Environ. Sci. Techno/. 1995,29, 2420-2428
GaslSolid Partitioning of Semivolatile Omanic Compounds to Model Atmospheric Sokd Surfaces as a Function of Relative Humidity. 1. Clean Quartz JOHN M. E. STOREY,+ WENTAI LUO, LORNE M. ISABELLE, AND J A M E S F . PANKOW* Department of Environmental Science and Engineering, Oregon Graduate Institute, P.O. Box 91000, Portland, Oregon 97291 -1000
Evaluating the relative importance of adsorption to particle surfaces vs absorption into organic material in particles for gas/particle partitioning in the atmosphere requires a comparison of relevant gas/solid partitioning constants with field-determined values. Gas/ quartz partitioning constants Kp (m3/pg) were measured a t 20 "C for clean quartz as a function of relative humidity (RH) for 11 semivolatile polycyclic aromatic hydrocarbons (PAHs) and n-alkanes. Increasing RH from -30 to -70% caused the Kp values to decrease by a factor of 10. With adsorption to the quartz surface as the only possible sorption mechanism, surface-area-normalized partition constants ( Kp,s,m3/m2)were calculated. For quartz, correlations of log Kp,swith the log of the vapor pressure were found to lie significantly below the corresponding lines for urban particulate matter. W e conclude that adsorption to mineral/oxide surfaces like clean quartz is not important in determining Kp values in urban air; such sorption may be important in ruralhemote environments.
Introduction Gas/particle (G/P) partitioning is an important process affecting the dry deposition, wet deposition, photolysis, and long-range transport of semivolatile organic compounds (SOCs) in the atmosphere. The 189 hazardous air pollutants listed in the Clean Air Act Amendments of 1990 include many SOCs, with these SOCs subject to different regulatory actions depending on whether they are primarily in the gas or particulate phase (1). Having a fundamental understanding of G/P partitioning will facilitate predictions * Corresponding author telephone: 503-690-1080; fax: 503-6901273. Current address: Health Sciences Research Division, Oak Ridge National Laboratory, P. 0. Box 2008, Oak Ridge, TN 378316113.
2420 m ENVIRONMENTAL SCIENCE &TECHNOLOGY / VOL. 29, NO. 9,1995
of the partitioning of SOCs under a range of environmental conditions. A useful parameterization of G/P partitioning is (2, 3)
where Kp (m3/pg)is a partitioning constant, TSP @g/m3) is the concentration of total suspended particulate material, and F (ng/m3) and A (ng/m3) are the P- and G-phase concentrations of the compound of interest, respectively. The symbols F and A originate in the common usage of a filter followed by an adsorbent to collect the P and G portions, respectively. Kp values depend on both temperature (T, Kl and relative humidity (RH, %) (2-4). When sorbing to urban particulate material (UPM),a given SOC will tend to exhibit values of Kp that are similar for given values of T and RH. The mechanisms underlying G/P partitioning of SOCs include simple physical adsorption onto particle surfaces (2,5,6)as well as phase partitioning into absorptive organic material (7). For a given aerosol material and for compounds within a given compound class, it can be shown that ( 7 )
where p t (Torr) is the subcooled liquid vapor pressure. The terms C,/pf and C2/pf represent the adsorptive and absorptive contributions to Kp, respectively. Expressions for the parameters C1and C2are given by Pankow (7). Within a given compound class (e&, PAHs, alkanes, etc.), Ci and Cz will be only weakly dependent on the compound. Thus, at a given temperature, we can expect that measured values of log (F/TSP)/A will tend to be correlated with log p f according to
log (FITSP)/A = rn, log p ;
+ b,
(3)
Within a given compound class, to the extent that Ci and CZ are independent of the compound, the slope m,will tend to equal -1, and the intercept b, will tend to equal log [Cl + C Z ] .The expression (FITSP)/A is used in eq 3 rather than Kp because (a) ambient values of F, TSP, and A may not be in full G/P equilibrium; (b) measured values of F, TSP, and A will generally be volume and time averages from a certain sampling interval; and (c) sampling artifacts may affect the data. The adsorption parameter C1 depends in part on the specific surface area a (m2/g)of the adsorbing solid (6, 7 ) . For the specific case of sorption to atmospheric particulate materials, we represent the specific surface area as UTSP (mZ/g);for UPM in Portland, OR, Shefiield and Pankow (8) have estimated that UTSP = 2.1 m2/g. The absorption parameter C, depends in part on the weight fractionf,, of the TSP that comprises the absorbing organic matter as well as the mean molecular weight of that organic matter (7). When adsorption is the governingsorption mechanism, a can be used to normalize Kp to yield the surface-areanormalized constant Kp,s (m3/m2)according to
0013.936X/95/0929-2420$09 OOiO
Z 1995 American Chemical Societv
clean
If adsorption dominates the G/P partitioning in a given situation (i.e.,C, = GI, then when field-derived values of (HTSPIIA are divided by anp, the resulting values should tend to agree at a given Tand RH with laboratorymeasured values of %.. for solid surfaces that are prevalent in ambient particulate material. The purpose of this work is to use comparisons of this type to consider the relative importance of adsorption as a mechanism for G/P partitioning in the atmosphere. Since oxide minerals (e.g., quartz, clays, silicates,flyash, etc.) provide a portion ofthe surface area for most samples of atmospheric particulate material, and since they can easily be baked cleaned of organic compounds, we selected quartz fiber filters (QFFs) as our first model material. We also note that quartz has been identified by X-ray and infrared spectroscopy as a constituent of airborne dust (91, and that it has been found in many samples collected at sites of the EPA Inhalable Particle Network (101. Since the degree of hydration of an oxide surface will change its adsorption properties, we included RH as a variable of interest.
ATD blank
cartridge
remove
aerosols
generator cartridges vent
uuu
OFF holder 2
ATD
to hood
cartridge
Effects of RH on GaslSolid Sorption RH has a strong effect on adsorption to mineral surfaces. For soils, the sorption of gaseous hydrocarbons decreases when going from very dry conditions (RH < 15%) to bigber humidities (111,in agreement with predictions based on multicomponent adsorption theory (12). However, as RH rises above 60-70% usually little further decrease in sorption to solids occurs; this behavior contrasts with predictions from some applications of multicomponent adsorption theory (e.g., ref 12) which predict an upswing in adsorption at high RH values. In the atmosphere, the Yamasaki dataset (13)seems to indicatethatsorptionofPAHstoUPMdecreases byafactor of -2 as RH increases from -40 to 95% (4). Since measured (NTSPVA values can contain contributions from both adsorption and absorption, the changes with RH observed in the Yamasaki dataset could have been due to effects on either orboth ofthe sorption mechanisms. Orthe observed RH effects may have been due in part to variability in some unknown property of the particulate matter that was itself correlated with RH. In the laboratory, working with UPM collectedinthe field,CothamandBidleman (14) compared the G/P partitioning of SOCs under very humid conditions (RH >95%) with those observed under less humid conditions. At 30 "C, sorption at RH ,9596 was significantly weaker than when RH = 35%. At 20 "C,data obtained in an RH range between 30 and 70% did not differsignificantly from that obtained at RH > 9 5 % the absence of an observable effect at 20 "C may have been due to the fact that some of the "lower" humidity values were actually quite high (e.g., RH = 70%).
Experimental Section Model SOCs and Model Solld. Each experiment was conducted using a fEed mass of QFF and the apparatus depicted in Figure 1. Each section of QFF was exposed to a continual flow of gas-phase SOCs until equilibrium was believed to be reached. Six n-alkanes were studied CV, C19,Cz0,Czl,Cz2,and CZ3. Five PAHs were studied 2-methyl phenanthrene (Me-Phen), fluoranthene (FLA), pyrene
FIGURE 1. Schematic diagram d experimental apparatus. (FYR). benzo[alfluorene (BaF), and benz[alanthracene (BaA).It was necessaty to consider analytical sensitivityas well as time to equilibrium requirements. For a compound with a high pe value, unless the gas-phase concentration orthemassofsorbent (ms,pglused was relatively high, the amount of compoundsorbed at equilibriumwouldnot have been large enough to be measured. For a compound with a low pE value, unless m,was relatively small, a long time would have been required to reach equilibrium. The minimum volume of air Vi,. (m9 that must be passedoverthesorbentto reachequilibriumis determined by Kp and m,. This is shown by expressing the partition coefficient as
where c, (ng/m3) is the gas-phase concentration. (We reserve A for field-measured values of the gaseous concentration.) Since the mass sorbed (ng) by the sorbent is to be delivered in V,i
c, = mass sorbed Vli"
Substituting eq 6 into eq 5, we obtain Vmin = Kpms
(7)
On the basis of the flow rate through the sorbent that was used ( F % 4.5-5.0 L/min), time periods tmi. (=Vmi,/R of 2-4 weeks were required to reach Vmi. for the least volatile compounds (Cz3 and benzialanthracene). Since V.i, represents the minimum volume required to deliver the mass sorbed at equilibrium, a volume larger than V.i, and a time longer than t,i, will be required to reach full equilibrium for any given compound. VOL. 29. NO. 9.1995 I ENVIRONMENTAL SCIENCE &TECHNOLOGY m 2421
The advantages of a QFF as a mineral sorbent material include the following: (a) crystalline quartz can be found in the atmosphere; (b) at -1 pm in diameter, the fibers in a QFF are similar in size to many atmospheric particles; (c) a QFF is easily handled and is well suited for exposure to a flow of gas-containing SOCs; (d) the fibers of a QFF are nonporous, making adsorption to the external fiber surfaces the only sorption mechanism; and (e) QFFs can be baked to achieve very low blank levels. Using krypton as the adsorbate, the specific surface area (UQFF)of the QFFs used here has been estimated to be 1.65 i 0.11 m2/g (15). Design and Operation of Apparatus. Each QFF was mounted in a stainless steel filter holder, and an N2 stream containing gaseous SOCs was passed through the filter. When equilibrium was believed to have been reached, each solid-phase concentration c, (nglpg) was measured along with each gas-phase concentration cg (ng/m3)).The parameter c, is analogous with the ratio FITSP. As noted above, cg is analogous with A. The partitioning coefficient Kp for each compound was therefore measured as the ratio CJC,(m31pg). The N2 gas came from the gas vent of a high-pressure liquid N2 (LN2) dewar. LN2 was used because it is less expensive than cylinders of high purity gas and because its low temperature affords low blank levels. The N2 was first passed through a large molecular sieve/activated carbon trap to remove contaminants and was then split into three flows: (a) -60 mL/min passed through a cartridge of TenaxTA (Alltech,Deerfield, IL) for measurement of the NZblanks by adsorption/thermal desorption (ATD);(b) 0.5-7 Llmin was humidified using two stainless steel impingers filled with deionized water and heated from below; (c)2-9 Llmin was split into four subflows using four rotameters and four needle valves. Three of the subflows passed through three parallel "generator cartridge" systems; the fourth provided dry gas, which could be used to adjust the RH of the combined flow that eventually passed over the QFFs. The ATD and generator cartridges were of Pyrex glass with a bed length and inside bed diameter of 8.25 and 0.95 cm, respectively. The outer diameter at each end of the cartridges was 1/4 in. (0.64 cm). The ATD cartridges contained -1 g of Tenax-TA. The generator cartridges contained 0.5-mm glass beads, which were coated with SOCs as described elsewhere (16). Two generator cartridges each were prepared for the high, intermediate, and low pfl compounds. Each pair of cartridgeswas connected together and was installed in one of the three generator subflow lines. Cartridges were connected using 1/4-in. (0.64 cm) Swagelok fittings. The high p&compounds were C,;, C19, and Me-Phen. The intermediate p f values were C ~ O1221, , FLA, PYR, and BaF. The low pf compounds were C22, CZ3, and BaA. The cartridge subflow containing the low pfl compounds was high relative to the flow through the cartridges containing the high pe compounds. The concentrations of all of the SOCs were within 2 orders of magnitude. The RH was monitored with a dewpoint hygrometer. The amount of dry Ne used depended on the desired RH. Two experiments were conducted at RH = 0.36%; two at RH = 6.5%;four at RH = 9.6%; one at RH = 26.7%;four at RH = 29.4%;one at RH = 48.5%; one at RH = 70.2%; and one at RH = 75.1%. Note that each experiment gave two Kpvalues for each compound. Typically, the RH remained constant to within about il%. 2422
ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 29, NO. 9. 1995
The stainless steel, 2.5 cm diameter filter holders (Gelman) were modified to accept Swagelok connections, cleaned with acetonelhexane, vacuum dried in an oven, and then fitted with Teflon seals. Each 2.5 cm diameter QFF (-30 mg) was punched from a 20 x 25 cm sheet of QAOT-UPfilter (PallflexCorp.), baked at 550 "C in a muffle furnace, inserted hot (-100 "C) into a holder, and then allowed to cool in the holder. Each of the two QFF filter holders received half of the humidified, SOC-containing gas. In some expcriments, the flows were balanced using restrictors (30 cm of 1.5 mm i.d. stainless steel). The face velocity at each filter was 20 cm/s. To measure the preand postfilter c, values, a small portion of the flow was diverted through an ATD cartridge at three locations: before the filter holders split and after each filter holder. The recombined flow (9 Llmin) passed through a rotameter and was vented to a hood. Most of the QFFs were equilibrated with the desired RH for -4 days before beginning exposure to the SOCs. The apparatus was located inside a large chamber to maintain the temperature at 20 i 0.2 "C for run times of 10 and 28 days. When pre- and postfilter cg values agreed within lo%,itwas hoped that the QFFs were at equilibrium with the prefilter cg values. After an additional set of cg values was obtained (with the filter holders still installed), the experiment was ended, and the QFFs were analyzed to determine the cs values for the various compounds. The typical values for cg (ng/m3)were as follows: Ci7, 5002000; C19, 60; C20, 80; CLI,25; C ~ L6;, C23, 5; Me-Phen, 5002000; FLA, 200; PYR, 60; BaF, 15;and BaA, 10. Typicalvalues for c, (nglpg)for the compounds were as follows: Cii, 10-3; cis,-10-4-10-7; cro,-10-3; c2],-10-3-10-2; cL2, -10-3-10-2; C27, Me-Phen, 10-4-10-1; FLA, -lo--'PYR, 10-4-10-3; BaF, and BaA, 10-3-10-2. For the experiments conducted at RH 230% (the data that will be emphasized in this work), when the areas of all sorbing SOCs were summed on the QFF surface, there was always less than 25% of a monolayer of coverage. Analyses. The 30-min backflush desorption (30 psi and 270 "C) of each ATD cartridge took place to a 25 m x 0.32 mm i.d., 0.25 pm film DB-5 capillary GC column at 0 "C ( 17). The column was interfaced to a mass spectrometer. After desorption, the pressure was reduced to 7 psi, and the GC was taken to 175 "C at 25 "Urnin. After 2 min, the GC was taken to 300 "C at 10 "C/min. Each QFF was sonicated 3 x with 10 mL of dichloromethane. The combined aliquots were reduced to 2 mL by K-D concentration (181,stored at -20 "C, and analyzed within 24 h. Prior to analysis, each extract was blown down under N2 to 200 pL and 2 pg of phenanthrene-dlowas added as an internal standard. The extracts were injected on-column (70 "C) onto the GC/MS. The temperature program was as follows: 70-175 "C at 25 "C/min, hold 2 min at 175 "C, and then to 300 "C at 10 W m i n . Also analyzed for each experiment were an ovenblank QFF and two blank QFFs spiked with 2 pg of each compound to determine recoveries. Blank QFF levels were always verylow. Recoveries of 85-1 15%were obtained for all compounds.
Results Sorption as a Function of RH. The measured Kp values (20 "C) obtained are given in Table 1 and plotted vs RH in Figures 2 and 3. The error bars are a total of one standard deviation (1 SD) wide for the replicate experiments. For each compound, the measured log Kp values remain
anes c23
c20
,
-7)
,
I
I
,
I
,
I
I
1
10 20 30 40 50 60 70 80 90 100
0
RH
(w)
FIGURE 2. Log Kp(m3/pg)vs RH (%) for sorption of six salkanes on quartz fiber filters (OFFs) at 20 "C.
e
M
1
\
-3i Ab-
V' -4
W
ka -5 l
m
"\
b BaA I
1
BaF
M
l
1
PAHs
PYR FLA
0
3
W M e - P h e A
0 N
I z l
-74 0
,
,
,
1
I
I
I
I
I
I
10 20 30 40 50 60 70 80 90 100
RH
(w)
FIGURE 3. l o g Kp(rn3/pg)vs RH (YO)for sorption of five PAHs on quartz fiber filters (OFFs) at 20 "C.
k-a I Y
cc
m
3 "E
1
essentially constant for RH values up to -30%. For RH values between -30 and -75%, there is a marked, approximately uniform rate of decline in log Kp for each compound; the d log Kp/d RH slopes average -0.020 and -0.025 for the alkanes and the PAHs, respectively. These slopes are consistent with results obtained by Goss (19) using a GC-based method to study the sorption of a number of relatively volatile compounds (e.g.,acetonitrile, diethyl ether, n-Cg,various dichlorobenzenes, and others) on quartz at 50-80 "C. For eight relatively nonpolar compounds at 70 "C and RH values greater than 30%, Goss (19) obtained d log K p / dRH slopes of about -0.03. Goss (19)estimated that equivalent monolayer coverage of water will be reached for a quartz surface when RH = 26%. This implies a BET constant of c = 8. For a silica gel surface (which is believed to be very similar to a quartz surface due to the usual presence of an amorphous surface layer on quartz (20)), Rhue et al. (21) estimated that c for sorption of water is -18 at -20 "C. Equivalent monolayer coverage of water on silica gel would then be reached when VOL. 29, NO. 9. 1995 / E N V I R O N M E N T A L SCIENCE &TECHNOLOGY
2423
TABLE 2
Poeled Mean log K,, (m3/pg)Values for Sorption on Ouartz Fiber Filters (QFFs) at 20 95% Confidence Intervals for Three Relative Humidity (RH, %) Rangesa
OC,
fl SD Values, and
log Kp (m3/pg),fl SD. and 95% confidence interval
compound
log pe (Torr)
c17
-3.42
c19
-4.43
c20
-4.93
C2l
-5.43
c22
-5.93
c23
-6.43
2-methylphenanthrene
-3.91
fluoranthene
-4.56
pyrene
-4.75
benza[alfluorene
-5.23
benz[alanthracene
-6.01
-30% RH
48.5% RH (48.5%) N=2
(26.7-29.4%) N = 10 n-Alkanes -5.51 i 0.20 -5.96 to -5.06 -4.42 i 0.08 -4.60 to -4.24 -3.91 i 0.11 -4.16 to -3.66 -3.33 f 0.16 -3.69 to -2.97 -2.72 f 0.20 -3.17 to -2.27 -2.26 i 0.11 -2.51 to -2.01
-6.14 NCb -4.93 NC -4.38 NC -3.72 NC -3.59 NC -2.65 NC
PAHs -4.93 i.0.15 -5.27 to -4.59 -4.41 f 0.11 -4.66 to -4.16 -4.35 =k 0.04 -4.44 to -4.26 -3.64 f 0.23 -4.16 to -3.12 -2.45 i 0.14 -2.77 to -2.13
-70% RH (70.2-75.1"/a) N=4
i. 0.10 i 0.01
rt 0.03 i 0.01 i 0.02 i. 0.01
-5.67 i 0.01 NC -4.95 i 0.02
NC -4.85 i 0.03 NC -4.14 i 0.02 NC -3.03 i 0.02 NC
-6.30 -6.59 -5.05 -5.72 -4.83 -5.12 -4.30 -4.78 -3.87 -4.57 -3.08 -3.43
i. 0.09 to -6.01 rt 0.21 to -4.38 i 0.09 to -4.54 i 0.15 to -3.82 i 0.22 to -3.17 i 0.1 1 to -2.73
-6.10 -6.74 -5.46 -5.84 -5.43 -5.94 -4.61 -5.02 -3.67 -3.96
i 0.20 to -5.46 f 0.12 to -5.08 f 0.16 to -4.92 i 0.13 to -4.20 i 0.09 to -3.38
a Number of relicates ( N ) given. NC = not computed because the calculated SD estimate for this RH range is believed to be lower than is realistic given the degree of scatter in the other data.
RH zz 19%. Both c = 8 and c = 18indicate that the equivalent of about four monolayers of water will be present when RH = 75.1'76, the highest RH used here. The steady decrease in sorption observed in our data as RH rises above 30% is therefore no doubt due to changes in the surface properties of the quartz as the amount of sorbed water increases above that for about one monolayer. The Goss (19)data do not show a constancy of sorption at low RH, but rather an increase in sorption as RH decreases below 30%. Similar results have been obtained for soils by Chiou and Shoup (11) and are predicted from theory (12). Thus, it seems possible that our low RH results are not correct. The Si02(s)surface is known to tenaciously retain some water. Even after outgassing under vacuum at 100 "C, Stober (22) found some water to be held by surface SiOH groups. (Whalen and Hu (23)reported avalue of five -SiOH groups/nm2 of quartz surface, which is about the same surface density for a monolayer of water molecules on quartz (241.) It is therefore difficult to keep a quartz surface free of adsorbed water. Any exposure of the QFF to ambient RH levels during the transfer from the muffle furnace to the filter holder may have allowed some adsorption of water to occur, with that water then not desorbing (adsorption hysteresis) when the QFF was equilibrated at a low RH. At RH ~ 3 0 % our quartz surfaces may therefore not have been as free of water as would have been the case if a baked QFF was exposed only to a given desired low RH. In the study by Goss (19),the sealed GC method used in conjunction with significantlywarmerstudy temperatures (and therefore faster water adsorption/ desorption kinetics) may have permitted the quartz surfaces at low RH to be more properly free of water and therefore more directly sorptive of organic compounds. Since RH 2424
ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 29, NO. 9,1995
A1 kanes n
bn
1
E
-4-
v
P.
-5M 0 3
Regression P a r a m e t e r s : b, m, -3Ora -1.09 -9.26 48.5% -1.11 -9.88 -70% -1.01 -9.72
-8
I
-7
I
-6
rz 1.00
0.98 0.98 I
-5
I
-4
log p: (torr) FIGURE 4. Log Kp (m3/pg) vs log /$(Torr) for sorption of salkanes on quartz fiber filters (QFFs) at 20 "C and three RH values.
values less than 30% are uncommon in the atmosphere, the uncertainty in our low RH data does not significantly affect the applicability of our results. In order to focus on the data for RH z -30%, the data have been placed in three groups: RH = 26.7-29.4%, RH = 48.576, and RH = 70.275.1%. The pooled data are given in Table 2 in three columns labeled -30%, 48.5%, and -70%. Sorption as a Function of pf and RH. The data in Table 2 are plotted vs log pf in Figures 4 and 5; the error bars are the 1 SD values from Table 2. The slope and intercept
4
E \
RH:-JOrs
-3n
m
Alkanes
3
2-
- 70%
Y
'
PAHs
48.5%
1-
-4-
E
0-
W
*
- 1-
R
-5-
M
- 2-
0
- 3- 4-7
-8
-7
-5
-6
-4
-5-
-3
log p i ( t o r r )
- 6-
FIGURE 5. Log Kp(m3/pg) vs log pi? (Torr) for sorption of PAHs on quartz fiber filters (QFFs) at 20 "C and three RH values.
-7-
TABLE 3
-81 I l l I I I -8 -7 -6 -5 -4 -3 - 2 - 1
Pooled Mean log K , s (m3/m2) Values at 20 "I:for Sorption on Quartz hber Filters ( O h ) for Three Relative Humidity (RH, %) Ranges
I
0
I
1
2
I
3
log p : ( t o r r ) FIGURE 6. Log Kp,*(m3/mZ)vs log /$(Torr) data at low, mid, and high RH for sorption of SOC n-alkanes on quartz fiber filters (QFFs) (from this work) together with data for VOC n-alkanes on silica (from ref
24).
0
c23
n-AIkanes -3.42 0.27 -4.43 1.36 -4.93 1.87 -5.43 2.45 -5.93 3.06 -6.43 3.52
2-rnethylphenanthrene fluoranthene pyrene benz[alfluorene benzlalanthracene
-3.91 -4.56 -4.75 -5.23 -6.01
c17 Cl9 c20
c21 c22
a
3--0.36
PAHs 0.85 1.37 1.43 2.14 3.33
1.40 2.06 2.19 3.13
-0.52 0.73 0.95 i.48 1.91 2.70
0.11 0.83 0.93 1.64 2.75
-0.32 0.32 0.35 1.17 2.1 1
0.85
The 1 SD values are the same as in Table 2.
2-
SO'S
PAHs and single ring aromatics
1n
"E '
m
E
W
2
0-
-1-
-2-3-
M
values ( i e . ,the mr and br values) and the r;! values for the regressions are given in the figures. Each of the regressions is quite linear with r;! 2 0.96. As expected (31,the m,values are in each case similar to -1, especially for the alkanes. At log pf = -6, for the alkanes, log Kp is reduced by -0.50 in going from RH = 30% to RH = 48.5% and by another -0.44 in going from RH = 48.5% to RH 70%. For the PAHs, at log pf = -5, the same changes in RH cause reductions in log Kpof -0.47 and -0.67, respectively. Thus, increasing the RH from -30% up to -70% causes about 1 order of magnitude reduction in Kp for both the alkanes and the PAHs studied here. Kp,sValues for SOCs on QFFs and Literature Kp,rValues for VOCs on Silica and Quartz. Taking CZQFF to be 1.65 mz/g, we have that log Kp,s(m3/m2)= log Kp
+ 5.78
(QFFs) (8)
and thereby obtain the Kp,svalues in Table 3 for the alkanes and PAHs sorbing to QFFs. The results are plotted in Figures
2
-4-
-5\
Regression Parameters: mrs brs r' low: -1.16 -3.8? 0.99 mid:. -1.13 -4.37 0.99 high: -1.11 -4.76 0.99
-0 -7 -6 -5 -4 -3 -2 - 1
0
1
2
3
log p i ( t o r r ) FIGURE 7. Log Kp*(m3/mZ)vs log pi?(Torr)data at low, mid, and high RH for sorption of SOC PAHs on quartz fiber filters (QFFs) (from this work) together with data for VOC single ring aromatics on quartz sand (from ref 18).
6 and 7. Plotted along with our SOC data for alkanes in Figure 6 are the 25.7,62.3, and 88%RH data for sixrelatively volatile n-alkanes (pentane through decane) for sorption to nearly pure silica (17.5 m2/g) derived from the 15 "C study of Dorris and Gray (25). The data were adjusted using VOL. 29, NO. 9, 1995 / E N V I R O N M E N T A L SCIENCE & TECHNOLOGY
2425
6
TABLE 4
Literature Values for log Kp,s(m3/m2)for WOCs on Silica and Quartz at 20 "6 for Three Relative Humidity (RH, YO) Ranges n-alkanes on silica (29 c5
CS Cl
CS c9
ClO
$5
25.7% RH
2.57 2.03 1.52 1.02 0.53 0.11
-6.60 -6.15 -5.70 -5.27 -4.83 -4.39
5
log Kp,* (m3/mZ) 62.3% RH 88% RH -N
-6.89 -6.48 -6.09 -5.80 -5.30 -4.91
-7.15 -6.79 -6.43 -6.07 -5.70 -5.35
log Pf (Torr)
30% RH
toluene m-xylene p-xy le ne chlorobenzene o-dichlorobenzene m-dichlorobenzene 1,2,3-dichlorobenzene 1,2,4-trichlorobenzene 1,3,5-trichlorobenzene naphthalene
1.32 0.78 0.81 0.96 0.02 0.23 -0.55 -0.49 -0.38 -0.70
-5.05 -4.50 -4.45 -4.99 -4.13 -4.38 NA -3.70 NA -3.36
-5.43 -4.84 -4.82 -5.35 -4.64 -4.79 -3.90 -4.19 -4.48 -3.85
4
E
Ambient, UPM
\
3- 3
NAa -5.30 -5.21 NA -4.87 -5.08 -4.19 -4.43 -4.73 -4.15
48.5%, QFF o
F2
3
1
0
-1
NA = not available.
\
-3oS, QFF
$
log Kp.r (m3/m2) 50% RH 70% RH
aromatics on quartz (19)
a
PAHs
Regression Parameters: m, -0.88 -2.19 -30% -1.20 -4.05 48.5% -1.27 -4.94 -70% -1.18 -5.07
UPM
3
-7
-6
-5
-4
-3
log p t (torr) 20 "C/ 15 "C factors (about 0.75) that were calculated based on their experiments at 21% water content; the results are given in Table 4. The adjusted 25.7,62.3, and 88%RH data sets were regressed together with our -30,48.5, and -70% RH data sets, respectively. The resulting three regression lines for low, mid, and high RH are given in Figure 6. The form of each regression is
1%
Kp,s
= mr,s 1%
PE + k
s
(9)
Compared to eq 3, for a given data set, we note that mr,s = m,,and br,s= b, - log(10-6a). The parameter b,,,is thus the specific-surface-area-normalizedintercept. Despite the differences in experimental methods, the exact natures of the solids, and the 10 order of magnitude range in pe, each of the combined data sets for the three RH values indicates remarkable linearity, with a slope close to -1.0, and z 0.99. Figure 7 provides a plot for the PAHs that is analogous to Figure 6. The additional VOC data that are plotted are from Goss (19) for the sorption on quartz of various lower molecular weight aromatics, including naphthalene. (Although naphthalene is usually not considered a VOC, for our purposes, it is quite volatile.) Goss' (19)data, given in Table 4, were obtained by extrapolation to 20 "C using equations given in that paper. Goss' (19) data for 30, 50, and 70% RH were regressed together with our -30, 48.5, and -70% RH data sets, respectively. The resulting three regression lines for low, mid, and high RH are given in Figure 7. As in Figure 6, good linearity is present in each of the three combined regressions, and the slopes are similar to those in Figure 6. The greater scatter in the VOC data in Figure 7 as compared to Figure 6 may be due to the fact that the VOCs in Figure 7 represent a relatively diverse mixture of compound types, including single ringed aromatics, chlorinated single ringed aromatics, and naphthalene. There is, nevertheless, considerable consistency between the SOC and VOC data groups. For example, for
+
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ENVIRONMENTAL SCIENCE &TECHNOLOGY / VOL. 29, NO. 9 , 1 9 9 5
FIGURE 8. Comparison of log 6,' (m3/n?) values obtained for sorption of PAHs on qua& fiber filters (QFR)at 20 "C and three RH values with ambient data for PAHs sorbing to UPM.
the low RH data (-30%), including the VOC data in the regression causes the slope to change by less than 5% (-1.20 to -1.15). Comparison of Kp,sValues for QFFs with Calculated Kp,sValues for UPM. As discussed above, a goal of this work was to determine Kp,svalues for QFFs and compare them with surface-area-normalized values of Kp for UPM. If adsorption governs G/P partitioning to UPM, based on Sheffield and Pankow's (8)estimate that UTSP % 2.1 m2/gfor UPM, by eq 4 we have
log KP,, (m3/m2) log Kp + 5.68 (UPM)
(10)
For PAHs sorbing to UPM under ambient conditions in Osaka, Japan,the Yamasaki data set (13)togetherwith PAH vapor pressure values consistent with those in Table 1 indicate that at 20 "C, log (F/TSP)/A= -0.98 log p&- 7.87. Taking @TSP 2.1 m2/g,we then obtain that log (FITSPII (AlO-'a~sp) = -0.98 log p& - 2.19. This line is plotted in Figure 8 along with the three regression equations from Figure 5 (after shifting the latter by -log(10-6aoFF)), For alkanes sorbing to UPM in Portland, OR, the Cls-C24 data of Hart (18) obtained using Teflon membrane filters for particle collection together with vapor pressure values consistent with those in Table 1 indicate that at 20 "C, log (FITSP)/A = -0.84 log pf - 6.93; taking UTSP % 2.1 m2/g,we obtain log (FlTSP)l(AlO-'%zTsp)= -0.84 log pf' - 1.25. For alkanes sorbing to UPM in Denver, CO, at 5 "C using vapor pressure values consistent with those in Table 1,we calculate that the data of Foreman and Bidleman (26) give log (F/ TSPIIA = -0.74 log pf - 6.71; taking UTSP = 2.1 rn'lg, we obtain log (F/TSP)/(A~O.'UT~~) = -0.74 log pf - 1.03. The lines for the Hart (18) and Foreman and Bidleman (26)data sets are plotted in Figure 9 along with the three regression
would not be the dominant mechanism. Other evidence that supports the absorptive interpretation for UPM would be the relatively low magnitude of the fdter adsorption artifact as studied by Hart and Pankow (27).
6
Alkanes 5 Ambient: N
Acknowledgments This work was supported in part by a grant from the U.S. Environmental Protection Agency's Office of Exploratory Research (U.S. EPAIOER) under Grant R822312-01-0.
4
E
\
3
3
Glossary
v)
Q w 2 0
d
1 Regression P a r a m e t e r s :
0 48.5% -7h
-1
3
1
-7
-1.09
-3.48
-1.11 -1.01
-4.10
-3.94 I
-6
t
-5
-4
I
log p t ( t o r r ) FIGURE 9. Comparisonof log Kp3(m3/mZ)values obtained for sorption of n-alkanes on quartz fiber filters (QFFs) at 20 "C and three RH values with ambient data for n-alkanes sorbing to UPM.
equations from Figure 4 (after shifting the latter by -1og(10-6aTsp)).For purposes of the comparisons in Figure 9, it is assumed that temperature effects on the UPM data were exerted mainly through p?. In Figure 8, for the 28 events in the Yamasaki UPM data set for PAHs, the average RH was 73%. The resulting UPM line is significantly higher than the QFF lines for RH = 48.5 and -70%. If alarger value of ~ T S is P assumed, say 10 m2/g, then the UPM line is lowered by 0.7 log unit, but is still about 1 log unit above the -70% RH QFF line. A similar situation is described in Figure 9 for the alkanes. Both of the UPM alkane lines lie significantly above the QFF alkane lines for RH = 48.5 and 70%. We note in this context that the average RH for the Hart (18) data set was -65%.
Conclusions Experiments with a range of PAH and alkane SOCs have yielded reliable Kp and Kp,svalues at 20 "C for sorption to clean quartz for RH values in the range -30--70%. Based on Figures 8 and 9, we conclude that sorption to UPM does not involve adsorption to surfaces like clean quartz. If adsorption to UPM is important, then there must be one or more surface types present that are much more sorptive than what we have observed for clean quartz. Considering the nonspecific nature of simple physical adsorption, this may be unlikely. At the same time though, we note that for rural/remote air masses wherein most of the TSP can be suspended soils and continental dust, adsorption to mineral surfaces may be important. Another interpretation of Figures 8 and 9 would be that absorption to the organic carbon component of UPM is significantly more important than adsorption to the available surface area. In this latter interpretation, the Kp,svalues calculated for UPM would not be real since adsorption
gas phase concentration of a compound in the atmosphere (ng/m3) specific surface area (m2/g) specific surface area of QFF (m'lg) specific surface area atmospheric TSP (m2/g) y-intercept in a log (FITSPIIA (or log Kp)vs log pf plot y-intercept in a log (FITSP)I(A10.6~~sp) (or log Kp3) vs log pi: plot gas-phase concentration (ng/m3) solid-phase associated concentration (nglpg) measured value of gadsolid partitioning coefficient (m3/pg) coefficient multiplying l/Efor adsorption component of Kp coefficient multiplying l/Efor absorption component of Kp particulate-phase associated concentration of a compound in the atmosphere (ng/m3) gaslparticle partitioning coefficient (m3/pg) surface-area-normalized gaslparticle partitioning coefficient (m3/m2) slope in a log (FITSP)/A (or log Kp)vs log pi:plot slope in a log (F/TSP)/(A~O-'UTSP)(or log ICp,&vs log pe plot mass of sorbent used in measurements of Kp subcooled liquid vapor pressure (Torr) quartz fiber filter relative humidity (%) semivolatile organic compound minimum time required to deliver the mass of compound that sorbs at equilibrium (min) total suspended particulate concentration in the atmosphere @g/m3) urban particulate matter minimum air volume to deliver the mass of compound that sorbs at equilibrium (m3)
literature Cited (1) Radian Corporation. Simultaneous Control of PM-IO and
Hauzrdous Air Pollutants: Rationale for Selection of Hazardous Air Pollutants asPotential Particulate Matter or Associated with Particulate Matter at Source Conditions; Final Report to U.S. EPA, Air Emissions and Atmospheric Research Laboratory, Research Triangle Park, NC; EPA/452-R-93-013;U.S. Government Printing Office: Washington, DC, June 1993;NTIS PB93-2163801 REB. (2) Yamasaki, H.; Kuwata, K.; Miyarnoto, H. Environ. Sci. Technol. 1982, 16, 189. (3) Pankow, J. F. Atmos. Environ. 1991, 25A, 2229.
VOL. 29. NO. 9.1995 / ENVIRONMENTAL SCIENCE 5 TECHNOLOGY
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Pankow, J. F.; Storey, J. M. E.;Yamasaki,H. Environ. Sci. Technol. 1993, 27, 2220. Junge, C. E. In Fate of Pollutants in the Air and Water Enuironments; Suffet, I. H., Ed.; Wiley: New York, pp 7-26. Pankow, J. F. Atmos. Enuiron. 1987, 21, 2275. Pankow, J. F. Atmos. Enuiron. 1994, 28, 185. Shefield, A. E.; Pankow, J. F. Enuiron. Sci. Technol. 1994, 28, 1759. Sowelim, M. A. Atmos. Enuiron. 1983, 17, 145. Davis, B.L.; Johnson, L. R.;Stevens, R. K.; Courtney, W. J.; Safriet, D. W. Atmos. Enuiron. 1984, 18, 771. Chiou, C. T.; Shoup, T. D. Enuiron. Sci. Technol. 1985,19,1196. Thibodeaux, L. I.; Nadler, K. C.; Valsaraj, K. T.; Reible, D. D. Atmos. Enuiron., 1991, 25A, 1649. Yamasaki, H.; Kuwata, K.; Miyamoto, H. Enuiron. Sci. Technol. 1982, 16, 189. Cotham, W. E.; Bidleman, T. F. Enuiron. Sci. Technol. 1992,26, 469. Turpin, B. J. Ph.D. Dissertation, Oregon Graduate Institute, 1989. Storey, J. M. E. Ph.D. Dissertation, Oregon Graduate Institute, 1993.
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Pankow, J. F.;Ligocki, M. P.; Rosen, M. E.; Isabelle, L. M.; Hart, K. M. Anal. Chem. 1988, 60, 1378. Hart, K. M. Ph.D. Dissertation, Oregon Graduate Institute, 1990. Goss, K.-U. Environ. Sci. Technol., 1992, 26, 2287. Kiselev, A. V. Proc. Int. Congr. Surf Act. Znd, 1957, 309. Rhue, R. D.; Pennell, K. D.; Rao, P. S. C.; Reve, W. H. Chemosphere 1989, 18, 1971. Stober, W. Kolloid 2. 1956, 145, 17. Whalen, J. W.; Hu, P. C. 1. Colloid Interfuce Sci. 1978, 65, 460. Kang, Y.; Skiles, J. A,; Wightman, J. P. 1. Phys. Chem. 1980, 84, 1448. Dorris, G. M.; Gray, D. G. 1. Phys. Chem. 1981, 85, 3628. Foreman, W. T.; Eidleman, T. F.Atmos.Enuiron. 1990,24A,2405. Hart, K. M.; Pankow, J. F. Environ. Sci. Technol. 1994, 28, 655.
Received for review February 28, 1995. Revised manuscript received J u n e 12, 1995. Accepted J u n e 13, 1995.@
ES950135I @
Abstract published in Advance ACS Abstracts, August 1, 1995.
