparent isosteric heat. It can be seen that with the exception of one outlying point (run No. 1). the DR isotherm does indeed offer a good correlation of the phenanthrene data. DR plots for the other volatile PAH considered in this work also show good correlation of the data, with slightly more scatter for the although benz(a)anthracene data. 3.5. Estimation
of ambient phase
distribution
As noted above, the relationships derived from the current work may be unique to the particular distriTable 5. Ratios of volatile PAH to phenanthrene Average ratio to PAH
RSD
(%)
phenanthrene
Anthracene Fluoranthene Pyrene Benz(a)anthracene Chrysene
0.14 1.0 0.46 0.25 0.32
15 15 17 36 37
9.0
r’=0.902
~=544030+00t2.ll4lE-OI b =-7.92720-02’7 1517E-03
80
70
(RTtn P/~12(kcal/mole)Z Fig. 3. Dubinin-Radushkevich isotherm for phenanthrene on ambient air particles.
butions of PAH found in these samples, and these relationships should be tested at sites having other overall compositions. The real utility of the DR approach described here lies in its potential for estimating PAH phase distributions and the extent of volatilization artifact formation based only on the data normally obtained with the PS-1 air sampler or similar systems. The DR isotherm specifies the relationship between the amount adsorbed and the vapor phase concentration. Therefore, if the total concentration of PAH is measured, Equation (7) can be solved for the relative amounts in the vapor and adsorbed phases. The volatilization artifact can then be estimated from the discrepancy between the calculated and measured concentrations of PAH on the particulate matter.
4. CONCLUSIONS
In any 24-h ambient air sampling study the temperature and the physical and chemical composition of the air being sampled can change appreciably during the course of the experiment. Furthermore, the phase distribution and even the chemical composition of the collected sample may change during the collection process as a consequence of the external changes. For example, an increase in temperature during the course of the sampling period would be expected to result in volatilization of species adsorbed on the already collected particulate matter. If the collected particulate mass forms a significant fraction of the final sample. the increase in temperature could have an appreciable effect on the apparent overall composition of the particulate matter. The collected sample, therefore, has properties that are uniquely averaged over the specific changes in variables that characterize each sampling period. In the absence of detailed time-dependent information on the temperature and ‘new sample’ composition, defensible relationships between the nature of the collected sample and that of the same material as it existed in the atmosphere cannot be developed. The best that can be done is to utilize fundamentally sound relationships to develop global guidelines. The current work indicates that 3- and 4-ring PAH exist to a large extent in the vapor phase in ambient air, and that appreciable artifact formation can occur during sampling as a result of volatilization of these compounds from filters. This finding is consistent with the conclusions of other authors, but the current work provides a means for distinguishing between artifact and vapor. The results are internally consistent when viewed from the perspective of thermodynamic equilibrium, and a means is provided for estimating vapor concentrations under other sampling conditions. The study also strongly suggests that oxidative losses of BaP associated with ambient airborne particulate matter due to reaction with O3 after collection are negligible.
Phase distribution and artifact fckrmation in ambient air sampling Disclaimer-Although the work described in this article was funded wholly by the U.S. Environmental Protection Agency through Contract No. 68-02-4127, it has not been subjected to Agency review. Therefore, it does not necessarily reflect the views of the Agency, and no official endorsement should be inferred. Mention of trade names or commercial products does not constitute endorsement or recommendation. REFERENCES
Appel B. R., Tokiwa Y. and Kothny E. L. (1983) Sampling of carbonaceous particles in the atmosphere. Atmospheric Environment 17, 1787-1796. Bidleman T. F., Billings W. N. and Foreman W. F. (1986) Vapor-particle partitioning of semivolatile organic compounds: estimates from field collection. Enuir. Sci. Technol. 20, 1038-1043. Grant R. J. and Manes M. (1966) Adsorption behavior of binary hydrocarbon gas mixtures on activated carbon. l&EC Fund. 5,490-498.
Grosjean D., Fung K. and Harrison J. (1983) Interactions of polycyclic aromatic hydrocarbons with atmospheric pollutants. Envir. Sci. Tech&. 17, 673479. Johnson M. D., Barton S. C. and Barton G. H. S. (1985) Development of a gas/particle fractionating sampler for chlorinated organics. Paper presented at 78th Annual
409
Meeting of the Air Pollution Control Association, Detroit, Michigan, June, 1985. Junge C. E. (1977) Fate of pollutants in the air and water environments-I. In Advances in Environmental Science and Technology (edited by Suffet I. H.), Vol. 8. WileyInterscience, New York. Lewis R. G. and Jackson M. D. (1982) Modification and evaluation of a high volume air sampler for pesticides and semi-volatile organic chemicals. Analyt. Chem. 54,592-594. Pace E. L. (1967) Adsorption thermodynamics and experimental measurement. In The Solid-Gas interface (edited by Flood E. A.), Vol. 1, Chapter 4. Marcel Dekker, New York. Sansone E. B., Tewari Y. B. and Jonas L. A. (1979) Predication of removal of vapors from air by adsorption on activated carbon. Enuir. Sci. Technol. 13, 1511-1513. Van Vaeck L., Van Cauwenberghe K. and Janssens J. (1984) The gas-particle distribution of organic aerosol constituents: measurement of the volatilisation artifact in HiVol cascade impactor sampling. Atmospheric Environment l&417430. Werner M. D. and Winters N. L. (1986) Predicting gaseous phase adsorption of organic vapors by microporous adsorbents. CRC Critical Rev. enuir. Control 16, 327-356. Yamasaki H., Kuwata K. and Miyamoto H. (1982) Effects of ambient temperature on aspects of airborne polycyclic aromatic hydrocarbons. Enuir. Sci. Technol. 16, 189.
M.
RIGGIN
Eli Lilly and Co., Indianapolis, IN 46285, U.S.A.
and ROBERTG. LEWIS U.S. Environmen~l
Protection Agency, Research Triangle Park, NC 27711, U.S.A.
(First received 23 January 1987 and in final form 25 June 1987)
Abstract-Laboratory and field sampling experiments were conducted to determine the phase.distribution of polynuclear aromatic hydrocarbons (PAH) in the ambient atmosphere, and to determine the potential for artifact formation due to volatilization and ozone (0,) reaction during normal sampling conditions. The study was conducted in two segments to investigate both summer and winter ambient temperature effects. The winter measurements reflect stronger association of PAH with the particulate phase than the summer data, but data from both seasons show appreciable filter losses due to volatilization of phenanthrene, anthracene, Buoranthene, benz(o)antbracene and chrysene. No evidence was found for volatilization of the heavier PAH, including benzo(e)pyrene, benzo(a)pyrene, indeno( 1,2,3-c,d)pyrene, ~nzo(g,k,i)~~lene and coronene. Although O3 reacted readily with particulate matter that was freshly spiked with PAH in the laboratory experiments, no evidence was found for reaction of 0s with particulate matter during the field sampling experiments. Key word index: Ambient air, PAH, phase distribution, isotherms.
1.
INTRODUCTION
The determination of concentrations of PAH in ambient air is of considerable importance to the ch~cte~~tion of air quahty. The task of sampling PAH is complicated by the fact that many PAH have equilibrium vapor concentrations that are considerably higher than their normal ambient air concentrations. This implies a temperature and concentration dependent distribution of such PAH between particulate and vapor phases, and also suggests the possibility for artifact occurrence due to volatilization during the sampling process (Junge, 1977). For consideration of human health risk assessment, it may be important to distinguish between the vapor and ~rticie-Lund PAH. Traditional sampling methods have used only filtration to collect ambient aerosol. More recently, the use of backup traps containing polyurethane foam (PUF) or other solid sorbents to collect vapor passing through the filter has become more widespread (e.g. Yamasaki et al., 1982; Van Vaeck et al., 1984). While this approach may permit total collection of PAH, it does not take into account the possibility of artifact formation as a result of either vaporization or condensation during the
sampling process. Furthermore, there is the possibility that the integrity of the collected sample may be altered by the reaction of PAH with reactive species, such as OS, during sampling. Other researchers have used vapor denuder systems to examine the questions of carbonaceous particle integrity (Appel et al., 1983) and s~plingofchlorj~ted organics (Johnson et al., 1985). This paper describes the use of a research level air sampler capable of operating at a flow rate of 15 l min-’ to investigate the phase distributions of PAH in the ambient atmosphere and to determine the extent of O3 reaction with ambient particulate matter under field sampling conditions. The field work was conducted outdoors in Columbus, Ohio, during two seasons to encompass typical sampling conditions of both summer and winter months.
2. EXPERIMENTS
2.1. Technical approach Preliminary laboratory experiments were conducted to evaluate the potential for both volatilization of PAH from tiher samples and reaction of PAH filter samples with 0s. In these experiments, National Bureau of Standards SRM-1649 urban particulate matter was loaded with selected PAH by a
404
ROBERT W. CWTANT
vapor spiking process. The particulate matter was dispersed in a nitrogen stream and was collected on quartz fiber filters. Sections of these filters were then exposed to various challenge conditions which included tests for vaporization and exposure to 0s as a function of time. For the field sampling experiments, two samplers were operated in parallel. The first sampler was a sampling train developed for semivolatile compounds (Lewis and Jackson, 1982). The commercial equivalent of this sampler is the General Metal Works PS-1 sampler. The train consists of a quartz-fiber filter and ~lyurethane foam (PUF) cartridge for series collection of particle-bound and vapor phase PAH. The second sampler was similar, but incorporated a multiple tube denuder (described below) to remove vapor phase PAH ahead of the filter. Filter and PUF samples from each of these samplers were analyzed by gas chromatography/mass spectrometry (CC/MS). Basically, the same sampler configuration was used for the field tests for 0s reactivity. However, in these experiments, the input air to the non-denuder sampler was spiked with sufficient 0s to increase the background level (ca 10-20 ppbv) to 180 ppbv. Sampling periods were nominally 24 h, with analyses being conducted for phenanthrene, anthra~ne, pyrene, guoranthene, benz(a)anthracene, chrysene, benzo(n)pyrene, benzo(e)pyrene, cyclopenta(c,d)pyrene, benzo(g,h,i)perylene, indeno( 1.2,3-c,d)pyrene and coronene. 2.2. Denuder assembly The denuder consisted of a parallel bank of seven stainless steel tubes, 61 cm long with the internal diameter of each tube being 1.5 cm. The assembly was interfaced with the sampling train using a 1S-cm transition section (see Fig. 1). The inner surface of each tube was cleaned and recoated with 5 g of either Dow Corning high vacuum silicone grease (for PAH vapor removal) or Sanford rubber cement (for 0s removal) before each sampling run. Initial tests of the denuder system were made using naphthalene as the challenge PAH. Air laden with naphthalene vapor was pumped through a single denuder tube at a rate of 2 / min- ‘, while the input and output concentrations were monitored by gas chromatography. Results of these tests showed an initial removal efficiency of 897; (theoretical efficiency = 92 %), and a capacity to remove several tens of pg without significant decline in removal efficiency. Tests of the OI removal efficiency were conducted using a challenge 03 concentration of 175 ppbv and a chemiluminescence monitor
er ui.
to determine input and output 0s concentrations. Results indicated that the initial removal efficiency of 9 1 y0declined to 85 ‘Y’after 24 h of continuous operation. 2.3.
Sample collection
and analysis
Filters were 104 mm QAST (quartz fiber) filters obtained from Pallffex. These were baked at 350°C for 6 h and were stored individ~lly between similarly treated watchglasses that were sealed with Teflon tape. PUF cartridges were cleaned by compression rinsing using 50 cycles in 8OOml toluene followed by 50 cycles in 10% diethyl ether in a-hexane and 50 cycles in acetone. The PUF plugs were then extracted in a Soxhlet-extractor with acetone ibr i 6 hand were dried under vacuum at room temperature in the dark. After drvina. each PUF plug was individually wrapped in hexane-washed aluminum oil&d was stored in a glass jar closed with a Teflon-lined cap. Typically, PUF plugs were used within 24 h after cleanup. All PUF and filter samples were extracted immediately after removal from the samplers. The filters were Soxhletextracted using 400 ml dichloromethane. The PUF cartridges were Soxhlet extracted with 800 ml of 10% ether in hexane for at least 16 h, or until the solvent in the top of the extractor was clear. After extraction, the solutions were reduced to 1 ml using a Kuderna-Danish apparatus. The extracts were then transferred to Chromflex tubes and were further concentrated to 100 ~1for filter samples and 200 ~1for PUF samples. Prior to analysis, the internal standard, 9-phenylanthracene, was added to yield a concentration of 1 pg ml-‘. A Finnigan 4500 GC/MS operated in an electron impact ionization-mode was used for analysis. A bonded-phase fused-silica canillarv column (Ultra No. 2, cross-linked 5 7’ phenylmethyl~ili~~e) was used in both the GC oven and the ionization source. Data acquisition and processing were performed using a Finni~n/~NCOS Model 2100 data system. Standard solutions containing the target compounds and the internal standard were prepared at four concentration levels (0.1, 0.5. 2.0 and 5.0 fig ml-‘) for generation of calibration curves, The MS was used to monitor the molecular ions of the target compounds. Identification of target compounds was based on detection of the molecular ion coupled with comparison of retention time relative to that of the internal standard. Detection limits were of the order of 0.05-0.1 ng mm3 depending on the particular compound and sample type
holder
Fig. 1. Sampler assembly.
405
Phase distribution and artifact formation in ambient air sampling 2.4. Data reduction The apparent vapor and artifact levels for each compound were calculated using the following equations: F,,=P-A” F,= PUF,
(1)
P-A,-A,
(2)
= V+ A,
(3)
PUFd=O.llY+A,+A,.
(4)
Here, F, PUF, P, Vand A refer to the amounts of each PAH associated with the filter catch, the PUF sample, the atmospheric particulate matter, the atmospheric vapor and the artifact, respectively. The subscripts d and nd denote denuder and non-denuder samplers, and subscripts n and e distinguish between the normal and excess artifact (that is caused by the use of the denuder). The vapor term (0.11 V) in the fourth equation is used to correct for the fact that the denuder efficiency was 89 %. (Based on calculated diffusion coefficients for the more volatile PAH, we estimate that the assumption of constant denuder efficiency introduces an error of no more than 3-5 % in the calculated vapor and artifact levels.) With several of the compounds that were found only at very low levels, these calculations are critically influenced by un-
certainties in the analytical results. Such cases are appropriately noted in the ‘Results’ section, and the total of vapor plus artifact data that were obtained from the nondenuder results alone were probably the more meaningful dam for those cases. 3. RESULTS AND DISCUSSION
3.1. Laboratory
experiments
Results of the laboratory experiments on spiked samples of SRM 1649 showed extensive volatilization (3&90x) of the more volatile PAH, including phenanthrene, anthracene and 1-methylpyrene at particle loadings of 10-20 pg g-’ when exposed to normal high-volume sampling flows (ca 2 e cm-’ min-’ for 24 h). On the other hand, BaP concentrations on the exposed particulate matter declined by no more than 10%. Results of the laboratory OJ exposures are illustrated by Fig. 2. In this experiment, all three of the PAH concentrations on the particulate matter declined rapidly over the first 8 h, but further exposure to 0s did not significantly reduce their concentrations. This implies ready reaction of O3 with PAH that is easily accessible at the surfaces of the particles but
physical protection particles.
of PAH
trapped
within
the
3.2. Field experiments 3.2.1. Ozone reactioity. Results of three field experiments to evaluate O3 reactivity with BaP during the sampling process are shown in Table 1. Although the data suggest the possibility of a small extent of reaction occurring in the non-denuder system, the agreement between pairs of BaP analyses is within analytical error, and no reaction is detected. This is in agreement with the findings of Grosjean et al. (1983). 3.2.2. PAH volatilization. In general, the lower molecular weight PAH including phenanthrene, anbenz(a)anthracene and fluoranthene, thracene, chrysene all displayed some tendency for volatilization and artifact formation during both the winter and summer sampling periods. However, no evidence was found for volatilization of the heavier PAH (5-ring and greater) during either sampling period. Tables 2 and 3 illustrate the measured and derived data obtained for one of the more volatile compounds, phenanthrene. Detailed data tabulations for the other PAH are available on request from the authors. Table 4 summarizes the results obtained for all PAH that were measured on a consistent basis for both the summer and winter runs. A cursory examination of the percentage vapor and artifact levels in Table 4 shows what appears to be considerable scatter in the fraction of any given PAH in the vapor phase. The overall vapor plus artifact values are, however, similar to numbers recently reported (Van Vaeck et al., 1984), and, as discussed further below, the observed variability can be related largely to temperature effects.
3.3. Artifact levels The variability of the apparent artifact levels is largely due to variations in temperature and the timing of these variations with respect to the sampling cycle. For example, samples collected during the winter tend to display lower artifact levels because the temperatures for those experiments were more nearly constant. Also, at the lower winter temperatures, changes in
Table 1. Ozone reactivity with ambient BaP*
8
a
Run No.
0.0-
la lb 2a 2b 3a 3b
0.6 -
0
I 4
I 6
I 12
I 16
I 20
Sampler
Sample volume (m?
BaP cont. (ng m-‘)
Denudert No denuder Denuder No denuder Denuder No denuder
20.7 20.6 22.2 21.6 18.9 19.6
0.39 0.38 0.81 0.77 0.86 0.83
24
Time (h)
Fig. 2. Effect of OJ on enriched PAH concentrations.
* 03 input to denuder sampler = 10-20 ppbv; OJ input to nondenuder sampler = 180 ppbv. t Denuder coating = rubber cement.
406
ROBERT W. COU.TANTet al. Table 2. Summary
of phenanthrene
phase distribution
Filter Run No. 1 2 3 4 5 6 I 8 9 10 I1 12 13
data measured
Q”C)
NDt
Dt
ND+
Dt
Total:
23.8 21.0 25.0 24.5 26.1 23.0 29.0 - 5.5 12.8 -1.0 3.0 - 2.0 4.8
69.5 13.5 66.0 59.5 138.0 87.0 91.5 94.5 118.0 114.0 74.5 81.5 106.0
4.20 0.75 0.72 1.30 1.40 0.89 0.60 4.50 2.30 10.1 5.70 4.70 6.70
3.10 0.75 0.56 0.89 1.00 2.10 0.65 2.70 3.20 6.10 2.00 3.60 2.80
50.7 80.6 146.0 41.2 138.0 68.2 83.2 18.1 95.8 73.4 84.8 59.3 94.9
39.6 250.0 31.9 0 84.2 46.0 48.4 9.9 63.8 24.5 32.8 30.6 42.2
54.9 81.4 147.0 48.5 139.0 69.1 83.8 22.6 98.1 83.5 90.5 64.0 102.0
* Total particulate loading. tphenanthrene concentrations (ng m ‘) associated with denuder $ Total phenanthrene concentration (ng m 3). 9:Vapor and volatilization artifact concentrations (ng m “). IIValues in error because of contamination of PUF sample.
Run No.
2 3 4 5 6 I 8 9 10 11 12 13
Capacity kvW* 60.4 10.2 10.9 21.9 10.1 10.2 6.56 47.6 19.6 88.6 76.5 57.7 63.2
air
PUF
Mass* (flgm-“)
Table 3. Phase distribution
in ambient
Art.?
V+A:
25 (-231) 87 109 43 34 46 49 35 70 68 52 62
68 (330) 13 - I2 56 65 53 31 62 18 25 41 32
92 99 100 97 99 99 99 80 98 88 94 93 93
* Particulate loading (W) of phenanthrene. t Vapor and artifact levels expressed as a percentage content of the sample. $ Sum of the vapor and artifact percentages. $Equilibrium vapor pressure of pure phenanthrene temperature. 11 Apparentisosteric heat of adsorption [see Equation
vapor pressure induced by temperature fluctuations are smaller. The timing of the sampling cycle also is important in determining the artifact level. For example, consider the hypothetical case where the total PAH content of the inlet air is constant, but the temperature rises continuously throughout the sampling experiment. In this case, particle-bound PAH collected at any instant during the sampling process would experience volatilization throughout the remainder of the run, thus increasing the artifact. For the opposite situation where temperatures continuously decline during the
13.6
(- 188.0) 127.0 52.9 60.2 23.3 38.6 11.1 34.6 58.8 61.9 33.1 62.9
(D) and non-denuder
data derived from ambient ments
W.t
Vapor 9:
air phenanthrene
Peg: (mg m-‘)
37.1 (268.O)ll 19.1 - 5.7 77.8 44.9 44.6 7.0 61.2 14.6 22.9 26.2 32.0
(ND) samplers.
measure-
4st” (kcal mole-‘) 6.81
1.40 1.01 1.60 1.51 1.94 1.21 2.51 0.033 0.38 0.062 0.11 0.054 0.14
of the total
Artifacta
s”ps 6.07 6.18 6.42 6.65 4.26 5.28 3.77 4.09 3.99 4.24
phenanthrene
at the average
sampling
(9)].
period, the volatilization artifact would be minimized. Obviously, real 24-h sampling cycles encompass a blend of these scenarios where volatilization may be appreciable during some periods and essentially nil at other times. Continuing this reasoning, the most severe artifact formation scenario would be associated with rising temperatures near the end of the sampling period. This suggests that a 24-h sampling cycle for PAH should normally be started and finished during the early morning hours so that only samples collected during the first 8-10 h would be subjected to rising temperatures within the sampler. sampling
Phase distribution and artifact for~tion
in ambient air sampling
Table 4. Summary of PAH vapor phase and artifact levels* Ranges Chemical Phenanthrene Anthracene Fluoranthene Pyrene ~~(=~nthra~ne Chrvsene Be&o(e)pyrene Benzo(a)pyrene Indeno( 1,2,3-c,d)pyrene
Benzo(g,h,i)perylene Coronene
Median
Vapor
Artifact
Vapor
Artifact
25-W $71 27-64 5-80 19-67 5-65 NAt NA NA NA NA
13-68 14-92 7-62 16-83 8-45 17-50 NA NA NA NA NA
39 26 43 43 26 28 NA NA NA NA NA
60 71 47 47 13 38 NA NA NA NA NA
*Ranges and median values expressed as percentages of the total amounts of each compound found. t NA = not applicable, no evidence of vapor or artifact found.
A complete description of the phase distribution of PAH requires the use of an adsorption isotherm that specifically relates theamount of the adsorbed phase to the vapor phase concentration. A variety of general isotherm forms have been developed for different applications. Some of these are based on sound, fundamental considerations, while others are purely empirical. The simplest theoretical relationship is the Langmuir isotherm, by which the fractional surface coverage is given as:
where P is the sorbate pressure and b is a constant. A number of authors have extended the utility of the Langmuir isotherm by expressing b as function of temperature, viz. Inb=a+E/RT
(6)
where E is taken as an adsorption energy. A more complex isotherm that specifically relates to both sorbate and sorbent properties was developed by Dubinin based on the Polanyi adsorption potential theory. This isotherm, referred to as either the Dubinin-Polanyi isotherm or the Dubinin-Radushkevich (DR) isotherm, is given by: W/W0 = exp-
(&‘f12)[RTln (P/Po)12
(7)
where W is the capacity for the sorbate at partial pressure P, Wo is the saturation capacity at the equilibrium pressure PO; B is a structural constant for the sorbent; and fi is the affinity coefficient for the sorbate. B is usually determined for a given sorbent by measuring the isotherm using a reference sorbate, and values of #I for different sorbates are found to be well correlated by their molar refractivities. Because of this ability to represent the sorption behavior of many sorbates in terms of a single set of reference parameters, the DR isotherm has also proven useful in
describing the simultaneous adsorption of multiple components (Jonas et al., 1979) (for an excellent
review, see Werner and Winters, 1986). Each of these isotherms has been used by various authors in describing adsorption of VOC by carbonaceous sorbents. All three yield similarly shaped curves, and, unless detailed measurements of the isotherm are made, choice between them is often arbitrary. The principal attraction of the DR isotherm is that it offers the possibility forextra~lation to other compounds once the m~surements have been made for one or two reference sorbates on a given sorbent. The Langmuir isotherm has been applied to volatilization of PAH (Junge, 1977; Yamasaki et al., 1982; Bidleman et al., 1986). The DR isotherm has been used extensively by Jonas and co-workers (op. cit.) for prediction of sorbent bed behavior. As noted above, the DR isotherm can represent adsorption of multiple sorbates in terms of a single set of sorbent parameters and the known molar refractivities of the sorbates. It is useful to note at this point that the bracketed term on the right-hand side of Equation (7) has units of (energy/mole)‘. The square root of this term is identical with the isosteric heat of adsorption, qs, (Pace, 1967). The total enthalpy of vaporization from the adsorbed state is given by: AH, = AH,+q,,
(8)
where AH, is the enthalpy of vaporization of the pure compound, and qst = RT In P,JPo.
(9)
Thus, the excess enthalpy of adsorption (or isosteric heat of adsorption) can be estimated from the ratio of the observed vapor concentrations to the equilibrium vapor concentrations of the pure PAH, and this in turn is directly related through the DR isotherm to the amount of PAH adsorbed on the particulate matter. It is important to note that this approach does not constrain the adsorption energy to be independent of
408
ROBERTW. COUTANTet al.
surface coverage and temperature, as is implied by the use of the modified Langmuir equation. Rather, the greater the amount of adsorbed PAH, the smaller qst is expected to be. Equation (7) is strictly intended for adsorption of single sorbates, but Grant and Manes (1966) have shown that it is equally applicable to mixtures of sorbates. provided that correction is made for the relative composition of the adsorbed phase. For the current samples, the relative compositions of the samples were quite constant, as can be seen in Table 5. Therefore, the applicability of the DR to the current data can be tested without correcting for compositional changes on a sample by sample basis. Figure 3 shows such a test for the phenanthrene data given in Tables 2 and 3. Here we have plotted In W (W expressed in ppmw) vs the square of the apparent isosteric heat. It can be seen that with the exception of one outlying point (run No. 1). the DR isotherm does indeed offer a good correlation of the phenanthrene data. DR plots for the other volatile PAH considered in this work also show good correlation of the data, with slightly more scatter for the although benz(a)anthracene data. 3.5. Estimation
of ambient phase
distribution
As noted above, the relationships derived from the current work may be unique to the particular distriTable 5. Ratios of volatile PAH to phenanthrene Average ratio to PAH
RSD
(%)
phenanthrene
Anthracene Fluoranthene Pyrene Benz(a)anthracene Chrysene
0.14 1.0 0.46 0.25 0.32
15 15 17 36 37
9.0
r’=0.902
~=544030+00t2.ll4lE-OI b =-7.92720-02’7 1517E-03
80
70
(RTtn P/~12(kcal/mole)Z Fig. 3. Dubinin-Radushkevich isotherm for phenanthrene on ambient air particles.
butions of PAH found in these samples, and these relationships should be tested at sites having other overall compositions. The real utility of the DR approach described here lies in its potential for estimating PAH phase distributions and the extent of volatilization artifact formation based only on the data normally obtained with the PS-1 air sampler or similar systems. The DR isotherm specifies the relationship between the amount adsorbed and the vapor phase concentration. Therefore, if the total concentration of PAH is measured, Equation (7) can be solved for the relative amounts in the vapor and adsorbed phases. The volatilization artifact can then be estimated from the discrepancy between the calculated and measured concentrations of PAH on the particulate matter.
4. CONCLUSIONS
In any 24-h ambient air sampling study the temperature and the physical and chemical composition of the air being sampled can change appreciably during the course of the experiment. Furthermore, the phase distribution and even the chemical composition of the collected sample may change during the collection process as a consequence of the external changes. For example, an increase in temperature during the course of the sampling period would be expected to result in volatilization of species adsorbed on the already collected particulate matter. If the collected particulate mass forms a significant fraction of the final sample. the increase in temperature could have an appreciable effect on the apparent overall composition of the particulate matter. The collected sample, therefore, has properties that are uniquely averaged over the specific changes in variables that characterize each sampling period. In the absence of detailed time-dependent information on the temperature and ‘new sample’ composition, defensible relationships between the nature of the collected sample and that of the same material as it existed in the atmosphere cannot be developed. The best that can be done is to utilize fundamentally sound relationships to develop global guidelines. The current work indicates that 3- and 4-ring PAH exist to a large extent in the vapor phase in ambient air, and that appreciable artifact formation can occur during sampling as a result of volatilization of these compounds from filters. This finding is consistent with the conclusions of other authors, but the current work provides a means for distinguishing between artifact and vapor. The results are internally consistent when viewed from the perspective of thermodynamic equilibrium, and a means is provided for estimating vapor concentrations under other sampling conditions. The study also strongly suggests that oxidative losses of BaP associated with ambient airborne particulate matter due to reaction with O3 after collection are negligible.
Phase distribution and artifact fckrmation in ambient air sampling Disclaimer-Although the work described in this article was funded wholly by the U.S. Environmental Protection Agency through Contract No. 68-02-4127, it has not been subjected to Agency review. Therefore, it does not necessarily reflect the views of the Agency, and no official endorsement should be inferred. Mention of trade names or commercial products does not constitute endorsement or recommendation. REFERENCES
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Grosjean D., Fung K. and Harrison J. (1983) Interactions of polycyclic aromatic hydrocarbons with atmospheric pollutants. Envir. Sci. Tech&. 17, 673479. Johnson M. D., Barton S. C. and Barton G. H. S. (1985) Development of a gas/particle fractionating sampler for chlorinated organics. Paper presented at 78th Annual
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Meeting of the Air Pollution Control Association, Detroit, Michigan, June, 1985. Junge C. E. (1977) Fate of pollutants in the air and water environments-I. In Advances in Environmental Science and Technology (edited by Suffet I. H.), Vol. 8. WileyInterscience, New York. Lewis R. G. and Jackson M. D. (1982) Modification and evaluation of a high volume air sampler for pesticides and semi-volatile organic chemicals. Analyt. Chem. 54,592-594. Pace E. L. (1967) Adsorption thermodynamics and experimental measurement. In The Solid-Gas interface (edited by Flood E. A.), Vol. 1, Chapter 4. Marcel Dekker, New York. Sansone E. B., Tewari Y. B. and Jonas L. A. (1979) Predication of removal of vapors from air by adsorption on activated carbon. Enuir. Sci. Technol. 13, 1511-1513. Van Vaeck L., Van Cauwenberghe K. and Janssens J. (1984) The gas-particle distribution of organic aerosol constituents: measurement of the volatilisation artifact in HiVol cascade impactor sampling. Atmospheric Environment l&417430. Werner M. D. and Winters N. L. (1986) Predicting gaseous phase adsorption of organic vapors by microporous adsorbents. CRC Critical Rev. enuir. Control 16, 327-356. Yamasaki H., Kuwata K. and Miyamoto H. (1982) Effects of ambient temperature on aspects of airborne polycyclic aromatic hydrocarbons. Enuir. Sci. Technol. 16, 189.