Environ. Sci. Technol. 1994, 28,1254- 1259
Desorption of Perylene from Combustion, NaCI, and Carbon Particles Dletmar Stelner and Heinz K. Burtscher' Laboratory for Solid State Physics, ETH Zurich, CH-8093 Zurich, Switzerland
The gadparticle distribution of perylene as a model PAH (polyaromatic hydrocarbon) is measured within a temperature range from 300 to 650 K. Perylene is adsorbed onto the core of combustion particles of different size (3080 nm in diameter) and origin (diesel engine and oil burner), carbon particles, and NaCl particles. The adsorbed perylene consists of a fraction x, that is nonremovable at temperatures up to 650 K and a fraction that can be desorbed at temperatures below 500 K. The mean desorption temperature T50% (50%of perylene still in the particulate phase) and the fraction 3c are increasing with increasing diameter of the particles in the case of combustion particles and carbon particles. 7'50% and x of the NaCl particles show no size dependence. The desorption enthalpy E d is close to the evaporation enthalpy of perylene. However, to model the experimental results by a two-step adsorption model, a distribution of E d instead of a single value has to be assumed. The results show that carbon particles may be used as a model for combustion particles when studying the adsorption of low volatile PAHs, whereas NaCl particles show considerable differences in their sorption properties.
Introduction Combustion processes emit a large number of particles. Associated with particulate emissions, a great variety of volatile and semivolatile compounds are released into the atmosphere. Among them the polyaromatic hydrocarbons (PAHs) are of special interest, because many of them exhibit direct mutagenic activity (e.g., benzo[alpyrene). Environmental PAHs may be divided into gas-phase and particle-phase fractions. In the literature (1-31, the ratio of these fractions is called the 'partitioning coefficient' Kp,e. Knowledge of Kp+is important to predict photochemical degradation or wet and dry deposition rates of the PAH. The fate of PAH molecules after inhalation also depends on the partitioning coefficient. In the past, a great number of field studies have been carried out where particle-phase and gas-phase PAH molecules were precipitated on particle filters and polyurethane foam plugs, respectively, as a function of the sampling temperature. Such studies, however, are subject to sampling artifacts such as adsorption to or desorption from the filter, or they are influenced by moisture on the filter ( 4 ) . These difficulties can at least be partly overcome by in situ measurement instead of filter sampling. A useful technique for this purpose is the photoelectron emission from particles (PE) (5),which allows the detection of the particle-bound PAH fraction (6). Burtscher and SchmidtOtt (7) used PE to monitor adsorption and desorption of perylene from carbon particles. Later, Niessner and Wilbring (6) adsorbed different PAHs on various model particles (NaC1, carbon, A1203, aerosil) and then studied the subsequent desorption in a thermodenuder by PE.
These laboratory studies also allow the extension of the temperature range significantly compared to field studies in ambient air. So far, various attempts for a theoretical description of the desorption process have been made: Yamasaki et al. (8)tried to apply a linear adsorption theory, proposed by Junge ( 9 ) , in order to find the correlation between the partitioning coefficient and the sampling temperature. Pankow (2) developed a more sophisticated two-state model, where he divides the particle-bound PAHs into two fractions: a removable one and a nonremovable one, the latter not participating in the equilibrium process between particle and gas phase. Storey and Pankow (10) used the data of Niessner and Wilbring (6) and applied a one-state desorptionmodel to interpret the data in terms of desorption enthalpies. In the present work, we want to close the gap between field studies carried out on actual environmental particles and laboratory studies on model particles. We adsorbed perylene (C2oH12), a 5-ring PAH, on the soot core of combustion particles of different size and origin (diesel, oil burner) and compared the results with those obtained for carbon and NaCl particles. In the second part, we apply Pankow's two-state model (2, 11) to derive desorption enthalpies of perylene from the mentioned kinds of particles. As large PAHs such as perylene have a low volatility, they already condense in an early phase during the cooling of the exhaust gas. This means that they will be adsorbed on the particle core below more volatile species such as water. To study the sorption properties of large PAHs, it therefore makes sense to remove the volatile fraction prior to the adsorption experiment. Experimental Procedures
* To whom correspondence should be addressed; e-mail address: AEROSOL@CZHETHSA.
In Figure la, the experimental setup for the evaluation of sorption properties of perylene on combustion and model particles is shown. Combustion particles are produced by an oil burner or a diesel engine. NaCl and carbon particles are investigated as model substances. NaCl particles are produced by nebulization and subsequent drying o f a diluted NaCl solution (a few milligrams of NaCl are dissolved in 100 mL of ultrapure water) (12). Carbon particles are produced by a spark discharge between two graphite electrodes in a nitrogen atmosphere (13). Afterwards, particles are charged by bipolar ion attachment. The ions are produced by a radioactive &source ("Ni). A subsequent differential mobility analyzer (DMA, manufactured by Hauke, Austria) selects one size class ( 1 4 ) .In the following, the term diameter (D)will hence always refer to the mobility diameter determined by DMA. Downstream of the DMA, the particles pass the first desorption unit (DUI), consisting of a tube furnace and a subsequent sink for the desorbed species (see Figure lb). The sink is realized by a water-cooledcharcoal layer inside a stainless steel cylinder. In order to remove the volatile fraction from the particles, the temperature of the desorption unit is set to 700 K. At this temperature, the particles are more or less reduced to the mere soot core
1254 Environ. Sci. Technol., Vol. 28, No. 7. 1994
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0 1994 American Chemical Society
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Figure 1. (a, top) Experimental setup. (b, middle) Desorption Unit. Volatile material is desorbgd in the heated section and subsequently removed by the activated charcoal adsorber. (c, bottom) Perylene adsorption. Perylene is evaporated in a tube furnace and mixed with the aerosol particles. In the following condenser the peryleneadsorbs on the particle surface. By the temperature in the furnace, the amount of evaporating materialand, thereby,the layer thickness can be varied.
(15,18). For the model particles, the desorption unit is operated at room temperature. After the first desorption unit (Dud, the particles are coated with perylene. The coating section (7) consists essentially of a glass tube equipped with a cavity containing perylene. A stainless steel screen separates the perylene from the flow. This part is heatable by a tube furnace. Behind the furnace, the tube is water-cooled to room temperature. Here, adsorption or condensation of the perylene takes place (see Figure IC). Subsequently the particles enter a second desorption unit (DU2), which is similar to DU1. The maximum temperature of DU2 is restricted to 650 K. The change in particle diameter caused by adsorption or desorption is monitored by a diffusion battery (DB) (19). This instrument was applied only in first experiments, where adsorption of perylene layers up to a thickness of 3 nm was investigated. In subsequent experiments, the particles are coated only with submonolayers, resulting in a relative change of the diameter of less than 1% , which is below the resolution limit of the diffusion battery. Adsorption and desorption processes are also monitored by photoemission, which is a more sensitive tool in measuring surface coverage of combustion or model particles by PAH. The sensor that is used in this experiment is described by Steiner et al. (15). The PE intensities given in the following plots are always normalized to the particle surface area, i.e., they correspond to the electron emission probability per unit surface area. The surface area is assumed to be proportional to 0 2 . With this setup, the following kinds of experiments have been performed (1) The dependence of the desorption properties on layer thickness is studied for carbon and diesel particles. (2) Experiments with model particles of
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6r (nm) Figure 3. Adsorption of perylene on diesel particles.The photoemission signal (PE) is plotted in arbitrary units (au) versus the radius increase (= perylene layer thickness) 6r.
different diameters are carried out to compare their desorption properties with those of combustion particles. (3) The influence of diameter and origin of combustion particles (i.e., diesel engine under different loads, oil burner) are investigated. (4) Experiments at different flow rates through the desorption unit, resulting in different residence times, are carried out to prove that the system is in thermal equilibrium.
Results Figures 2 (carbon particles, D = 16.5 nm) and 3 (diesel particles, 50 % load, D = 66 nm) show results of the coating experiments, which were carried out to determine the probing depth of photoemission of different particles. PE saturates at a perylene layer thickness of 6r = 1.3 nm in the case of the diesel particles and at 6r = 2 nm in the case of the carbon particles. This is comparable to the results who found 6r = 2 nm of Burtscher and Schmidt-Ott (3, in the case of the carbon particles. The term 'saturation' denotes the fact that, in spite of growing thickness of the perylene layer, electron emission probability per unit Environ. Sci. Technol., Vol. 28, No. 7, 1994
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Flgure 5. Desorption of perylene from NaCl particles of different size. 100.00
Flgure 4. Desorption of perylene from diesel particles with different initial coverage accordingto in Figure 3. The PE signal is normalized to 100% for the initial coverage.
carbon-particles
surface area remains constant. This can be explained by exclusive photoemission from the perylene layer. In Figure 3, the first desorption unit (DU1) was set to 700 K, i.e., the "naked"soot core (18)of the diesel particles was coated with perylene. Three points Pi-3 are marked along the curve. PI and Pp are situated at the steep increase of the PE signal. P3 denotes the saturation plateau. The desorption behavior starting from different layer thicknesses (corresponding to Pi-3) is shown in Figure 4. The PE signal correspondingto the initial coating is normalized to 100% for convenient comparison of all curves. After a flat section, the PE signal steeply decreases with increasing temperature of DU2. At a temperature 2'50% (ca. 390 K), 50% of the initially adsorbed perylene has desorbed. At higher temperatures, the curve approaches a constant value different from zero. This means that a fraction x of perylene exists on the surface which cannot be removed at temperatures below 700 K. Similar observations have been made by Loepfe et al. (161, who analyzed the desorption of anthracene from soot particles as a function of temperatures, and by Siegmann (13,who studied benzene adsorption on graphite monocrystals. and the nonremovablefraction x are the characteristic values used to describe the sorption properties of particles of different size and origin. The two curves PI and Pp are identical within the experimental error. Hence, the partitioning into removable (100% - x) and nonremovable fraction x and 2'60% are also independent of the initial coverage with perylene, as long as the initial coverage is small enough (in the submonolayer region). This does not hold for desorption from P3 (more than one layer). In this case, the removable part increases from 75 to 90%. Although the absolute amount of the nonremovable adsorbed perylene is highest in this case, the ratio x is lower compared to PI and PZas only the first layer contains nonremovable material. In the following experiments, the initial coating of the particles is in the range between PI and Pp. Figure 5 shows the desorption of perylene from NaCl particles of 27 and 60 nm in diameter. The nonremovable fraction is very small; 2'50% is 333 K. The larger particles (60 nm) are four times more concentrated than the smaller ones. Nevertheless, the two curves are congruent within the experimental precision. This indicates that the desorption 1256
Environ. Sci. Technol., VoI. 28, No. 7, 1994
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Flgure 7. Desorption of perylenefrom combustion particlesof different size.
properties of the NaCl particles are independent of particle size and concentration. While desorption from carbon particles is also not affected by particle concentration, it is influenced by particle diameter. 2'60% is rising from 335 to 423 K, and x is increasing from 5 to 25 % for the transition from the small (D = 21 nm) to the large (D = 75 nm) particles (see Figure 6). Figure 7 shows four desorption plots of combustion particles of different size produced by a diesel engine (D = 30,42, and 55 nm) and an oil burner (D = 80 nm). Tsow and x are also increasing with increasing diameter.
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According to Figure 8, the load of the diesel engine does not affect the desorption properties of the particles. Particles of comparable diameter produced at 25% load (D= 55 nm) or at 50% load (D= 60 nm) show the same behavior. I t can be seen that x and T ~ o xdepend only on particle diameter for soot core and C particles. In Figure 9, results obtained with two different residence times of the particles in the desorption unit are plotted. This shows that the residence time has no influence on desorption, indicating an equilibrium between the gas and particle phases in the heater part. Measurements with other particle sizes corroborate this result.
Discussion The experiments with the NaCl particles of different size and concentration show that the nondesorbable fraction x is negligible and that desorption is independent of size and Concentration. The particle surface concentration differs by a factor of 20 between the two experiments in Figure 5, if the surface is estimated by 0 2 . Nevertheless the two desorption plots in Figure 5 are identical. As readsorption of perylene onto the particles-if existent-would depend on the particle size and manifest itself as a nondesorbable fraction x , this result shows that readsorption of perylene can be excluded. The gas-phase fraction is absorbed by the charcoal; the particle phase is "frozen" and accessible for measurements a t room temperature. The time between removal of the gas phase and
subsequent P E measurement is very short (4s). Further desorption of perylene from the particles can thus be neglected. Rounds and Pankow (20) discuss the effect of PAH moleculesinside the particle that can diffuse to the surface, which can delay the desorption process. This effect can be more pronounced for larger particles (longer distance to the surface). If the residence time in the desorption unit is too short for all PAHs to diffuse to the surface, this could explain the size dependence of the mean desorption temperature 2'50% and the nonremovable fraction. However, this can be excluded as !I'm% and x are independent of the residence time in the denuder, as experiments with different residence times (Figure 9) show. The gas and particle phases are in thermal equilibrium. Together with the knowledge that readsorption can be neglected, this showsthat the high temperature equilibrium concentration of the particle-bound perylene is frozen and accessible after the desorption unit. This will be essential for the following model. Extended (Two-State) Adsorption Model. Untilnow the mean desorption temperature 2'60% and the nonremovable fraction x have been used to describe desorption from the various model and combustion particles. Partitioning of PAH molecules between the gas and particle phases is discussed by numerous papers, both field studies and laboratory experiments (6,8,21,22). Yamasaki et al. (8) determined partitioning coefficients as a function of sampling temperature and applied Langmuir's adsorption concept. If this simple adsorption model is applied, very poor agreement between our experimental data and the model is obtained. Pankow (2) developed a more sophisticated model in which he divides the particle-bound PAH concentration into a removable fraction and a nonremovable fraction. Pankow and Bidleman (11)applied this model to interpret the data of Yamasaki et al. (8). To explain experiments with perylene coated carbon particles, Burtscher and Schmidt-Ott (7) suggested dividing the particulate PAH concentration into an adsorbed and a condensed state. In the following, we adapt Pankow and Bidleman's (2, 11)modified adsorption concept on our data (eqs 1-6). In eq 1,the total particulate PAH concentration Ftis divided into a volatile fraction, Fe, taking part in thermal equilibrium and a nonremovable fraction x (subscript t stands for total; e stands for equilibrium):
Ft = F, + x
(1)
Together with the perylene gas-phase concentration A,, Fe, and x sum up to 100% (eq 2):
A,
+ F, + x = 100%
(2)
Following Pankow (2, 3), we define a partitioning coefficient Kt between Ft and A, (eqs 3a and 3b):
FtITSP Kt = Ae
log(1OO - X ) (3b)
TSP denotes the total suspended particulate mass; m, is the slope of the Arrhenius plot which is proportional to Environ. Sci. Technol., Vol. 28, No. 7, 1994
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the desorption enthalpy Ed (eq 4); b, is the y-axis interception of the Arrhenius plot and depends on the sorbing surface A ~ s p(cm2/gg)of the particles, the typical vibration time of adsorbed PAH molecules (to),and their molecular weight M (252 g/mol) (eq 5); T is the absolute temperature (K) [see Pankow (2,3)l. Equation 4 gives the relation between m, and Ed; R is the gas constant. For a convenient evaluation Of Ed, T i s % eq , 4, which changes the result by less replaced by T ~ o in than 1.5%:
- d(logK,) =-- Ed mP-
d(l/T)
T 2.303R 2 X 2.303
4
3l
2
(4) 20
15
25
30
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By estimating to with 10-13 s (23)and ATSPwith 0.025 cm2/gg ( 2 4 ) ,eq 5 yields log b, = -17.1. By analysis of a large number of field studies, Pankow (3)finds log b, = -18.48. If we let b, float a good mathematical fit is obtained; however, values for b, up to -3 result, which makes no sense. As no A ~ s pvalues for our particles are available to determine b, straightforwardly, we decided to use Pankow’s value (3)as a compromise, being aware that these are values for urban aerosol not combustion particles:
TO TSP(T) = TSP(To)-?;
Envlron. Sci. Technol., Vol. 28, No. 7, 1994
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(6)
In eq 6, TSP(T)denotes the total suspended particulate mass which depends on the actual temperature T and the initial temperature TO. TSP(T0) was estimated by the aerosol concentration and the diameter of the particles assuming a density of 2 g/cm3 and spherical geometry. Photoemission from particles yields the total particulate PAH concentration (removable and nonremovable fraction). We determine Ft from the P E signal normalized to 100%at the initial surface coverage; A, is 100%- Ft.This is similar to Storey and Pankow (IO), who applied a onestate Langmuir adsorption concept to interpret the data of Niessner and Wilbring ( 6 ) . They investigated various PAHs adsorbed onto NaC1, carbon, aerosil, and A1203 particles. Our value for x is obtained from the experiment; mp is fitted by a Marquardt-Levenberg least square algorithm (25,26). Figure 1.0shows an Arrhenius plot of the experimental data and the best-fit curve for small NaCl particles. The fit is acceptable a t lower temperatures (lOOO/T> 2.5). For higher temperatures, we find a large discrepancy between the model and the experimental data. The fit for combustion particles (diesel particles, D = 55 nm) is worse (Figure 11). The agreement between fit and experimental data is poor over the whole temperature range, Table 1 gives an overview of the results obtained for combustion and carbon particles. As predicted by Pankow et al. ( 2 3 ,the values of the desorption enthalpy are very close to the value of the evaporation enthalpy of perylene (Ev = 138 kJ/mol). Thus, the desorption enthalpy of perylene from the surface of perylene droplets, carbon particles, and combustion particles is similar. Desorption enthalpy of perylene from NaCl particles is smaller than 1258
Figure 10. Experimental result ( 0 )and calculation with the two-state adsorption model (-) of perylene desorption from NaCl particles.
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-6
1 .0
1.5
2.0
2.5
35
3.0
1000/T (1/K) Figure 11. Experimentalresult (0)of perylene desorption from diesel particles and calculation with the two-state adsorption model using a single Ed (-) or a set of three Ed’s (- -).
-
Table 1. Results of Two-State Fit to Combustion, NaCl, and Carbon Particles origin of particles
f (% )
mp
NaC1, D = 27 nm NaC1, D = 60 nm carbon,D = 20 nm carbon,D = 75 nm diesel, D = 30 nm diesel, D = 42 nm diesel, D = 55 nm diesel, D = 60 nm oil burner, D = 80 nm evaporation enthalpy
5 7
6571 6462 7004 7384 7385 7477 7217 7317 7407
a
7 16
10 15 22
19 25
desorption enthalpy (kJ/mol) 124 123 133 140 140 141
138 138 140 1380
From ref 28. I _ -
__
for carbonaceous particles, indicating a weaker bonding between these particles and the perylene. However, as the agreement of the fit is poor, the values of the desorption enthalpy are rough estimates. The discrepancy between fit and Arrhenius plot is caused by a broadening of the actual desorption plot, i.e., the decrease of the particulate PAH fraction as a function of temperature is not abrupt enough. Broadening of the plot due to the readsorption processes or a nonequilibrium state of the system can be excluded (see above).
However, broadening of the plot could be explained by a distribution of desorption enthalpies. Keeping in mind the irregular surface of the combustion particles, this is indeed more adequate than assuming a sharp desorption enthalpy as it is found for noble gases on metal surfaces, for example. In order to test this assumption, we replace the single E d of Pankow’s model (2,11) by three Ed’s. We assume that 60% of the adsorbed perylene molecules occupy a site with a mean desorption enthalpy Ed*, 20% occupysiteswith l.lEd* and20% occupysiteswith0.9Ed*; respectively. Surface diffusion of perylene from one site to another is excluded. Figure 11compares the result of the 3-fold E d fit with the single E d fit. Ed* is estimated to be 127 f 11kJ/mol, which is still in good agreement with the evaporation enthalpy (Ev = 138 kJ/mol). It is obvious that the triple model shows a much better agreement with the experimental data than the single E d model. Replacing the discrete Ed by a continuous distribution may therefore be a promising way of modeling desorption processes of PAHs from the surface of combustion particles, explaining the relatively slow decrease of the adsorbed perylene concentration as a function of temperature. The significant increase in x and E d with particle size, however, is not explained. One explanation might be an increasing probability for fissures and cracks in larger (more agglomerated) particles, but for a conclusive interpretation, the experimental data are so far insufficient. Desorption experiments with anthracene by Loepfe et al. (16) also yield a considerable amount of closely bound material, desorbing only at high temperatures. However, these experiments yield no size-related information. Experiments studying the adsorption of PAHs on graphite under well-defined conditions are in progress. First results also yield an unexpected high fraction of strongly bound material, the amount of which sensitively depends on how the adsorption takes place ( I 7). These experiments indicate that the intercalation of PAH may be of importance, which could be more efficient on larger particles. Acknowledgments We wish to thank H. C. Siegmann for many fruitful discussions and for his support of this project and S. ChristDiserens, who carried out the first experiments on partitioning of perylene on combustion particles in our laboratory. The financial support of the WaBoLu I1 Project of the Federal Institute of Technology of Zurich is kindly appreciated.
Literature Cited (1) Pankow, J. F. Atmos. Environ. 1987,21, 2275. (2) Pankow, J. F. Atmos. Environ. 1988,22, 1405. (3) Pankow, J. F. Atmos. Environ. 1991,25A, 2229. (4) Thibodeaux, L. J.; Nadler, K. C.; Valsaray, K. T.; Reible, D. D. Atmos. Environ. 1990,25A, 1649. (5) Burtscher, H.; Schemer,L.; Siegmann,H.-C.;Schmidt-Ott, A.; Federer, B. J. Appl. Phys. 1982, 53, 3787. (6) Niessner, R.; Wilbring, P. Anal. Chem. 1989, 61, 708. (7) Burtscher, H.; Schmidt-Ott, A. J. Aerosol Sci. 1986, 17, 699. (8) Yamasaki, H.; Kuwata, K.; Miyamoto, H. Environ. Sci Technol. 1982,16, 189. (9) Junge, C. E. In Fate of pollutants in the air and water environments, part 1; Suffet, I. H., Ed.; John Wiley and Sons: New York, 1975. (10) Storey, J. M. E.; Pankow, J. F. Atmos.Enuiron. 1992,26A, 435. (11) Pankow, J. F.; Bidleman, T. F. Atmos. Environ. 1991,25A, 2241. (12) Liu, B. Y. H.; Lee, K. W. J.Am. Ind. Hyg. Assoc. 1975,36, 861. (13) Schwyn, S.; Garwin, E.; Schmidt-Ott, A. J. Aerosol Sci. 1988, 19, 639. (14) Reischl, G. P. Aerosol Sci. Tchnol. 1991, 14,5. (15) Steiner,D.; Burtscher, H.; Gross, H. Atmos.Environ. 1992, 26A, 997. (16) Loepfe, M.; Burtscher, H.; Siegmann, H. C. Water, Air, soil Pollut. 1993, 68, 177. (17) Siegmann, H. C. Personal communication, 1994. (18) Steiner,D.; Burtscher, H. Water, Air, Soil Pollut. 1993,68, 159. (19) Scheibel, H. G.; Porstendorfer, J. J.Aerosol Sci. 1984,15, 673. (20) Rounds, S. A.; Pankow, J. F. Environ. Sci. Technol. 1990, 24, 1378. (21) Valerio, F.; Pala, M. Fresenius J.Anal. Chem. 1991,339, 777. (22) Steiner, D.; Diserens, S.; Burtscher, H.; Siegmann,H.-C. J. Aerosol Sci. 1990,21 (Suppl. l),S27. (23) Redhead, P. Vacuum 1962,12, 203-211. (24) Corn, M.; Montgomery, T. L.; Esman, N. Enuiron. Sci. Technol. 1971,5, 155. (25) Marquardt, D. W. J. SOC.Ind. Appl. Math. 1963,11,431. (26) Press, W. H.; Flannery, B. P.; Teukolsky, S. A.; Vetterling, W. T. Numerical Recipes; Cambridge University Press: Cambridge, U.K., 1986. (27) Pankow, J. F.; Bidleman, T. F. Atmos.Environ. 1991,26A, 1071. (28) Hoyer, H.; Peperle, W. Z.Elektrochem. 1958, 61.
Received for review August 25, 1993.Revised manuscript received March 28, 1994. Accepted April 5, 1994.’ @
Abstract published in Advance ACS Abstracts, May 15, 1994.
Envlron. Scl. Technol., Vol. 28. No. 7. lQg4 1259
