Research Communications A Large Gas-Phase Stripping Device to Investigate Rates of PAH Evaporation from Airborne Diesel Soot Particles RICHARD M. KAMENS* AND DANA L. COE† Department of Environmental Sciences and Engineering, University of North Carolina, Chapel Hill, North Carolina 27599
Introduction The phase in which a toxic semivolatile organic compound (SOC) exists strongly influences its fate in the atmosphere and its potential health impact. Since the late 1970s, gasparticle partitioning theories have been based upon Langmuirian adsorption, which assumes equilibrium conditions (1-4). For this theory to work, it is necessary that SOCs be able to rapidly “off-gas” from the particles to maintain equilibrium as particles dilute from their sources. To provide insights into the kinetics of mass transfer between the gas and particle phases, evaporation rates of SOCs from particle surfaces are needed. In this study, we investigated the evaporation rates of selected polycyclic aromatic hydrocarbons (PAHs) from fresh diesel soot particles. The approach utilized a large gas-phase stripping device (LGPS) designed to remove gas-phase PAHs over time scales of seconds. This minimized condensation so that evaporation rates could be measured directly. Preliminary data from this study suggest very fast evaporation rates. The possibility also exists for SOC particle off-gassing artifacts inside the sampling denuders. This would occur as SOCs were removed from the gasphase.
Experimental and Analytical Techniques Experiments were performed under darkness using two Teflon film environmental chambers at the University of North Carolina’s (UNC at Chapel Hill) outdoor environmental chamber facility near Pittsboro, NC. One chamber was a large 190 m3 A-frame structure (5), and the other was an octagonal-based 25 m3 chamber with a conical top (6). Diesel particles were added and stored in the outdoor chambers and then were drawn into the inlet of the LGPS. Semivolatile PAHs were measured in the gas and particle phases both upstream and downstream of the LGPS. A 1967 Mercedes sedan (Model 200D) engine was the source of diesel emissions for the first experiment, and a 1978 Mercedes sedan (Model 300SD) was used for the second experiment. Both cars were operated at 1000-2000 rpm, generally idling with occasional and momentarily applied loads. An aluminum electrical conduit (5.1 cm i.d., length 5-10 m depending on the chamber) transferred air and diesel emissions between the diesel engines, chambers, and LGPS. A calibrated highvolume air sampler motor was used to control the flow rate through the LGPS. During the experiments, the flow rate * Corresponding author e-mail:
[email protected]. † Present address: Sonoma Technology Inc., 5510 Skylane Blvd, Suite 101, Santa Rosa, CA 95403.
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was monitored by its pressure drop across a calibrated orifice in back of the high-vol blower. Particle size measurements were performed with an electrical aerosol analyzer (EAA, Thermo Systems, Inc., Model 3030, St. Paul, MN). LGPS Design. The LGPS was designed as a parallel plate denuder to permit high flows and low pressure drops (Figure 1). Two gas-phase denuder stripper units, each containing 55 parallel plates, were placed end-to-end inside the LGPS. Activated carbon-impregnated quartz fiber high-vol filters (Schliecher and Schuell No. 508, Keene, NH) were selected as the adsorptive plate material because of their large capacity (7). The LGPS housing was fabricated from galvanized sheet metal. The housing had a length, width, and height of 107 × 35 × 25.4 cm. The inlet expanded from a 5-cm pipe to a tapered sheet metal flange, which then mated with the 35 × 25.4 cm housing. A flow straightener consisting of tightly packed open glass tubes (each 1.3 cm i.d. × 25.4 cm in a 20.3 × 27.9 × 25.5 cm aluminum frame box) was placed upstream between the parallel plates and the inlet of the LGPS. The plate height and width facing the flow were 16.5 × 24.4 cm; the length was 25.4 cm. Each stripper unit was enclosed in an aluminum frame, 20.3 × 27.9 × 25.4 cm long. The width of a carbon filter paper plate was 0.05 cm, and the spacing between the plates was 4 cm. With the internal support, this gave a total interstitial volume for the two stripper units of 17.95 L. The LGPS was designed such that, at the flows used, the Reynolds number in the parallel plate region remained below 400. Inlet and Outlet Gas and Particle-Phase LGPS Measurements. Samples called upstream or inlet samples were taken from a port in the conical inlet of the LGPS, which is located just upstream of the flow straightener (Figure 1). Outlet samples were taken from a void volume 23.4 cm downstream from the second stripper unit. The volume of the void was 11.9 L and served to mix the effluent from the stripper units. The sampling flow rate for the chamber experiments was ∼20 L/min and was corrected to standard temperature and pressure (0 °C, 1 atm). Sample times were 5 min for the high concentration experiment, and 20 min for the low concentration particle experiment. Sampling lines, connectors, and filter holders were made of Teflon or glass (minimum i.d. of 1.27 cm). The low Stokes numbers even for 1-µm particles permitted sampling at right angles to the stripper flow without measurable effects on the particle size distribution (8). Each sampling train (5, 9) consisted of a 40-cm five-channel primary, or top, denuder for collection of gas-phase PAHs, followed by a 47-mm Teflon-coated glass fiber filter (type T60A20, Pallflex Products Corp, Putnam, CT). Air passing through the filter then entered a secondary denuder or polyurethane foam (PUF) adsorbent. The sum of the filter extract and the extract from the lower adsorbent or denuder was used to measure particle-phase PAHs. A denuder-filterabsorbent system was used to avoid reported sampling artifacts when using a filter-absorbent sampling system (5, 7, 10). The denuders were an adaptation of the two-channel XAD-coated denuder system originally developed by Gundel and co-workers (11). Denuders were field-extracted and reused multiple times without recoating (5, 9). Samples were spiked with deuterated internal standards and worked up and analyzed via gas chromatography-mass spectrometry according to previously discussed procedures (5, 9).
S0013-936X(96)01083-8 CCC: $14.00
1997 American Chemical Society
FIGURE 1. Exploded idealized illustration of large gas-phase stripping device (LGPS), not to scale.
Results and Discussion LGPS Performance. The overall performance of the system rested on the ability of the LGPS to pass particles and simultaneously remove SOCs from the gas phase. Cheng and Reist (12, 13) summarized mathematical solutions for diffusion at right angles to a flow between two parallel plates. For this study, diffusion coefficients were calculated three different ways based on molecular weight and liquid molar volumes (14). These ranged from 0.054 to 0.062 cm2 s-1 at 25 °C for the PAH fluorene. Using the Cheng relationships and an assumed accommodation or sticking coefficient of 1, the predicted LGPS collection efficiency for fluorene was 100% at a flow rate of 2.8 L/s (6.4 s in the stripper units) and 97% efficient at a flow of 13.7 L/s (1.3 s in the stripper units). In actuality, we observed a 98% gas-phase collection efficiency for fluorene at 2.8 L/s, and this declined to 86% at 13.7 L/min. It was possible to use fluorene in the diesel exhaust for these estimates, since measurements showed that it was almost entirely in the gas phase. Although this suggests some breakthrough, the stripper units as designed were fairly efficient at removing fluorene from the gas phase. For these same flows, one would estimate that 86% of the 0.01 µm, 96% of the 0.03 µm, and 99% of the 0.1 µm particles would pass through the denuder at a flow rate of 2.8 L/s. EAA particle size measurements at the inlet and outlet of the LGPS showed diesel particles to have count mean geometric diameters of 0.10-0.14 µm with geometric standard deviations of 1.5-1.7. Filter mass measurements at the inlet and outlet of the LGPS, depending on the flow rate, showed that 7-15% of the particle mass was lost as particles moved through the LGPS. EAA inlet and outlet comparisons supported this observation. Because particle losses in the LGPS were different at each flow, PAH masses on the particles were normalized to the total particle mass and reported in units of nanogram of PAH/milligram of particles. This facilitated comparisons between the inlet and outlet because differences would reflect only particle off-gassing. To evaluate the ability of the LGPS to remove gas-phase SOCs, the collection efficiency of selected PAHs was determined by passing raw diesel exhaust (diluted by approximately a factor of 2) directly from the car tailpipe to the LGPS. Comparisons for the inlet and outlet gas-phase PAH concentrations for 6-10 s residence time in the stripper zone revealed that the most volatile compounds [naphthalene (Nap), acenaphthylene (Ace), and fluorene (Fl)], were effectively removed by the stripper carbon-coated walls (Figure 2). As volatility decreased, however, the relative amount of gas-phase PAH removed appeared to decrease. This observation, however, is explained by the particle-phase PAH data (Figure 3). Comparisons of particle-phase PAH (normalized for particle mass) at the inlet and outlet of the LGPS showed that more than 90% of the volatile PAHs [see fluorene (Fl) and phenanthrene (Phe) in Figure 3] were lost from the particles as the particles passed through the LGPS. Even the less volatile pyrene (Py) and fluoranthene (Fla) still exhibited
FIGURE 2. Removal of gas-phase PAHs in the large gas-phase stripping device. Naphthalene (Nap) concentrations were divided by 4 and phenanthrene (Phe) concentrations were divided by 2.
FIGURE 3. Removal of particle PAHs in the large gas-phase stripping device. a 30-50% particle loss, while the least volatile benz[a]anthracene (BaA) and chrysene-triphenylene (Chy) showed no loss. As particles left the parallel stripper zone and entered the void volume, semivolatiles continued to off-gas because the gas phase had been removed in the stripper zone. Since this off-gassed material was not removed in the void volume, it therefore was measured by the samplers as gas-phase material at the outlet of the LGPS. Under the conditions used, mid-range volatile compounds (subcooled vapor pressures ∼10-5 Torr) such as fluoranthene and pyrene entered the large stripper predominantly in the particle phase. A small release from the particles contributed substantially to the gas phase. This occurred in the void volume without any further gas-phase removal. Hence, the observed gas-phase concentration of these compounds measured at the outlet sampler appeared to be “high” as compared to higher volatility compounds like naphthalene and fluorene. Given the much lower volatility of BaA and chrysene, almost no mass was transferred to the gas-phase. Particle Off-Gassing at Different Residence Times in the LGPS. Two experiments at different particle concentrations and temperatures were conducted to see if these conditions
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FIGURE 4. Removal of gas-phase fluorene (Fl) and gas and particlephase phenanthrene (Phe) in the LGPS at different residence times from a warm high-concentration particle experiment. influenced particle off-gassing rates. The first experiment used a 25 m3 chamber and the 1967 diesel car. The temperature inside the chamber and LGPS remained stable at ∼23 °C for the entire evening. The relative humidity was greater than 90%. The initial particle concentration in the chamber after the addition of diesel exhaust was 35.7 mg/m3, and it decreased to 12.5 mg/m3 in 2.8 h. This decrease was due to particle losses to the chamber walls. In the second experiment, particles were added from the 1978 diesel car to our 190 m3 chamber. The temperature decreased from 10 to 7 °C; and the relative humidity increased from 78 to 88% over 4 h. The particle concentration started at 0.77 and declined to 0.38 mg/m3 over this period. In the warm, high-concentration particle experiment, the flow rate through the stripper was varied between 13.7 and 2.8 L/s. This gave residence times in the stripper zone of 1.3-6.4 s. After spending time in the stripper zone, the particles also spent additional time in the void volume. For example, after 6.4 s in the stripper units, the particles spent another 4.3 s in the void volume. The sum of these two times was called the total LGPS time. Figure 4 shows the observed or measured percent removal of gas- and particle-phase phenanthrene between the LGPS inlet and outlet at different total LGPS residence times. Three samples were taken at the shortest total LGPS time (shown as an average in Figure 4) at 1.2, 3.2, and 4.0 h after the addition of diesel exhaust to the chambers. One sample was taken at LGPS times of 3.6, 5.5, and 10.7 s. These occurred at 1.9, 2.8, and 3.6 h after the addition of diesel exhaust. Uncertainty estimates ((2 σ) are shown for particle-phase phenanthrene and were similar for the gas-phase PAHs. Most of this uncertainty was due to imprecisions in flow measurements, filter masses, and sample workup procedures. At the longest total LGPS time of 10.7 s almost 93% of the gas-phase phenanthrene was removed between the inlet and outlet of the LGPS. As total LGPS times decreased from 10.7 to 2.2 s, the observed gas-phase phenanthrene removal declined from 93% to 67%. Approximately 15% of the phenanthrene was initially associated with particles. Over this same change in LGPS times, the corresponding phenanthrene removal from the particles went from 83 to 67% of its initial particle-phase loading. Fluorene, which partitioned almost completely to the gas phase (∼99% was in the gas phase), had a higher observed gas-phase collection efficiency than phenanthrene, because its removal was not confounded by particle off-gassing (Figure 4). In the cooler low particle experiment, the flow was varied so that the time in the stripper units ranged from 0.7 to 2.3 s. This gave total LGPS times of 1.0-3.6 s. At total LGPS times of 3.6 and 1.0 s, 90 and 70% of the of the gas-phase fluorene was removed. This is consistent with the LGPS gasphase removal rates for fluorene in the previous experiment. At a total LGPS time of 3.6 s, 40% of the particle phenanthrene mass was released, and at 1.0 s, 20% of the particle phenanthrene and 50% of the particle fluorene were released. Results from this and the previously described experiment
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showed that significant particle off-gassing occurred, even at total LGPS times as low as 2.2 and 1.0 s. This corresponds to 1.3 and 0.7 s in the stripper units. These observations raised concerns about possible particle off-gassing in the denuder samplers, since these residence times were 0.3 s. Particle Off-Gassing in the LGPS and Denuder Samplers. To estimate particle off-gassing rates in the entire system (the LPGS and denuder samplers), a three-compartment model was used. The stripper units, void volume, and denuders in the sampling system each represented individual compartments in the model. It was assumed that particle off-gassing in each of the compartments could be approximated as a first-order process. The residence times for each of the compartments were known, and outlet concentrations (as measured by the sampler) could be represented as the product of first-order expressions derived for each of the three compartments, as follows:
Cout ) Cine-(λp1t1+λp2t2+λp3t3) Cin is the concentration in the particle-phase entering the LGPS, and Cout is the particle concentration measured at the sampling denuder. t1, t2, and t3 are the residence times of the three compartments; λp1, λp2, and λp3 represent first-order rate constants for particle off-gassing in the stripper, void, and sampler compartments. The model assumes that at zero time in the stripper (and the other compartments), there would be no gas-phase removal and the moles of a given compound per particle mass entering that compartment would equal the moles per particle mass leaving that compartment. This was experimentally addressed for the LGPS by removing the stripper units but leaving the flow straightener in place. Under these conditions, measurements of particle PAHs at the inlet and outlet of the LGPS gave essentially the same concentrations. The λp rate constants in each compartment could have been separately treated and numerically calculated. It was reasonable, however, to assume that initially all three λp values were similar. This was because the λp values in each compartment would be a function of the depleted SOC gas and how far the system was from equilibrium at a given time (see ref 5, eq 6). Since the stripper units and sampling denuder compartments both had high gas-phase removal efficiencies and low SOC gas-phase concentrations, their λp values should have been similar. In addition, as particles entered the void volume, the gas phase was also low, and initially it should have a λp similar to λp1 and λp3. However, as residence times increased, the gas-phase concentration in the void volume increased due to particle off-gassing in the void. Accordingly, estimated off-gassing rate constants (λp2) from the particles in the void compartment decreased with time as the system approached gas-particle equilibrium (5). To accommodate this phenomenon, λp2 was assumed to decrease linearly from its initial value with time in the void. Figure 5 illustrates the model results for particle-phase phenanthrene. For these calculations, it was assumed that the entire particle fraction was available for rapid release from the particles. In reality, a small non-exchangeable fraction exists, although it is difficult to estimate. If a rough estimate of 5 or 10% is used, however, model PAH fits would improve for the longest LGPS resident times when considerable amounts of particle phase are released (Figure 5). When all three compartments started with the same initial rate constants, best fit λp values for phenanthrene and fluorene in the first experiment were 0.37 and 0.51 s-1. In the second, cooler experiment they were 0.27 and 0.43 s-1. Although lower (but of similar magnitude) rate constants were observed for the cool vs the warm experiment, we could not determine from the limited data if temperature, particle concentration, or errors associated with measurements were responsible for this observation.
Longer time scales have been suggested for higher molecular weight compounds such as chrysene and benz[a]anthracene (16).
Acknowledgments This work was supported by a grant from the National Science Foundation to the University of North Carolina (ATM 940848, Dr. Sherry O. Farwell project officer) and a gift from the Ford Motor Co. to the University of North Carolina, under the direction of Dr. Dennis Schuetzle. The valuable comments of Michael Strommen were greatly appreciated.
FIGURE 5. Model (line) fit to phenanthrene particle data (boxes) in the warm diesel experiment. Point at zero time is estimated and represents loss in the sampling denuder. In the model calculation, the predicted outlet concentration exiting the void volume was subtracted from the predicted concentration measured by the denuder sampling system. This gave an SOC particle loss for the sampling denuder. In the high concentration experiment, 12% of the particle phenanthrene and 14% of the particle fluorene were lost from particles as they traversed the channels of the sampling denuder. This loss is reflected in Figure 5, where at time zero, the loss in the sampling denuder is the y intercept. In the cooler and lower particle concentration experiment, denuder sampling losses of 8 and 12% for these two compounds were estimated. Results from these experiments are preliminary, and off-gassing rate constants were developed from particle concentrations that were much higher than those found in ambient atmospheres. One must be concerned from these observations, however, that a potential for such losses exists for low concentration systems, especially if long denuder residence times are used. The ability of particles to rapidly off-gas in the absence of a complementary gas phase (such as in the stripper units) may explain why under certain conditions Pankow and Bidleman and others (1-5, 15) have observed the atmosphere to be close to partitioning equilibrium. As particle emissions travel from their sources and dilute, they experience a “gaspoor” environment with respect to gas-particle equilibrium. The ability of freshly emitted diesel particles to rapidly compensate suggests that, in a short period (s to min) and at moderate temperatures, compounds in the volatility range of fluorene to phenanthrene may re-approach equilibrium.
Literature Cited (1) Yamasaki, H.; Kuwata K.; Miyamoto H. Environ. Sci. Technol. 1982, 4, 189-194. (2) Bidleman, T. F. Environ. Sci. Technol. 1988, 22, 361-367. (3) Pankow, J. F.; Bidleman, T. F. Atmos. Environ. 1991, 25A, 22412249. (4) Pankow, J. F.; Bidleman, T. F. Atmos. Environ. 1992, 26A, 10711080. (5) Kamens, R.; Odum, J.; Fan, Z. Environ. Sci. Technol. 1995, 29, 43-50. (6) Kamens, R. M.; Guo, Z.; Fulcher, J. N.; Bell, D. A. Environ. Sci. Technol. 1988, 22, 103-108. (7) Eatough, D. J.; Wadsworth, A.; Eatough, D. A.; Crawford, J. W.; Hansen, L. D.; Lewis, E. A. Atmos. Environ. 1993, 27A, 12131219. (8) Hangal, S.; Willeke, K. Environ. Sci. Technol. 1990, 24, 688-691. (9) Fan, Z.; Chen, D.; Birla, P.; Kamens, R. Atmos. Environ. 1995, 29, 1171-1181. (10) McDow, S. R.; Huntziker, J. J. Atmos Environ. 1990, 24, 25632571. (11) Gundel, L. A.; Lee, V. C.; Mahanama, K. R. R.; Daisey, J.; Stevens, R. K. Atmos. Environ. 1995, 29, 1719-1733. (12) Cheng, Y. In Air Sampling Instruments, 7th ed.; Hering, S. V., Ed.; ACGIH: Cincinnati, 1989; pp 405-419. (13) Reist, P. C. Aerosol Science and Technology, 2nd Ed.; McGrawHill, Inc.: New York, 1993. (14) Schwarzenbach, R.; Geschwen, P.; Imboden, M. Environmental Organic Chemistry; John Wiley and Sons: New York, 1993; pp 195-197. (15) Bidleman, T. F.; Billings, W. N.; Foreman, W. T. Environ. Sci. Technol. 1986, 20, 1038-1043. (16) Allen, J. O.; Dookeran, N. M.; Smith, K. A.; Sarofim, A. F.; Taghizadeh, K.; Lafleur, A. L. Environ. Sci. Technol. 1996, 30, 1023-1031.
Received for review December 31, 1996. Revised manuscript received February 18, 1997. Accepted March 4, 1997. ES961083F
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