Environ. Sci. Technol. 1993, 27, 2441-2449
Design and Evaluation of a Novel Diffusion Separator for Measuring Gas/ Particle Distributions of Semivolatile Organic Compounds Barbara J. Turpln, Shl-Ping Llu,t Kathryn S. Podolske, Marcos S. P. Gomes, Steve J. Elsenrelch,t and Peter H. McMurry'
Particle Technology Laboratory, Department of Mechanical Engineering, University of Minnesota, 111 Church Street SE, Minneapolis, Minnesota 55455 ~~~~~
A new gadparticle diffusion separation sampler was developed with the purpose of definitely measuring semivolatile organic compounds (SOCs) in atmospheric gases and particles. A laminar flow separator is able to separate a known fraction of the gas phase from the aerosol because gases diffuse orders of magnitude faster than particles. This fraction of the gas phase is collected downstream of the separator, extracted with supercritical fluid, and analyzed by gas chromatography/mass selective detection. The particulate phase is the difference between measured total and gas-phase concentrations. The separator performance agreed well with theory. Ultimately, we plan to use several separators in parallel to collect semivolatile organic compounds to further the understanding of gas/particle partitioning and the fate of semivolatile species in the atmosphere. Introduction The atmospheric lifetime and fate of pollutants depend in part on their distribution between gas and particulate phases. Many laboratory and field studies have been undertaken to better understand the gadparticle distributions of semivolatile organic compounds (1-1 1 ) . However, accurate assessment of gadparticle distributions is hampered by gas-phase adsorption on filter media and volatilization from collected atmospheric particles. The goal of this research was to develop a sampling system which is not subject to sampling artifacts. This system will be used to evaluate the accuracy of traditional gas/ particle distribution measurement techniques and to study gas/particle partitioning. Particles are typically collected on a high volume glass or quartz fiber filter. The gas phase is collected downstream of the filter on polyurethane foam (PUF), Florisil, XAD-2 macroreticular resin, or Tenax resin ( 1 , 2,4-9,11, 12). Unfortunately, gas-phase organic compounds adsorb on glass and quartz fiber filters, and this artifact can cause the particulate phase to be significantly overestimated. The magnitude of the adsorption artifact for total particulate organic mass has been recently investigated (1317). Adsorption of individual semivolatile organic compounds (SOCs) on filter media has been observed by Krieger and Hites (18)and Foreman and Bidleman (19). Changes in ambient air quality during sampling can also cause additional adsorption or volatilization of material from the sampling filter, resulting in a weighted average filter sample which gives more weight to ambient conditions toward th,e end of the sampling period. In addition, a pressure drop across the filter can induce volatilization
* To whom correspondence should be addressed.
t Gray Freshwater
Biological Institute and Department of Civil and Mineral Engineering, University of Minnesota. 0013-936X/93/0927-2441$04.00/0
0 1993 American Chemlcai Soclety
of organic compounds from the filter. Eatough et al. (20) have reported large volatilization losses from unspeciated semivolatile organic carbon. Schwartz et al. (21) interpreted differences in the composition of 6-h composite samples and concurrent 24-h samples as loss due to volatilization of moderately polar compounds. Volatilization losses of individual polycyclic aromatic hydrocarbons (PAHs) and other SOCs have also been reported (3, 17,22-261,and evaporative losses from impactor and filter deposits have been calculated theoretically (27). Sampling artifacts can also result from chemical reaction of gases with the filter or collected material. This has not been explored fully, but some oxidation reactions with collected PAHs have been investigated (28-33). Although it is recognized that gas- and particle-phase measurements of organics are influenced by sampling artifacts, the redistribution of compounds between the gas and adsorbed phases during sampling is very complex because of the interaction of the many species present including water vapor, and the effect of sampling artifacts on measurements of individual organic compounds is not well understood. These various sampling artifacts result in large gas/particle partitioning uncertainties. For this reason development of a new sampling system, not subject to sampling artifacts, is needed. Several investigators (3,17,18,20,34,35)have developed gas and particle sampling systems for SOCs which remove the gas phase using a denuder; the particulate phase is collected on a filter downstream of the denuder, and blowoff from the particulate phase is collected on a gas trap (Tenax, XAD, PUF, etc.) downstream of the filter. The particulate phase is considered to be the sum of concentrations from the filter and gas trap downstream of the denuder. A parallel sampler, operated without a denuder, gives the total compound concentration, and the gas phase is the difference between total and particulatephase concentrations. The denuder-based sampling system is clearly an improvement over the filter and adsorbent collection method, especially for measurement of the particulate phase. Residence times of denuders are short (0.1-3.2 s), minimizing the redistribution of SOCs between the gas and particulate phases. Although removal of the gas phase substantially increases volatilization losses from particulate material collected on the filter (27),the volatilized material is subsequently collected on an adsorbent and correctly assigned to the particulate phase. Several limitations of current denuder-based systems were discussed by Kaupp and Umlauf (36). The largest limitation is that denuder performance for a single compound depends on the concentrations of all the compounds present includingwater vapor because compounds compete in the adsorption process. Since atmospheric samples contain a multitude of gas-phase organic compounds, it is difficult to determine the compound adsorption effiEnviron. Sci. Technoi.. Vol.
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cienciesfor atmospheric sampling. Recognizing this, Lane et al. (34)evaluated the denuder collection efficiencies for lindane and hexachlorobenzene for low and high relative humidity (RH) and low and high temperature. A t 25 "C removal efficiencies were 98-100% regardless of RH, but at 41 O C efficiencies were 60439% (and possibly RH dependent) for hexachlorobenzene and less than 20% for lindane. Until quite recently extraction efficiencies for denuders have been quite low, and gas-phase concentrations have been obtained by difference instead of direct measurement. Progress toward direct extraction of denuder material is currently being made by Krieger and Hites (18),Gundel et al. (37),and others. High extraction efficiencies can now be obtained for several compounds after careful preparation and coating of denuders. Krieger and Hites (18) have had success with a technique to thermally desorb denuder-collected gases directly onto a high-resolution glass capillary chromatographic column. Like the denuder, the sampler described in this paper avoids volatilization and adsorption artifacts that affect filter measurements. Whereas denuder measurement systems measure the particulate phase directly, this sampling system measures the gas phase directly, resulting in a more accurate gas-phase measurement. Also, calculation of ambient concentrations from measured concentrations is dependent only on the diffusivity of the compound and is not complicated by the complexities of multicomponent adsorption. Because recent analytical developments are reducing the detection limits for the analysis of PAHs, low flow rate samplers (1-5 L/min) are now becoming practical. Advancements in detector technology and reductions in blank levels are ongoing. Direct coupling for atmospheric analysis will require the development of additional online sample cleaning procedures to avoid column deterioration, but ultimately will result in substantial reductions in detection limits. The low flow rate diffusion separator described in this paper can be used as part of a system of parallel separators for atmospheric sampling at current levels of detection. Separator Design
A schematic of the diffusion sampler is shown in Figure 1. The microetched, perforated stainless steel flowstraightening screen (Buckbee-Mearsscreen 2-1-8)shown
in the core flow inlet section is needed to dampen flow disturbances. A flow rate of 3 L/min is maintained through the separator with core and annular exit flows of 1.5 L/min. Clean, particle-free air is pushed into the separator through the core inlet at a flow rate set between 0 and 3 L/min, and ambient aerosol is drawn into the annular region to make up the balance of the 3 L/min flow. The optimal core inlet flow rate is 1.5-1.7 L/min. Clean air and ambient aerosol join downstream of the core inlet section and flow parallel to each other through a 21-cm-longdiffusion zone. As the aerosol travels through the diffusion zone, the gas phase diffuses into the core flow. The diffusion separator is able to separate and collect a fraction of the gas phase from ambient aerosol because gases diffuse orders of magnitude faster than particles. The separator is designed such that particles from ambient aerosol in the annular flow do not penetrate into the clean particle-free core flow, and yet the ambient gas phase in the annular flow has sufficient time to diffuse into the particle-free core flow. 2442
Envlron. Scl. Technol., Vol. 27, No. 12, 1993
Clean, particle-free air (Core Met) flow straightening screen
Ambient air
I
Core inlet section
-
holes
+
(Annular inlet)
,953 cm
t
annulus 5.72 cm
I21 cm
Diffusion Zone
Ii0q
splitter exit
Annular exit flow (1.5 Iprn)
Core exit flow (1.5 Ipm) Flgure 1. Schematic of diffusion separator including core Inlet sectlon and stainlesssteel flow-straighteningscreen. Total flow rate 18 3 Llmln, and several separators can be operated in parallel.
A known fraction of the gas phase, determined from the diffusivity of the gas, penetrates into the core flow and is collected on a PUF plug at the core exit. The adsorbed gas phase on each PUF plug is then extracted with supercritical fluid COZand analyzed by gas chromatography/mass-selective detection (GC/MSD). The ambient gas-phase concentration of each target compound is determined from this measurement. The remainder of the gas phase and the particulate phase exits in the annular flow. Measurements of the particulate phase from this port would be subject to the same uncertainties as a conventional filter-adsorbent sampler. The diffusion separator was designed with special attention to flow stability. The pressure drop over the annular inlet section (-0.14 g/(cm s2)) is greater than the dynamic head at the entrances to the separator (-1.1 g/(cm s2)) to assure that the annular flow will distribute evenly around the annulus. Although the bulk of the flow is laminar, local disturbances could result in substantial mixing of particles into the core flow. To avoid local disturbances, annular and core inlet flows converge at the tapered tip of the core inlet section. The Falkner-Skan solution (38) for stability of a flow past a wedge predicts that for a 1.5-lpm core inlet flow rate a taper of less than 1 8 O is needed to prevent the flow from separating from the wall; the critical Reynolds number (Re) for flow separation calculated with respect to displacement thickness (Re)is about 100. With this in mind, we designed the inlet section with a 15' taper (Re = 10-20).
We used an experimental approach to minimize flow mixing due to shear at the confluence of the annular and core inlet flows. Two inlet sections were built, one providing equal core and annular velocity gradients at the flow confluence (smaller diameter inlet section) and the other providing equal mean velocities across the core and annular flows (larger diameter inlet section). For both designs, separator Reynolds numbers were less than 100, and the Kolmogorov theory (39) predicted that if eddies were formed in the inlet or at the flow confluence, they would be immediately damped. In other words, eddies associated with turbulent energy generation and eddies associated with turbulent energy dissipation are of the same order of magnitude. Particle penetration tests showed that the larger, equal mean velocity inlet design is preferable. The poor performance of the smaller diameter, equal gradient inlet section suggests that when this inlet was used, the flow expanded from the smaller diameter, faster core inlet flow into the annular flow and eddies were formed at the confluence. The separator performance was fairly insensitive to the design of the flow splitter at the separator exit. We chose a design identical to the smaller diameter inlet section. Initial testing of the system without use of a flowstraightening screen showed that the growth of small disturbances in the diverging core flow inlet section resulted in substantial mixing in the separator. Insertion of the microetched stainless steel flow-straightening screen downstream of the diverging flow dampened those disturbances, dramatically improving separator performance. We simulated the separator flow field using an axisymmetric, laminar, steady-state, incompressible flow model and a finite differences partial differential equation solver (40). The separator was described by greater than 2400 control volumes which were more closely spaced near the annularlcore flow boundary (from inlet to exit). Since the model cannot identify unsteady disturbances, this exercise is only a tool to identify steady recirculation patterns. The model showed that the velocity profiles develop rapidly at the inlets; no flow separation or permanent recirculation zones were observed. Concentration profiles for gas-phase PAHs and 0.05pm particles were calculated by solving the diffusion equation for fully developed laminar flow,assumingsteadystate, incompressibleflow with a constant diffusivity.Since measured diffwivities for PAHs are generallynot available, we calculated diffusivities by the method of Fuller, Schettler, and Giddings (41). This method adds atomic diffusion volumes and adjusts for ring structure. It is believed to be accurate within 10% of measured values (41, 42). The resulting equation is
:>I
u -dc = D -I - rd- dc +[ r dr( dr) Zdz The solution to this equation is of the form
where @ = C/CO, 71 = rlro, and 5 = Z/Zchar. The characteristic length, Zchar, was taken to be equal to the mean speed multiplied by the characteristic time to diffuse to the wall, or Zchm = 2vZro2/D,where D is the diffusivity, ro is the radius of the separator, and uz is the flow velocity. This results in a separation of variables giving the following
,o 0.8 5
0.7
/
50-
/
z
I
I
00 00
'
' 01
__ z = 30 L__-___-
cm
--I--
C2
0 3 04 0 5 0 6 07 'raction of =low Ins de Rod us r
08
C9
1 0
Flgure 2. Theoretical concentration profiles describing the diffusion of gas-phase phenanthrene for z = 0, 10,20, and 30 cm. At z = 0, 100% of the gas-phase PAHs and partlcies are In the annular flow. In contrast no visiblechange in the concentratbn profileoccurs between z = 0 and z = 30 for 0.05-pm particles (not shown). The x axis describes the radius inside which the given flow fraction fails.
equation for the eigenvalues (Bn)and eigenfunctions (I?,,): 1
(;>'I..
(3) R," + -I?; 71 + 8,2[ (1- s2) + =o where Pe is the Peclet number. As boundary conditions
for gas-phase diffusion, the concentration profile at the inlet (CO) was given as a step function, the concentration at z = m was set equal to 0, and the concentration gradient was set equal to 0 at the wall and at r = 0. This equation was solved using a Runga Kutta scheme (43). The eigenvalues and other important quantities are given by Siege1 et al. (44). For the case of particle diffusion, the boundary condition at the wall is different (c(z,ro) = 0), resulting in different eigenvalues and eigenfunctions. These are given by Tan (45). The F,, are dependent on the inlet concentration profile. They were solved for a large Peclet number (>loo); for this case axial diffusion (diffusion in the z direction) can be neglected. Peclet numbers for particle diffusion are on the order of lo5. Peclet numbers for gas-phase diffusion of PAHs range from 90 to 129 for naphthalene and benzo[alpyrene, respectively. Thus, while the assumption that axial diffusion is negligible is not strictly valid for gas-phase diffusion, it should yield a reasonable approximation. The F, values satisfy the equation
at the inlet. For our problem, the inlet profile ~ ( 7=) 0 at the inlet of the core flow and y(9) = 1at the inlet of the annular flow. The program used to calculate the theoretical concentration profiles was a modification of a program written by Stolzenberg (46). Figure 2 shows theoretical normalized concentrations of phenanthrene at 25 "C between the center line ( r = 0) and the separator wall ( r = ro) for separator diffusion lengths ( z ) of 0, 10, 20, and 30 cm. The origin of z ( z = 0) is taken to be the flow confluence (see Figure 1). The radial location is described in Figure 2 by the fraction of the flow falling inside radius r. A t z = 0, the core flow is composed of particle-free clean air and the annular flow is ambient air containing particles and gases. The diffusivity of 0.05-pm particles is 2.4 X 10" cm2/s,whereas the diffusivity of gas-phase phenanthrene is 0.054 cm2/s, Envlron. Sci. Technol., Vol. 27, No. 12, 1993 2443
1:
100
0.433 0.390 h
E 0.346 " 2 0 303
.......................
II
30.260 L
2 0.21 6 $0173 0.1 30
......... Napthalene Phenanthrene Benzo-a-pyrene
L
.g 0.057 0.043
oooot 0
'
' 4
'
' 5
'
' 12
'
' ' ' ' ' ' ' 16 20 24 25 Temperature C
'
' 32
'
' 36
'
-
40
-
10
10
40
Flgure 3. Theoretical calculations of diffusion factors as a function of temperature for naphthalene, a low molecular weight PAH; phenanthrene, a mldrange molecular weight PAH; and benzo[a]pyrene, a high molecular weight PAH. The theoretical diffusion factors give the fraction of the ambient gas phase whlch Is collected downstream of the separator in the core exit flow assuming no flow mixing. Diffusion factors were calculated for a separator whlch has a dlffusion zone 21 cm In length and a cwe Inlet flow rate of 1.7 Llmin. For such a flow configuration, complete diffuslon corresponds to a diffusion factor of 0.433. The percent of complete diffusion Is shown on the right.
3 orders of magnitude greater. When z = 20 cm, the diffusion of gas-phasephenanthrene into the core exit flow is 67 % complete. In contrast, significant (>5%) diffusion of 0.05-pm particles does not occur until z > 3000 cm. (At the separator lengths shown in Figure 2, the concentration profile for 0.05-pm particles is identical to that shown for phenanthrene at z = 0.) Therefore, diffusion of particles into the core flow is negligible; avoiding the mixing of clean and ambient flows was the most critical design challenge. The theoretical diffusion factors for three representative PAHs are shown as a function of temperature in Figure 3. The diffusion factor (DF) is the fraction of the ambient gas phase which is collected downstream of the separator in the core exit flow. The calculated theoretical diffusion factors assume no flow mixing and therefore describe the extent of diffusion into the core flow. Diffusion factors will be used to calculate the ambient gas-phase concentration (CA) from the measured concentration in the core exit flow (CJ:
CA = C,/DF
(5)
The diffusion factors of Figure 3 are calculated for a separator which has a diffusion zone 21 cm in length and a core inlet flow rate 1.7 L/min. Since the total separator flow is fixed at 3 L/min, the annular inlet flow rate for these calculations was 1.3 L/min, and the core exit flow concentration after complete diffusion would be 43.3 % of the ambient concentration. In this separator, diffusion of water vapor is complete, but complete diffusion of naphthalene and higher molecular weight PAHs would require longer residence times. The figure shows calculated diffusion factors for three PAHs: naphthalene, a low molecular weight PAH found mostly in the gas phase; phenanthrene; and benzo[alpyrene, a high molecular weight PAH found almost entirely in the particle phase. The major uncertainty in the calculated diffusion factors is the 10 % uncertainty in the calculated diffusivities. This translates to about a 17% uncertainty in the diffusion factor. Figure 3 shows that atemperature change between Oand 40 OC results in about 10%variation in the diffusion 2444
Envlron. Scl. Technol., Vol. 27, No. 12. 1993
factor. Because this variation is small, we can calibrate the separator with diffusion factors measured at a single temperature. Temperature adjustments can be made based on theory. It is possible that volatilization of organic compounds from atmospheric particles could occur in the air stream during the 2.6-sresidence time in the diffusion zone, where particles are exposed to reduced gas-phase concentrations. The evidence below suggests that the characteristic times required for such redistribution are long relative to the transport time in the separator. As a lower limit, we calculated the characteristic time for redistribution assuming that diffusion-limited mass exchange is the ratelimiting step. If "intraparticle" transport or surface desorption is slower than gas-phase diffusion, longer characteristic redistribution times would result. The flux of molecules away from the particle surfaces due to diffusion is (47)
dc dt = -2rD$(c
- ceq) X (1
1+Kn + 1.71Kn + 1.333Kn2)NW,)
dD, (6)
where Kn = 2110, is the Knudsen number, I is the mean free path of gas, D, is the particle diameter, D is the diffusivity in air, N is the no. of particles/volume, c is the concentration (molecules/volume), Ceq is the gas-phase concentration at particle surface, and, il(D,) = n(D,)/N is the normalized number distribution. This transitionregime expression is valid for the entire range of Knudsen numbers. Gas-phaseconcentrations are reduced by about a factor of 2 as a result of diffusion into the clean core flow. In order to estimate the maximum impact of redistribution between gas and particulate phases, we assume that this dilution occurs instantaneously at the point where the aerosol and clean core flows merge, so that c(0) = c,/2. Integrating over D, and solving for C(t), c(t) = ceq - c.9 2 exp[-2rDNIt]
(7)
where 1+Kn
)DpND,) dD,
I = Lm(l+ 1.71Kn + 1.333Kn2
(8)
The characteristic time to achieve redistribution is
We calculated characteristic times using normalized data from a grand average of 93 number distributions (0.000875-2.22 pm) measured between 0600 and 1200 Pacific daylight time in Pasadena, CA, in 1969 and published in Whitby et al. (48). The calculations assume that the SOC size distribution is comparable to the size distribution of the aerosol. For naphthalene (D= 7.02 X lo4 m2/s)and benzo[alpyrene (D = 5.13 X 10-8 m2/s) and concentrations typical for a clean continental air mass ( N = lo3~ m - ~characteristic ), time scales for diffusion-limited mass exchange were 9 X lo2and 1.3 X lo3s, respectively. For a polluted urban environment (N = lo6 cm-9, they were 9 and 13 s, respectively. If redistribution were limited by gas-phase diffusion, redistribution would only be important during the 2.6-9 transport through the separator if
particle concentrations exceeded 4 X lo6 cm-3, Although Larson and Taylor (49) demonstrated that the loss of material from NHdN03 particles exposed to reduced gasphase concentrations was limited by gas-phase diffusion, Gerde and Scholander (50) predicted that the time scale for gas-phase diffusion and adsorption of PAHs onto clean particles is 2 orders of magnitude smaller than the time scale for compound diffusion within the particle. According to these authors, redistribution of PAHs would be limited by the rate of “intraparticle diffusion”. Gerde and Scholander (50) predicted a characteristic time of about lo6 s to reach equilibrium between gas-phase PAHs and clean particles. Rounds et al. (52) measured blowoff rates from particle-loaded filters exposed to a flow of clean nitrogen. They found that the measured diffusion-reaction time scales were about 106 longer than that predicted by a model in which intraparticle, sorption-limited diffusion takes place through air-filled pores in the particles. Unless preferential flow paths had developed in the filter, this suggests a slower process of intraparticle diffusion analogous to diffusion through a liquid or solid. When the effective intraparticle diffusivity suggested by the experiments is used in the theoretical calculations, diffusionreaction time scales are on the order of 600 s for 0.009-pm particles and 106 s for 0.25-pm particles. These results suggest that the kinetics of gas/particle partitioning are slow relative to the transport time in the separator. For these reasons we expect redistribution in the separator and the collected gas phase to be negligible. Although adsorptive losses of PAHs in the separator are expected to be small, we have not yet investigated their effect.
Performance Testing Penetration of particles from the annular to the core flow was evaluated by sampling an aerosol from a variablevolume polyethylene sampling chamber and counting the particles in the core and annular exit flows using two TSI 3760 condensation nuclei counters (CNC). For all experiments core and annular exit flows were both maintained at 1.5 L/min with critical orifices, and the clean, particle-free, core inlet flow was varied from 0 to 3 L/min under the control of a calibrated laminar flow meter and Magnehelic pressure gauge. The annular inlet drew aerosol from the sampling chamber to provide the balance of the 3-L/min flow. Three types of aerosol were supplied to the sampling chamber: (1)partially filtered ambient laboratory aerosol, (2) polydisperse sodium chloride aerosol, and (3) monodisperse sodium chloride aerosol. The laboratory aerosol was partially filtered in order to supply particle number concentrations within the response range of the CNCs. The sodium chloride aerosol was prepared by atomizing a 0.01 76 aqueous solution of sodium chloride with a Collison atomizer and drying with a silica gel diffusion drier. For a monodisperse aerosol the atomized sodium chloride aerosol was charged using a low-ionconcentration Ni-63 charger (52) and an aerosol of known size was selected using a differential mobility analyzer (53). As mentioned earlier, the addition of a microetched stainless steel flow-straightening screen in the core inlet section reduced particle penetration dramatically. With the addition of the flow-straightening screen we achieved excellent performance for the larger, equal mean velocity inlet section, and therefore this inlet design was selected
3.’
-
I
i
c o r e ,?le- f o w r0.e ( l p l :
Figure 4. Particle penetration into the core exit flow (number in core/ total number) as a function of the core inlet flow rate for the optimal separator design. Experimental results for a polydlsperse aerosol (squares)and theoretical performance assuming no diffuslon (he) are shown. Performance for 0.06- and 0.41-pm aerosols (not shown) were essentially identical.
over the smaller, equal velocity gradient inlet section. Figure 4 shows particle penetration (number in core/total number) as a function of the core inlet flow rate for the selected design. The squares indicate experimental data for the polydisperse laboratory aerosol, and the line shows the ideal separator performance. The actual performance is very close to ideal, with 50 % particle penetration at 0 L/min and less than 1%particle penetration at a core inlet flow rate of 1.5 L/min. (Ideally particle penetration would be 0 at 1.5 L/min.) At 1.7 L/min the particle penetration was 0.36 % . The performance of the separator for monodisperse 0.06- and0.41-pm aerosolswas essentially identical to the performance for polydisperse aerosol. The excellent performance of this prototype separator suggests that we might be able to extend the length of the diffusion zone, to allow more complete diffusion of PAHs and eliminate the dependence of the diffusion factor on diffusivity for at least some PAHs. We used the experimental apparatus of Figure 5 to find the diffusion factors for several PAHs. Gas-phase PAHs were collected on polyurethane foam (PUF) plugs 5.5 cm long and 1.5 cm in diameter. Ultrapure carrier (UPC) air was further purified by adsorption of gases on two consecutive PUF plugs (PUF1 and PUF2), and any particles present in the UPC air or generated in the flow control devices were eliminated by a 47-mm Gelman QAOUP quartz fiber filter (Ql) downstream of the PUF plugs. This formed the clean core inlet flow, which was set at 1.7 L/min for all diffusion factor calibration runs. PUF2 was analyzed for PAHs and served as a clean air blank. A 47-mm quartz fiber filter (Q2)was placed upstream of the annular separator entrances, and 1.3 lpm of particle-free air containing typical ambient concentrations of PAHs was drawn into the annular inlet. Field samples were taken outside the Particle Technology Laboratory in Minneapolis, MN, and conditioned to approximately 25 OC. Two PUF plugs in series (PUF3 and PUF4) collected gases at the annular exit and two more (PUF5 and PUF6) at the core exit. PUF4 and PUF6 were used to check for breakthrough. PAH concentrations from the annulus and core were used to calculate the diffusion factors. In Envlron. Scl. Technol., Vol. 27, No. 12, 1993 2445
Flgure 5. Schematic showing experimental setup for diffusion factor calibration. PUF = polyurethane foam plug, QF = quartz fiber, UPC = ultrapure carrier.
addition to the clean air blank (PUFB), a field blank was assigned for each calibration run and traveled with the calibration samples. PAH mass measurements which were below the detection limits (3aof blank) and data for PANS with greater than 25% breakthrough were not used. Breakthrough was defined as
breakthrough,,,e,,uF,
-- PUFG PUF5 + PUFG
This criterion excluded naphthalene through acenaphthene. With XAD-2, a more retentive adsorbent, less breakthrough occurred but blank values were substantially higher. We are exploring the use of EMPORE disks (3M, Inc.) to expand the range of PAHs we can measure while keeping detection limits low. Prior to sampling, the experimental apparatus was cleaned by sonicating for 30 min in a mild liquinox solution and 30 min in deionized water and rinsing with dichloromethane to remove machining residues. The quartz fiber filters were baked a t 600 "C for 2 h. The PUF plugs were Soxhlet extracted for 24 h twice in acetone and twice in petroleum ether. They were stored in glass bottles at 0 "C until use. All solvents used in this study were pesticide grade (Baxter Co., Chicago, IL). Three background samples were collected by flowing 3 L/min of clean air into the core inlet for 4, 20, and 24 h. These runs showed that background PAH concentrations downstream of the diffusion separator were indistinguishable from the blank, and therefore no contaminants were present in the separator. We performed a particle penetration test before and after each calibration run to check the performance of the separator. A rigorous flow control and leak-testing routine was also performed. Six 20-24-h samples of particle-free ambient air were collected outside the Particle Technology Laboratory to serve as diffusion factor calibrations. 2446
Envlron. Sci. Technol., Vol. 27, No. 12. 1903
The PUF plugs were stored at 0 "C immediately after collection. Each PUF sample was extracted with 60 mL of supercritical C02 fluid modified with 5 % methanol at 4500 psi and 70 "C. The supercritical fluid extractions were performed with a syringe pump (ISCO, Inc., Model 260D, Lincoln, NE) and a 4.95-mL Keystone vessel extractor (Keystone Scientific Co., Bellefonte, PA). The supercritical extract was then depressurized using a 50pm-i.d., 10-cm-longcapillary column (PolymicroTech, Inc., Phoenix, AZ) and was collected in 5 mL of hexane. Methanol was first removed by partitioning to 6 mL of deionized water. The extract was concentrated to 1mL by nitrogen blowdown and then eluted with 5 mL of 10% methylene chloride in hexane from a cleanup column consisting of 0.2 g of sodium sulfate and 0.8 g of 5% deactivated silica acid (BIO-SIL A 100-200 mesh, BIORAD, Richmond, CA). This column removed polar and high molecular weight contaminants from the air sample and PUF matrix. The elution was further concentrated to 50 FL under a purified nitrogen stream. Five deuterated internal standards (20 ng of dlo-acenaphthene, dlophenanthrene, dlo-pyrene, dlz-benzo[eIpyrene, and d ~ z benzo[ghi] perylene) were added after nitrogen blowdown for PAH quantitation. The concentrated extract was analyzed by a HewlettPackard 5890 gas chromatograph equipped with a 5970 mass spectrometer operated in selected ion monitoring mode. Separation was achieved with splitless injection at 290 "C into a 30-m-long, 0.25-mm-i.d., DB-5 chromatographic column (J&W Scientific, Inc.) with a 0.25-pm film thickness and a linear velocity of 26.5 cm/s at 290 "C. The oven temperature increased from 50 to 290 "C at a rate of 10 "C/min, and it remained at this temperature for 10 min. For each calibration run, known amounts of target compounds and deuterated surrogate PAHs (20 ng of denaphthalene, dlo-fluorene, dlo-fluoranthene, and d12perylene) were spiked into a PUF blank for an analytical recovery analysis. Over the course of the calibration runs, five analytical recovery analyseswere performed to monitor recovery efficiencies over the entire chemical analysis procedure. Absolute recovery efficiencies for all target PAHs were determined by dividing the recovered masses of target PAHs in the five spiked PUF samples by the masses injected into the samples. Figure 6 shows the means and standard deviations for the absolute recovery analyses. The absolute recoveries are reasonably good (most 70-80% 1. Low molecular weight PAHs have lower recoveries due to their high volatility, but the surrogate PAH recoveries well represent the selected PAHs in any given molecular weight range. These PAHs were corrected by the surrogate compound recovery efficiencies to yield relative recovery efficiencies as shown for the average of the calibration samples in Figure 7. PAH mass measurements falling below the detection limits and PAHs with greater than 25 9% breakthrough were eliminated from the calibration data sets. Breakthrough was typically a problem for PAHs with molecular weights below about 160. Limits of detection typically ranged from 20 to 50 pg of injected mass for all PAHs. Figure 8 summarizes the results of six calibration runs. The circles represent the average experimental diffusion factors for each PAH. The experimental diffusion factor is a function of the residence time in the separator, the degree of mixing between annular and core flows, and the
loo 90
9 40
2o
t
1
i
Figure 6. Average analytical recovery efficiencies for six caiibration samples. Error bars are 1u of the mean and show the variation in the six calibration experiments. Compoundsthat were not weii represented by the surrogates were excluded from the analysis at this point. Compounds from left to right are &naphthalene, naphthalene, %-methylnaphthalene, acenaphthylene, acenaphthene, &fluorene, fluorene, 1-methylfluorene, phenanthrene, anthracene, 2-methylphenanthrene, 4,5-methyiphenanthrene, 1-methyiphenanthrene, dimethylphenanthrene, &fluoranthrene, fiuoranthene, and pyrene. 120
I
Figure 7. Average PAH recoveries corrected by the surrogate compound recovery efficiencies for the six calibration samples. Error bars are 1u of the mean. PAH mass measurements failing below the detection limits and PAHs with greater than 25% breakthrough were eliminated from the data set.
diffusivity of the species, which depends on temperature. The solid squares represent the theoretical diffusion factors assuming diffusion but no mixing. The DF for each PAH was calculated from the blank-corrected concentrations of PUF plugs 3-6: DF =
(PUF5 + PUFG - 2B) (12) (PUF3 + PUF4 + PUF5 + PUFG - 4B)
where B refers to the average blank. The error bars on the experimental results are 1 standard deviation from the mean and show the variation of the experimental data. The uncertainties in the theoretical values are about 17 % and are dominated by the uncertainty in the PAH diffusivity. The agreement with theory is quite good. The uncertainty in the measured diffusion factors is dominated by the analytical uncertainties and can be further reduced
15 t
10
l5
Figure6. Experimentaldiffusion factors (circles) and theoretical diffusion factors assuming no flow mixing (soild squares). Error bars are 1u of the mean and show the variation in the six calibratlon experiments. Uncertainties in theoretical diffusion factors are about 17 % due to the 10 % uncertainty in the calculated diffusivities.
by longer sampling times or by operating several diffusion separators in parallel. Conclusions We developed a low flow rate sampling system for the purpose of avoiding redistribution between the gas and particle phases during the sampling of semivolatile compounds such as PAHs and polychlorinated biphenyls (PCBs). The main components of the sampling system are diffusion separators, operating in parallel, which facilitate collection of a fraction of the gas phase for analysis by supercritical fluid extraction and gas chromatography/ mass selective detection. This gas phase measurement system will be combined with a measurement of the total compound concentration, and the particle-phase concentration will be measured by subtraction. Although increasing sampler flow rates would reduce the fraction of the gas phase in the core exit flow, shorter collection times can be achieved by operating several separators in parallel. The separator performance is highly dependent on the design of the core flow inlet section, and the use of a flow-straightening screen to dampen flow disturbances was essential. When the design was optimized, particle penetration into the core exit flow was negligible. The diffusional transport of gas-phase PAHs from the annular flow containing ambient gas and particle concentrations to the clean core flow agreed well with theory. A longer separator is currently being tested. Continued improvements in flow stability might make it practical to build a separator long enough to allow complete diffusion of at least some PAHs. When the separator is used for ambient sampling, care must be taken to assure that annular and core flows enter the separator at the same temperature. Particle penetration into the core exit flow is a measure of flow mixing and should be measured as a routine quality control test. We will use this sampling system to evaluate the magnitude of sampling artifacts encountered with traditional samplers and to perform field and laboratory studies to further the understanding of gas/particle partitioning of PAHs and PCBs. Envlron. Scl. Technol., Vol. 27, No. 12, lS93 2447
Acknowledgments This research was sponsored in part by t h e Environmental Protection Agency (Grant EPA/R 816672-01-0) and t h e University of Minnesota Supercomputer Institute. Nomenclature C
CA cc CO
Ceq
CNC D DP DF DMA
Fn GC
I Kn 1 MSD
A(Dp) N PAH PCB Pe PUF r r0 Rll
Re RH SOC t UPC VZ
Z
zchar
Pn
Y(9)
9
CL P
9 ?char
F
gas-phase concentration (molecules/cm3) ambient gas-phase concentration (ng/cm3) gas-phase concentration in core exit flow (ng/cm3) inlet gas-phase concentration equilibrium gas-phase concentration a t the particle surface (molecules/cm3) condensation nuclei counter diffusion coefficient (cm2/s) particle diameter (pm) diffusion factor ( C c / C ~ ) differential mobility analyzer numeric constants that satisfy eq 4 for the given inlet conditions gas chromatography integral described in eq 8 Knudsen number (21/0,) mean free path of gas (pm) mass selective detection particle number distribution (no. of particles/(m3 pm)) n(D,)/N particle number concentration (no. of particles/ cm3) polycyclic aromatic hydrocarbon polychlorinated biphenyl Peclet number (2rouJD) polyurethane foam radial distance from center line separator radius (cm); ro = 1.410 cm for prototype eigenfunctions of eq 3 Reynolds number (2rovplp) relative humidity semivolatile organic compound time ultrapure carrier gas axial flow velocity (cm/s) axial distance from flow confluence characteristic axial distance, which is the mean speed times the characteristic time to diffuse to the wall; 2ur02lD eigenvalues of eq 3 inlet gas- or particulate-phase concentration profile atz=O dimensionless radial variable, r/rO viscosity density dimensionless concentration, c/co characteristic time for gas-phase concentration (c) to reach the equilibrium gas-phase concentration at the particle surface (ceJ dimensionless axial variable, Z/Zchar
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