Environ. Sci. Techno/. 1995, 29, 43-50
Some Observations on Times to Equilibrium for Semivolatile Potycydic Aromatic Hydrocarbons RICHARD KAMENS,* JAY ODUM,+ AND ZHI-HUA FAN Department of Environmental Sciences and Engineering, University of North Carolina, Chapel Hill, North Carolina 27599
The relative time scales at which semivolatile gas and particle PAH approach equilibrium under both moderate and cool temperatures were investigated. Combustion particles were added directlyfrom a diesel car and a wood stove to a 190 m3 outdoor Teflon film chamber. The rate of migration of a gas-phase semivolatile PAH to combustion particles at a warm outdoor temperature was explored by volatilizing solid deuterated pyrene (dlO-py) in a hot injector (200 “C) into the rural background air in the chamber atmosphere. After 2 h, diesel exhaust was added to the chamber. Results show an initial rapid migration of dlO-pyfrom the gasto the particle phase in an attempt to re-establish equilibrium. The relative closeness to equilibrium was monitored by calculating the equilibrium constant Kpovertime as measured by PAHpad (PAHga, x TSP). As gas-phase PAH concentrations changed in the chamber, due to wall losses, particle off-gassing occurred so that Kpwas reasonably constant over time. Under cool outdoor conditions (-1 to -4 “C), PAH loss from the particle phase could not keep up with gas-phase PAH wall losses, and the system departed from equilibrium. Kinetic simulations suggested that tens of hours would be required to reestablish 90% of equilibrium concentrations for compounds like phenanthrene and pyrene once they had departed from equilibrium in the particle phase by 34 and 18%, respectively.
Introduction The phase in which a semivolatile organic exists in the atmosphere will strongly influence its atmospheric fate. Over the past 15 years, much of the research on the atmospheric gas-particle distributions of semivolatile compounds has been based on Langmuirian gas-solid equilibrium theory (1-9). Three important relationships have been developed. The first by Junge in 1977 (1) estimated the fraction of a semivolatile compound (4) in the “particle phase” as a function of saturated vapor pressure (Pin Torr), the concentration of aerosol surface area (Oj in cm21 cm3 air), and a constant cj:
4 = ejcj/(p+ ejcj)
(1)
Junge started with incomplete particle coverage of adsorbing semivolatiles. Under these conditions, the Langmuirian isotherm becomes a special case of the more general BET isotherm of Brunauer, Emitt, and Teller (10). Pankow (5) evaluated the constant c, such that c, = 760 hRnV,, where h is the BET constant or approximately e(Qd-QJ’)IRT, R is the gas constant in kcal/(deg mol) in the BET term and in cm3atml (degmol) in the preexponential term, T is the absolute temperature in K, Q d is the heat of desorption from a surface in kcal/mol, Qvis the heat of vaporization of the pure substance in kcallmol, and Nsis the moles of sorption sites per cm2 of aerosol. The impact of saturated vapor pressure and available particle surface area on gas-particle phase equilibrium partitioning can be illustrated with this relationship. For example, estimates of the aerosol fraction for three polycyclic aromatic hydrocarbons (PAHs),phenanthrene (Phe), pyrene (Py),and benzo[alpyrene(BaP),using three different particle concentrations, 10,100, and 500pglm3,and three different particle sizes (assumed here for simplicity to be monodispersed) are shown in Table 1. A difference of 3 kcallmol for (Qd - Qv), as suggested by Pankow (.), and subcooled liquid vapor pressures (PLo), as originally suggested by Bidleman et al. (3), were used. The influence of particle surface area is most apparent for compounds like pyrene, which have vapor pressures in the range of Torr. At very low particle concentrations, these compounds are almost entirely in the gas phase, while at very high particle concentrations (500pg/m3)they appear mostly on particles. A second relationship based on equilibrium theory was reported by Yamasaki and co-workers (2) in 1982. They used Langmuirian adsorptionldesorptiontheory to describe the gas-particle distribution of urban PAH as a function of temperature and aerosol concentration (TSP):
log Z$ = log (C,I(C,ITSP))= - ~ ( 1 / 2 J + b
(2)
The Yamasaki et al. term { C,/ (C,lTSP)} may be thought of as a partitioning or equilibrium constant, where C, is the gas phase concentration and C, is the particle phase concentration. TSP is assumed to be directly related to the * To whom correspondence should be addressed e-mail address:
[email protected]. t Present address: Environmental Quality Laboratory, Caltech, Pasadena, CA 91125.
0013-936W95/0929-0043$09.00/0
0 1994 American Chemical Society
VOL. 29, NO. 1, 1995 / ENVIRONMENTAL SCIENCE & TECHNOLOGY 43
TABLE 1
Fraction in Aerosol Phase at Different Aerosol Concentrations at 25 "C aerosol concn bg/m3)b
$ pyrene $ BaP henanthrene ($1 x I W 4Torr). (3.3 x Torr)' (9 x lo-* Torr)a
10pg/m3(0.5) IO0 pg/m3(0.35) 500 pg/m3(0.25)
0.002
0.03 0.32 0.77
0.03 0.17
0.93 0.99 1 .oo
a Subcooled saturated vapor pressure at 25 "C computed from ref 23; particledensity= 1.25. Size in micrometers is given in parentheses.
Langmuirian number of sites on the aerosol surface, a necessary assumption in the derivation of this relationship, but a significant oversimplification for urban particulate matter. Pankow (5,s)later showed that compounds of the same class have almost the same y-intercept band that the SlOpea= -Qd/2.303R - Ta,b/4.606,where QdiSthe enthalpy of desorption. The third relationship illustrated the association between partitioning of a class of semivolatiles and their subcooled liquid vapor pressures, PL' (3-7):
+
log (K,) = log (Cp/(Cg x TSP)} = -m log (PLo) b (3) where the slope at equilibrium, as predicted by Pankow and Pankow and Bidleman, takes on a value of - 1 (5- 7, 9). The term Kp (Kp=Ky - 1)is also an equilibrium constant between a concentration of a semivolatile, C, in the gas phase, which interacts with a concentration of adsorption sites [assumed to be proportional to TSP) on the particle phase to yield a concentration of Cp in the particle phase. This equilibrium then represents the following process: C,
+ sites
kon
Cp
(4)
Pankow (11) has recently shown for liquid particles (absorption) how log Kp can have the same functional relationship to log P L O as for adsorption processes (eq 3). In case of absorption, the constant in eq 3 contains the reciprocal of the activity coefficient for a semivolatile molecule in a particle liquid phase. This has considerable significance because there is mounting evidence that diesel and wood combustion particles at moderate temperatures have an exterior liquid organic layer (11-14). All of the above relationships can be applied to atmospheres that are in equilibrium with respect to the partitioning of semivolatiles. Unfortunately, however, these relationships do not give a sense of the time scales that are required to establish equilibrium. In this paper, we explored the time that it takes for semivolatile PAHs to approach equilibrium under both moderate and cool temperatures.
Experimental Section A 190-m3and a 25-m3 outdoor 5-mil Teflon film chamber
located at the University of North Carolina's (UNC) smog chamber facility in Pittsboro, NC, (12,15,16) were used for this study. The chamber leak rate, including sampling in the 190-m3chamber, was about4-5% per hour as measured with sulfur hexafluoride (SF6) as a tracer. Combustion emissions were added directly as a single injection to the chambers under darkness from either a diesel car (16) or a wood stove (12). Before entering the 190-m3chamber, emissions passed through a W5charge neutralizer so that fine charge-free particles enter the chamber. Chamber 44
1
ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 29, NO. 1. 1995
initial particle concentrations ranged from 500 to 1300pgl m3. Half-lives of fine particles in the chambers ranged from 11 to 13 h. Rural North Carolina air was used to purge the chamber before the start of an experiment. Background aerosol concentrations range between 10 and 30 pg/m3. Chamber background particle PAH were generally below 5 ng/m3. Background gas phase naphthalene ranged between 50 and 150 ng/m3. After the addition of combustion emissions, chamber concentrations were 1-2 orders of magnitude higher than background levels. Chamber particles and gases were drawn through a vertical 3 m x 15 mm i.d. glass manifold (1.5 m of this extended above the chamber floor) and passed into the sampling lab under the chamber. The sampling manifold between the chamber and the laboratory was insulated with 1 in. of glass fiber insulation. Two sampling systems were used. The first was a conventional 47-mm filter cartridge (with a Teflon-impregnated glass fiber filter, PallflexT60A20) followed by a 9 x 4 cm polyurethane foam (PUF)cartridge. It was operated at a flow of 18-20 Llmin. The second sampling system consisted of a novel semivolatile glass denuder followed by a 47-mm Pallflex Teflonimpregnated glass fiber filter, which was followedby another denuder. This type of denuder was recently developed by Gundel and co-workers (17) and employed the direct coating and adhesion of submicron XAD particles on the sandblasted walls of the denuder without any additional binders or glues. After each sample Gundel et al. removed XAD from the denuder wall via sonication (17). The sampling denuders used in this study were 40 cm long (University Research Glassware, Carrboro, NC) and consisted of five annular channels spaced 0.1 cm apart. The inside wall of the outermost channel had a diameter of 2.4 cm. XAD-4 resin particles (Supelco, Bellfonte, PA) were ground with a standard laboratory mortar and pestle, sieved through 400 wire mesh screen, and finally Soxhlet extracted. A slurry of the fine XAD particles in hexane was then added to avertically positioned denuder and permitted to drain at a flow of -8 mL/min. This was repeated six times. Sample flow rates were typically about 20 L/min to maintain laminar flow inside the denuder. At this flow, residence time in the denuder was 0.3s. Samplingdurations of 20-30 min were used. After a new XAD coating, three to six sampling-rinse cycles were necessary to ensure that XAD particle loss during sampling and extraction was not significant. We reported preliminary results that indicated the denuders can be used for aminimum of 20 times without recoating and without a loss in collection efficiency (9599%) for compounds in the volatility range of acenaphthylene through pyrene (16). In this study, characterization tests showed that 100% of chamber gas phase deuterated pyrene appeared on the first 40 cm of the denuder. Denuder samples were solvent extracted directly in the field by rinsing the denuder four times with 30 mL of a 50:30:20 volume mixture of hexane-acetone-dichloromethane. This was done by capping the denuders after adding the rinse solvent and rotating it back and forth in an axial direction 20-30 times. Prior to denuder solvent extraction, 5 p L of a deuterated PAH internal standard mixture &naphthalene, dlo-phenanthrene, dlo-fluoranthene, d12-chrysene,and d12-BaP)were added to the top of the denuder. These compounds covered the volatilityrange of the PAHs sampled from the chamber. After extraction, the walls of the denuder were immediately dried with a dry nitrogen stream, and the denuders were reused without
recoating. Before storage, complete nitrogen drying of the denuder walls was necessary, because residual solvent tended to breakdown the small glue spots used to hold the channels of the denuder in place. At the flow rates used in this study, we observed a -5% lower particle concentration collected on the denuder system filter compared to the fdter in the conventional fdter-PUF system. Other estimates from our experiments using amounts of deuterated pyrene on particles collected on denuders suggested 5-10% particle loss. PAH gas and particle data were corrected for a 5%particle loss to the denuder. We assumed that light compounds (naphthalene through pyrene) on particles collected on the denuder walls were efficiently extracted. Higher molecular weight compounds like BaP were not extracted because we did not find detectable concentrations in the denuder extracts. After continual use, "clean" solvent-extracted denuders sometimes exhibited high hank values in the extraction solvent. This is especially true for phenanthrene, pyrene, and fluoranthene after exposure to ve'y high gas phase concentrations such as those found in direct diesel source sampling. Constant monitoring of extraction blanks is necessary. After sampling, filters were transported hack to UNC in glass 28 x 50 mm jars with Teflon-fitted screw caps at 0 "C temperatures. Samples were extracted within 24 h from the time of collection. Deuterated naphthalene, phenanthrene, fluoranthene, chrysene, and henzo[alpyrene in dichloromethane were added to filtersin Soxhlet extractors just prior to solvent extraction. Filters and PUF samples were extracted in Soxhlet extractors with MeClz for 12-18 h. Filter solvent extractswere reduced to 1OOpLwith microSnyder columns and a dry gentle nitrogen stream. PUF and denuder extracts were concentrated by rotary evaporationto2mLandfunherconcentratedhyanitrogenstream to 100 pL. Analyses were performed directly on the unfractionated extract. PAH were separatedwitha Hewlett Packard 5890(II) gas chromatograph on a J&W30-m DB5 column and analyzed with a Hewlett Packard 5971A MSD mass spectrometer. Identities were referenced to authentic standards of PAH in SRM 1647b available from the US. Institute of Standards and Technology (NIST). Repeated analysis of a 1:20 diluted 1647h sample on this system gave a precision of 3Z5% PAH (2 relative standard deviations). For this study, eleven 0.5-3-mg diesel soot particle samples (NTIS SRM 16501taken through the entire workup scheme gave PAH precisions (2 RSD), which were generally 325% orless (phenanthrene He%,fluoranthene &9%,BaAHl%, chrysene *7%, henzofluoranthenes fl1%, BaP +25%, and indeno [1,2,3-cdlpyrene 3~16%).
Results and Discussion Migration h m Gas to the Particle Surface under Moderate Temperatures. The rate of migration of a gas phase PAH to the particle phase was monitored by vaporization of deuterated pyrene (dlO-py) into the background air of the 190-m3chamber at 920 PM on a warm July evening. After the addition of dl0-py to the chamber air, a small amount of dl0-py appeared on chamber background air particles. At 1210AM, diesel exhaust was added directlyto the chamber such that an initial concentration of 929 pg/ m3 particles was established after 5 min of adding diesel exhaust. The geometric count mean diameter of the particles 10 min after entering the chamher (EAA measwements) was 0.073 pm with a geometric standard deviation
500
3000
Phe ~2500
400
-2000 300
. ? 0
E
-1500 0
200
-
100
-
E
P -1000
dlo-py
i
dlo-py
~joo
0
10
8
12 am 2 Time in Hours (EDT)
4
6
FIGURE 1. Gas phase PAH behavior in a 190-d outdoor chamber under warm dark conditions with dilute diesel exhaust.
150
I
Particle PAH
d1O-PY
I
1.2
7
h
0
E 100 ?
0 10
11
12
lam
2
3
4
5
6
Time in Hours (EDT)
FIGURE 2. Particle associated PAH and TSP behavior in a 190-d outdoor chamber under warm dark conditions with dilute diesel exhaust.
of 1.71. As will he shown later, the surface to volume ratio of particles in the chamber does not substantially change over the course of 9 h. During diesel particle sampling of this experiment, the outdoor chamher temperatures ranged from 24.3 to 22.2 "C, and the relative humidity was approximately 65%. The behavior of gas phase semivolatilePAH is illustrated in Figure 1. After its addition to the gas phase, dl0-py initially declined in the chamber atmosphere at rate of -20%/h. This was 4-5 times faster than the air exchange rate in the chamber as measured with SFBand indicated a loss of dl0-py to the chamber walls. Immediately after the addition of diesel emissions to the chamber, dieselrelated gas phase PAH appeared and then began to slowly decline. The initialgas phase loss rate ofdl0-pyand diesel fluoranthene were similar. As illustrated in Figure 1, the loss of phenanthrene was slower than that of fluoranthene and suggested that the PAH loss rate to the walls was related to vapor pressure. Particle PAH are shown in Figure 2. Compounds like henz[alanthracene (BaA) and henzofluoranthenes (BFs), which were predominantly in the particle phase, were lost at the same rate as TSP. The first particle sample taken after the addition of diesel emissions contained high levels ofdl0-pyandindicatedaveryrapidmigrationofgas phase dl0-py to the particle phase. This was followed by slower particle dl0-py uptake as the system equilibrated. Gas phase dl0-py did not, however, exhibit a sudden decline after the addition of diesel particles, even though the new appearanceofparticlephasedl0-pyrenewasalmostathird VOL. 29. NO.
1.1995 I ENVIRONMENTAL SCIENCE &TECHNOLOGY
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45
of the gas phase dl0-py (Figures 1 and 2). This suggested that chamber wall dl0-pyrene was quickly released to help establish a dl0-py gas and particle phase equilibrium. Of interest was the number of effective collisions (or the accommodation coefficient a) that take place between the gas phase dl0-py and the surface of the diesel aerosol. If we assume that the concentration that was in the gas phase represents the total number of molecules that were colliding with the particle surface in the chamber, then eq 5 will
.
-
.............................. .
0.01
-
.
~ . ~ . . ~.~ ~ . . . ~ ~ ~
~
0.Wl
represent the time rate of change of PAH as a function dl0-py concentration, absolute temperature (I),molecular weight (MI,and aerosol surface concentration [surfl. Ris theuniversalgasconstant (8.33 x 107 ergK-'mol-').When the particle surface [surf]doesnot change, the rate constant for this process is (RT12nM11'2[surfla,where a is the accommodation coefficient, which can be thought of as the net fraction of effective collisions. According to Frey and Corn (18). the specific surface area of diesel soot as determined by scanning electron microscopy ranges from 28 to 50 m2/g. FAA particle s u e measurements for this experiment gave a particle surface of 1.7 x cmZ/mL. Dividing this by the initial particle concentration of 929 pg/m3 suggested a specific particle surface of -18 m2/g. It was assumed initially that gas phase dl0-py migrated to the fresh particles and that particle off-gassingof dl0-py was very small. Setting the change in the gas phase dl0-py concentration equal to the amount that appeared in the particle phase for the first point in Figure 2 and using an effective initial gas phase dl0-py concentration of -350 ng/m3 (walls gas phase) and the range of diesel particle specificsurface areas above gave values for a which ranged from -1 x to 4 x This means one out of every 2500- 10 000 collisionsof gas dl0-pywith an aerosol surface resulted in effective particle sorption of dl0-py. Equilibrium at Moderate Temperatures. Given the recent work of Pankow (11). Kp can be taken to represent the equilibrium partitioning of gas phase semivolatileswith liquid layer particles as well as particles with a solid exterior. Of interest is the stability of Kp over time for a variety of PAH of differentvolatilities. The denuder sampling system as used in this study provides reasonable gas and particle phase information for compounds that are not entirely in the gas or particle phase. Under warm conditions for compounds that are predominately gas phase such as naphthalene (Nap),acenaphthene (Acnl,and lluorene (FI), even a slight breakthrough in the top denuder of say 0.52%wiUcauseaverylargeoverestimateoftheparticlephase concentration. For lowvolatility compounds such as benz[alanthracene (BaA), if the fraction of particulate matter collected on the denuder wall (5%) is extracted, it will similarly confound the gas phase BaA measurements. Both the denuder and conventional sampling system gavesimilartotalgas and particle phase concentrationsfor PAH ranging from naphthalene through chrysene. The conventional sampling systemwas on average 0.02%higher than the denuder system (range of +25%). For phenanthrene (Phe), anthracene (Ant),fluoranthene (Fla),pyrene (Py),anddl0-py, however, theconventionalsamplesystem gave particle associated concentrationsthat were more than twice that derived from the denuder sampling system (at
+
4%. ENVIRONMENTAL SCIENCE &TECHNOLOGY I VOL. 29.
NO. 1.1995
12
1 am
2
3
4
5
6
Time in hours (EDT)
+Fla
+Phe-FI
+BaA
FIGURE 3. Plot of the logs of the equilibrium constant Kp over time forPAH wilhdifferemvolatilitiesinawarmdarkchamberexperinem with diesel exhaust.
the flow rates and concentrations used). Even in light of this positive PAH particle phase sampling artifact (1% the conventional sampling system (given the breakthrough problems of the denuder system) provided a much better estimate than the denuder system of particle PAH concentrations for PAH which were almost entirely in the gas phase. For compounds that were predominately in the particle phase like BaA, we felt the best estimate of their gas phase concentrations (given particle loss to the denuders) could be made from the PUF concentrations from the conventional sampling system. A plot of the partitioning equilibrium constant Kp, for PAH of different volatilities over time is shown in Figure 3. Kp was represented as PAHpan/(PAHgaa x TSP), where the PAH particle and gas phase concentrations. (PAHpartand PAH& have units of ng/m3 and TSP has units ofpg/m3. Kp remained relatively constant over time. Although data are morelimited, similar trends were suggestedunderwamn dark conditions for wood smoke in our 25-m3 outdoor chambers (12). For example, Kp values for fluoranthene over a 3-h period (in order) were 0.23,0.29,0.27, and 0.35. The temperature was in the same range (26.2 to 23.4 "C) as the above-described warm diesel experiment. The relative humidity was close to 100%. In this experiment, however, the wall loss rate of gas phase PAH was much faster than in the diesel experiment in the 190-m3chamber because of the higher chamber surface to volume ratio in the 25-m3chamber. This necessitated a correspondingly faster release of particle PAH to the gas phase (in the 25-m3 chamber compared to the 190-m3 chamber) in order to maintain equilibrium. In the above described diesel experiment, the logs of Kp for seven PAH were also regressed vs log PL' (Figure 4). We used the subcooled vapor pressure constants determined by Yamasaki (20)whichemployedthe gas chromatographic relative retention time technique of Hamilton (21). The plots of Kp vs log PLoover time gave slopes that remained closeto-1 (-0.98, -0.93, -1.01,-0.97. -0.97)withZvalues of 0.97 to 0.99. The standard errors of the slopes ranged from0.07 to0.04. AUofthe slopeswerewithinonestandard error from one another. Intercept values also remained constant at a value of -7.52 % 0.12 (average error of they estimate) and also could not be distinguished from one another. Similar slopes were also obtained using the subcooledvapor pressure data ofHinckleyet al. (221,which
12:32am to 4 5 8 am
-1 to -4Oc
600
2000
-1 BaA .....................................
.......................
a n
g-4
Phe &Ant
...................
500
.......
-
I
0
O
.....
...................................
I
6pm
L 12am 2 Time in Hours EST
8
10
Acn
-6 -C -6
-4
-5
-2
-3
log P," in torr
6
FIGURE 6. Comparison of the loss of gas and particle phase pyrene with time in the 190-3 outdoor chember under darkness at -1 to -4 O C with dilute wood smoke. Particle PAH loss
FIGURE 4. Plot of log & vs log subcooled liquid vapor pressure (log for regression lines from all five samples from e warm dark chamber expriment with diesel exhaust Date points (squares)from the third sample are also plotted.
w)
-1 to -4Oc
50
140
1
120
-.
2000
-a0 _60 --
1500
100
m E
40
9
s
W 30
-
5
O 4
0
1000
P $ I
K
w
"1 i 20
20
m
E L
500
0
0
6pm 10
8
10
12
2am
4
6
8
Time in Hours EST
Q 8
IO
12am
2
4
6
Time in hours FIGURE 5. EAAsurface to volume ratio (/rm-')for dilute wood smoke particles in the UNC 190-I# outdoor chamber.
were also based on the Hamilton methodology. That slopes of all of the log Kp vs log PL" lines remained fairly close to one and intercepts did not change suggested that measured semivolatile PAH remained in equilibrium, even though loss rates of gases and particles to the chamber walls were different. Time to Equilibrium under Cool Conditions. A dark outdoor chamber experiment with wood combustion emissions was conducted to measure times to equilibrium under cool outdoor conditions. Overcast conditions were selected, which resulted in relatively constant ambient temperatures over the course of the experiment (-1 to -4 "C), The initial particle concentration was 502 pg/m3, and it declined to 320pg/m39 h later. Throughout most of this period, the EAA count geometric mean diameter ranged from 0.1 to 0.09 pm with a geometric standard deviation of 1.57to 1.51. As shown in Figure 5, the estimated particle surface to volume ratio from the EAA count measurements did not substantially change with time. The difference between the loss rates for pyrene in the gas and particle phase after wood smoke emissions were added to the chamber is illustrated in Figure 6. Gas phase pyrene disappeared from the chamber atmosphere much faster than particle associated pyrene. This effect was present, but less pronounced for phenanthrene (not shown).
FIGURE 7. Behavior of different volatility particle phase PAH in the 190-m3outdoor chamber under darkness at -1 to -4 "C with dilute wood smoke.
In Figure 7, one can also see that more volatile compounds are released faster from the particles than less volatile compounds to accommodate the gas phase loss to the wall. Three samples at the beginning, middle, and end of the experiment were averaged to give average PAH concentrations at three different times. Plots of log Kp vs log P L O for five monitored gas and particle PAH (acenaphthylene(Ace), F1, Phe, Fla, and Py) were then made. The first averaged sample had a time of 1O:Ol PM, which was 1 h after the addition of wood emissions to the chamber. The slope of the log Kp vs log PL"line was close to 1 (-1.08), suggesting that at the start of the experiment partitioning was close to equilibrium. With time, however, the slope became more and more negative; after 3.8 h of aging the slope decreased to -1.25, and at 5:39 AM the next moming (8.4 h after the start),it had decreased to - 1.44. 9 values for the regression for these three lines were 0.99. The slopes for the initial and last samples (1and 8.4 h, see Figure 8) were statistically different from one another at the 95% confidence level (~kt~.~ The , ~slope ) . at 3.8 h was statistically different from the one at 8.4 h but not from the initial sample. The intercepts of these lines decreased from -7.79 in the initial sample to -9.36 in the final sample. Again, the intercepts of the lines from the first and last were statistically different at the 95% level, while the intercept at 3.4 h only differed from the 8.4 h intercept. The combined effect of observed slope and intercept changes in the log Kp vs log P L O plots strongly suggested that the system departed further and further from equilibrium as it aged. This was largely due, VOL. 29, NO. 1, 1995 I ENVIRONMENTAL SCIENCE &TECHNOLOGY
47
-1
,
I
1.08 hours
the rate constants for PAH particle loss or gain. At equilibrium, for a closed batch system without any particle loss, the gas particle interaction in eq 7 gives a simple equilibrium constant where &, is equal to [PAHpIeql [PAHgleq and also equals konlkoffwhere [PAH,],, and [PAHgleq are the equilibrium concentrations of the particle and gas phase PAH. From eq 3, &,ITSP = Kp . At any time t, [PAHpIeq [PAH,],, = [PAH,] [PAH,], which leads to
+
-d[PAH,lldt=
after 8.4 hours slope= -1.44 -6
-5
-4
+
-3
-2
-1
{[PAHpl - [PAHpl,ql{koff+ konl
(8)
The units of [PAH,] can be nglvol of air or nglmass of particles or nglliquid volume of particles. Given that [PAH,],, is constant, integration of eq 8 yields
log P," in torr FIGURE 8. log rCp vs liquid vapor pressure plots for dilute wood smoke for averaged initial and final samples in a lW-m3 outdoor chamber under darkness at -1 to -4 "C.
given the low temperatures, to the inability of particles to release PAH at a fast enough rate to keep up with the loss of gas phase PAH to the chamber walls. At temperatures above 10 "C, Ross et al. (14) suggested that the organic material on diesel soot particles may be in the liquid state. Wood soot particles have ahigh phenolic and methoxyphenolic content (231, and most of these compounds are solids at -1 to -4 "C. A similar Kp vs log P pattern was observed when we used the solid PAH vapor pressures (PI of Sonnefeld et al. (24). The initial slope was -0.93, and it decreased to a value of -1.25. The fits to these lines were not quite as good at the log 9" lines (0.96 > $ < 0.99), and the slopes of the first and last samples were only statistically different from one another at the 70% confidence level. That sub-cooled vapor pressures give better linear fits in log Kp vs log P plots is consistent with the original observations of Bidleman and co-workers (3).
To explore the times required to re-establishequilibrium, two different kinetic treatments were undertaken. In the first, we assumed that the observed net particle semivolatile off-gassingrate (kobs) was a function of the relative closeness to gas particle phase equilibrium, where APAH,, is the change in concentration in the gas and particle phase needed to establish equilibrium and APAH is that part of the concentration change that has already occurred by time t. [PAH,] is the bulk PAH particle phase concentration at time t. This relationship assumed that as the system approached equilibrium, the observed rate coefficient kobs for the net loss to or from the particles would correspondingly be attenuated (eq 6). Representing the decay as pseudo-first-order permitted an estimate of kobs:
In the second case, PAH loss rates from the gas and particle phase were considered separately. We assumed a simple expression for the change in particle concentration for either a solid or liquid particle, such that
where again [PAH,] is the bulk particle phase concentration, [PAH,] is the gas phase concentration, and koff and kon are 48 1 ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 29. NO. 1.1995
where [PAH,,] is the particle PAH concentration at t = 0 and [PAH,] is the particle concentration at time t, The particle PAH (Le,, [PAH,]) concentration data adjusted for particle loss to the walls can be fit with a pseudo-first-order expression such that
Combination of eqs 7,8, and 10 and solvingfor koffyields
At any time t after the addition of emissions, actual PAH, and PAH, can be obtained from the observed data or calculated from the observed first-order rate constants and initial conditions. kobs is measured, and Kqis calculated. This permitted an evaluation of koe in eq 11. In the above cool temperature chamber experiment, we started with an initial gas phase pyrene concentration of 564 ng/m3, a particle phase pyrene concentration of 1714 ng/m3, and a TSP of 502 pglm3. Based on the observed rates of decay, after 3 h of aging (and departing from equilibrium), the pyrene gas phase concentration would be 254 ng/m3, the particle pyrene would be 1426 ng/m3, and TSP would be 431 pglm3. Using the initial concentrations to calculate Kp gave 3-h pyrene equilibrium concentrations of 468 ng/m3 in the gas phase and 1212 ng/m3 in the particle phase. This created a pyrene particle phase excess of +18% pyrene and a deficit of pyrene in the gas phase of -46%. Re-establishing equilibrium would necessitate that 214 ng/m3 of pyrene be transferred from the particle to the gas phase. For phenanthrene, we calculated that 346 ng/m3 would be needed in the gas phase, which translated to a 34% excess in the particle phase. To analytically estimate the time it would take to approach some percentage of the equilibrium value (eq 9),one needs to hypotheticallyimpose on the system no further chamber wall losses of gas or particle phase PAH,since the difference in the gas and particle phase loss rates is what moved the system out of equilibrium. Knowledge of kobs and the change in gas phase PAH concentration (APAH,,) needed to bring the gas phase into equilibrium permitted a numerical solution of eq 6. Calculation of koff (eq 11) and kon (Le., kon = k0&,) permitted a similar solution of eq 7 and allowed eq 9 to be solved for the times to equilibrium. The approach to equilibrium for these two differently
350 -
300
250
-
-
phenanthrene
1
OY 0
5
chamber, due to wall losses, particle off-gassing occurred so that Kp was reasonably constant over time. The slope of log Kp vs log PLoplots were close to 1 and also did not change with time. These observations suggest under warm ambient temperatures (22-25 "C) that fresh combustion semivolatiles most probably equilibrate rapidly between the gas and particle phase as they dilute in the atmosphere. Under cool outdoor conditions (-1 to -4 "C), particle loss from the wood smoke particle phase could not keep up with PAH gas phase losses to the walls, and the system departed from equilibrium. Kinetic simulations indicated that tens of hours would be required to re-establish 90% of equilibrium for compounds like phenanthrene and pyrene once they had departed from equilibrium in the particle phase by 34 and 18%,respectively. More volatile compounds like fluorene and acenaphthylene required much shorter times to equilibrium. The same behavior is expected for dilute diesel exhaust systems. These findings suggested that at 0 "C temperatures, once a deficit in the gas phase results from dilution or PAH gas phase reactions, long periods are required for the system to return to equilibrium for compounds like phenanthi-ene and pyrene.
10
15 20 Time in Hours
25
30
35
FIGURE 9. Phenanthrene and pyrene approach to equilibrium at -1 to -4 "C using eq 6 (thin line) and eq 7 (dark line). The calculated equilibrium concentration for phenanthrene was 346 ng/m3 and for pyrene was 214 ng/m3.
derived kinetic treatments is graphicallyillustrated in Figure 9. Both approaches give similar results and show that -20 and -40 h respectively are required for phenanthrene and pyrene to re-establish 90%of their equilibrium conditions. Fluorene, which was close to the pivot point of the log Kp vs log POLplots in Figure 8, required a much shorter time (1.5 h), and acenaphthylene required 2.5 h. The striking observation, however, is that under cold conditions an 18% excess in particle pyrene could require tens of hours to return to equilibrium. As mentioned above, this approach is overly simplistic, but it gives a sense for the approach to equilibrium time scale. An additional but very important observation was that both koff and kon decreased with time by about 40% between a 3- and a 7-h calculation. This is most likely due to the fact that the process is not first order and is actually limited by diffusion through the particle as a function of changes in particle liquid layer viscosities. As a result of this study, we have developed an inner particle diffusion model (25) to describe semivolatile losses and gains from and to an aerosol surface. Preliminary implications of this study suggest that inner particle diffusion and surface mass transfer coefficients vary with temperature. Higher diffusion and surface mass transfer coefficients permit a very rapid approach to equilibrium at warmer temperatyres. At the cool temperatures of this experiment, the converse appears to be true.
Summary and Conclusions In this paper we explored the time scales that it takes for semivolatile PAH to approach equilibrium under both moderate and cool temperatures. Combustion particles were added directly from a diesel car and a wood stove to a 190-m3 outdoor Teflon film chamber. The rate of migration of a gas phase semivolatile PAH to combustion particles at a warm outdoor temperature was investigated by volatalizing solid deuterated pyrene (dlO-py) in a hot injector (200 "C) into the background air chamber atmosphere. After 2 h, diesel exhaust was added to the chamber. Results show an initial rapid migration of dl0-py from the gas to the particle phase in an attempt to re-establish equilibrium. An accommodation coefficient of the order of 1 x to 4 x was estimated. Closeness to equilibrium was monitored by tracking the equilibrium constant Kp over time as measured by PAHp,,/(PAHga, x TSP). As gas phase PAH concentrations changed in the
Acknowledgments This work was supported by a gift from the Ford Motor Co. to the University of North Carolina, under the direction of Paul Wgoar and Dennis Schuetzle;CooperativeAgreement CR.819675fromthe US. EPA, under the direction ofWilliam Wilson and Larry Cupitt; and a research grant (R816678) from the office of Exploratory Research U.S. EPA, under the direction of Deran Pashayan. We are extremelygrateful to Drs. Lara Gundel and Joan Daisey from Lawrence Berkeley Laboratories, who shared with us their newly developed denuder technology long before they had a chance to publish this very important work. We also acknowledge the efforts of Jian-xin Hu, Jianbo Zhang, and Heejeong Latimer, who helped collect and analyze some of the data presented in this study.
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Received f o r review March 1, 1994. Revised manuscript received September 6, 1994. Accepted September 27, 1994.@
ES9401252 @
Abstract published in AduunceACSAbstracts,November 1,1994.
