PB92-164979
Phase Distributions of Airborne Polycyclic Aromatic
Hydrocarbons in Two U.S. Cities
Battelle, Columbus, OH
Prepared for:
Environmental Protection Agency, Research Triangle Park, NC
L
J
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TECHNICAL REPORT DATA
1. REPORT NO. J.
«. TITLE AND SUBTITLE
Phase Distributions of Airborne Polycyclic Aromatic
Hydrocarbons in Two U. S. Cities
T. AUTHOR(S)
Robert C. Lewis, AREAL, EPA, RTP; Thomas J.
Jane C. Chuang, Patrick J. Callahan,
Robert W, Coutant, Battslle Columbus, Ohio
Kelly,
9 PERFORMING ORGANIZATION NAME AND ADDRESS
Battelle
SOS Ring Avenue
Columbus, Ohio 4J2O1
11. SPONSORING AGENCY NAME AND ADDRESS
Atmospheric Research and Exposure Assessment Lab-RTP
Office of Research and Development
U.S. Environmental Protection Agency
Research Triangle Park, NC 27711
PB92-164979
5. REPORT DATE
2/28/92
6.PEJLFOAMINO OSOANttATJON CODE
Battelle
I.POLFORMINQ ORGANIZATION REPORT
NO.
N/A
10 PROGRAM ELEMENT NO.
Aioi/c-ie/oi
1 1 . CONTRACr«3RA]Tr NO .
68-DO-0007
13 TYPE OF REPORT AND PERIOD COVERED
Conf. Proceedings 6/91-2/92
14. SPONSORING AGENCV CODE
EPA/600/09
IS. SUPPLEMENTARY NOTES
16. ABSTRACT
Measurements of tha ambient air concentrations of 16 polycyclic aromatic
hydrocarbons (large, carbon-and-hydrogen-containing molecules derived from fossil
fuels, wood burning, and other sources) were made in Boston, Massachusetts, and
Houston, Texas. Since these chemicals may exist in the air both as vapors and
associated with particles, a special type of sampler was designed to determine the
distributions between these phases. This "denuder" sampler, which denudes the air
of vapors and collects only particles, was operated next to a traditional sampler
at each site from August 27, 1991 until August 27, 1992. Phase distributions were
determined on a seasonal ba odel was tested to determine if
these distributions could b A^j^jS^^ (** al **•**•* ot a compound in the
atmosphere may determine ho /\ 7-, j&aj ne *nd now toxic it is when
breathed. Conequently , it lA*^" methods to measure phase
distributions and apply thai £xUV/ &*j 1"? JO£j/ r in urban areas.
"'
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PB92-UU971
IU-118.09
Phase Distributions or Airborne Polycyclic
Aromatic Hydrocarbons in Two U.S. Cities
Robert G. Lewis
U.S. Environmental Protection Agency
Methods Research and Development Division (MD-44)
Research Triangle Park, North Carolina 27711-2055 USA
Thomas J. Kelly, Jane C. Chuang,
Patrick J. Callahan, and Robert W. Coutanc
BATTELLE
Columbus Operations
505 King Avenue
Columbus, Ohio 43201-2693
REPRODUCED flv
U.S. DEPARTMENT OF COMMERCE
NATIONAL TECHNICAL INFORMATION SERVICE
SPRINGFIELD. VA. 22161
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IU-IIB.09
INTRODUCTION
Polycyclits aromatic hydrocarbons {PAHs} have long been known u> exist in the atmosphere
distributed between the vapor and panicle-associated phases.1'1 Traditional air sampling systems
consisting of particle filters followed by vapor traps may efficiently collect the total of txnh phases.
hut do not provide accurate data on the proportions of the chemicals in each phase, due to
volatilization of the paniculate-phase PAHs fiom the fillers to the vapor trap."1 The magnitude of the
volatilization artifact may be very large, depending on meteorological conditions (especially
temperature) and the residence time of the panicles on the filter during sampling.5
Accurate determination of the physical state of airborne PAHs is needed to understand
atmospheric fate and transport phenomena and to assess potential health risks. To address this need.
a high-volume compound annular denuder (HVCAD) sampler was developed and shown to be
efficient at separating vapor phase PAHs from particles.6 Field evaluations of the HVCAD. involving
collocaied sampling with (he traditionally-designed PS-1 sampler, were conducted during summer and
winter periods in Columbus, Ohio.5 These investigations showed that while 95% of PAHs with vapor
pressures greater than 10"' kPa were collected in the vapor trap of the PS-l sampler, samples
collected with the HVCAD sampler indicated that the median proportions of the compounds in the
vapor phase at the time of sampling averaged 40 to 70% over the two seasons.J'7 The volatilization
artifact was determined to range from 6 to 92% depending on compound vapor pressure and air
temperature, which ranged from daily means of-5.5° to + 29CC.
[n order to obtain more extensive data covering all four seasons, sampling was carried out in
two U.S. cities with substantially different climates. The results of this study, conducted between
August 1990 and August 1991 in Boston, Massachusetts, and Houston, Texas, are discussed in this
paper.
EXPERIMENTAL
Sampling Sites and Schedules
The two sampling shes chosen for this work were formerly pan of the U.S. EPA Toxics Air
Monitoring System (TAMS) network. One site was located in downtown Boston and ihe other about
20 km east of downtown Houston in Deer Park. Both sites are classified as industrial, although the
Boston site is impacted heavily by automotive traffic and genera] urban sources, while the Houston
site is in a semi-rural area downwind of large oil refineries and chemical plants. The means of the
average daily ambient temperatures measured at the two sites on sampling days are presented by
season in Table I, along with the means of the daily minimum and maximum temperature readings.
The samplers were placed in secured areas on roof lops at both sites, about 12 m above ground
in Boston and about 3 m above ground ir. Houston. Samplers were operated approximately every
Uth day, commencing at 0600 local time and continuing for 24 hours. Flow rate checks were per-
formed immediately before and after each sampling period, Flow calibrations were performed on the
samplers prior to the study and again in the field at the midpoint of the study. Collocated duplicate
samples and field blanks were taken periodically, and field spike recoveries were determined at the
start of the study (see below).
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JU-I1B.09
TABLE I. Ambient temperature daia(t> for the two field sampling sites in *C.
Season
Summer
FaJI
Winter
Spring
Mean
24
15
1
9
Boston
Minimum
IB
2
-14
1
Maximum
37
29
13
IB
Mean
28
19
12
20
Houston
Minimum
23
4
1
10
Maximum
40
34
24
21
(a) Data are means of the minimum, maximum, and average daily temperatures recorded at the
monitoring sites on days when PAH samples were collected.
Air Sampling
The basic air sampler used in these studies is the General Metal Works PS-1 sampler equipped
with a 104-mm Pallflex 2500-QATUP quaru-fiber particle filter followed by a glass vapor trap con-
taining 55 g of Supel-Pak-2 precleaned Amberlite XAD-2 resin.8 This sampler has been shown to be
70-100% efficient at collecting a broad spectrum of PAHs.9 This sampler was operated simultane-
ously with a collocated identical sampler equipped with a high-volume compound annular denuder.5
The inner walls of the HVCAD were coated with Dow Corning high vacuum grease to an average
layer thickness of 69 jim. This denuder has been shown to be 89 to 99% efficient for the target
compounds.6'10 The basic (non-denuder) sampler was used to determine total PAH concentration,
while the collocated denuder-equipped sampler was used to determine panicle-associated PAHs (the
two samplers were placed 0.6 m apart). Vapor phase concentrations were calculated by subtracting
denuder results from those obtained with the basic sampler Both samplers were operated at nominal
flow rates of 110 L/min, with acruaJ flows measured at the time of sampling. Sealed ftlter/XAD sam-
pling modules and precoated denuder assemblies were shipped to and from the two monitoring sites
by overnight delivery service. For this procedure, specially-modified ice chests were used that held
each component in a stable, protected manner and permitted the addition of ice packs for the return
trip to Columbus, Ohio, for chemical analysis. Cleaned XAD cartridges spiked with naphthalene-da.
acridine-d9, and chrysene-d,2 were initially transported to and from the sites to serve as field controls.
Recoveries of the latter two compounds were essentially identical to those obtained from laboratory
controls, while that of the more volatile naphthalene-dg was about 20% less (76% versus 96%).
These results indicated minimal losses of collected PAHs as a result of the sample shipping
procedures. Collocated duplicate samples taken with the PS-1 samplers showed good agreement for
measured concentrations of all the target PAHs, with an overall mean relative standard deviation of
11.5% in Boston (range 0.37 to 28.6%), and 14.9% in Houston (range 0.03 to 45.3%).
Analytical Method
For all but two sets of samples from both sites, the corresponding filter and XAD-2 samples
were combined and Soxhlet extracted with dichloromethane (DCM) for 16 hours. For two samples
from each site, the filter and XAD-2 samples were extracted separately using DCM in order to
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IU-I1B.09
determine artifact values. The DCM extract was concentrated by Kuderna-Danish (K-D) evaporation
and the extract was analyzed by gas chromatography/mass spectrometry (GC/MS) to determine PAH.
A Finnigan TSQ-4S GC/MS/MS operating in GC/MS selected ion monitoring (SIM) mode, and an
(NCOS 2300 data system, were employed. Peaks monitored were the molecular ion peaks, and the
characteristic fragment ion peaks of the target compounds. The GC column was an DB-5 fused silica
capillary column (30 m x 0.21 mm. 0.25 urn film thickness; Supelco). The GC temperature was held
at 70°C for 2 minutes, then programmed to 290°C at lO'C/minuie, and held at 290°C for 20
minutes. Identifications of target compounds were based on their GC retention times relative to the
internal standard (9-phenylanthracene). Quantification of the target compounds was based on
comparisons of the respective integrated ion current responses of the target ions to that of the
corresponding internal standard, with average calibration responses of target compounds generated
from standard analyses. The estimated precision for the analytical method is 10%. The estimated
detection limit for the target compounds was 0.01 ng/m3 based on 150 m3 of air sampled.
Data Analysis
The vapor-phase and panicle-bound levels of each target compound were calculated using the
following equations:
P + V
FXd = P -I- (l-E)V
(FXnd - FXd)
E
(I)
(2)
(3)
(4)
where
FXd =
P
V
E
Measured concentrations from filter and XAD-2 combined sample for non-
denuder sampler.
Measured concentrations from filter and XAD-2 combined sample for
denuder sampler.
Estimated concentration of panicle-bound target compound.
Estimated concentration of vapor-phase target compound.
Calculated denuder adsorption efficiency for each target compound.
RESULTS AND DISCUSSION
If we classify semivolatile organic chemicals as those with saturation vapor pressures at 25 "C
that fall between 10'2 and 10'8 kPa4-", the 18 PAHs targeted in this study ranged from nearly volatile
(naphthalene, 8.7 x 10"3 kPa) to nonvolatile (coronene, 1.5 x 10"13 kPa). Theoretically, the former
should exist in the ambient atmosphere primarily, if not exclusively, in the vapor phase, while the
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IU-HB.09
latter should exist essentially in the panicle-associated phase. The first direct measurements of the
phase distribution of PAHs using denuder sampling indicated that substantial proportions of 3- and 4-
ring PAHs exist in the ambient air as vapors1. The median vapor phase distributions in summer and
winter measurements in Columbus, Ohio, ranged from 28 to 43% for six targeted PAHs having
estimated vapor pressures between 10"* and 10'* kPa. However, collocated non-denuder PS-1
sampling showed the major fractions of these compounds were found in the vapor trap due to post-
collectirn volatilization from the panicle filter. Since our earlier work was limited both temporally
and geographically, the more extensive field studies reported here were undertaken in hopes of
clarifying some of the questions regarding phase distribution and to provide additional data that may
aid in prediction of the distributions.
The ambient air concentrations of the target PAHs were determined by the non-denuder sam-
ples and are summarized for the two sites in Table It. The most abundant PAH (vapor phase and
paniculate phase) found in air from both Boston and Houston sites was naphthalene, and (he least
abundant PAH was dibenzo[a,h]anthracene for most samples. Differences in ambient PAH concentra-
tions were observed between the Boston and Houston sites. In general, higher concentrations of most
4- to 7-ring PAH were found in Boston. Note that in the winter the concentrations of most 2- to 3-
ring PAH showed the reverse trend, indicating higher concentrations in samples from Houston than in
those from Boston. Similar concentration ranges were observed at both sites during summer, fall,
and spring. The Boston site, located in downtown Boston, can be classified as an urban area with
heavy traffic. The Houston site is located in a suburban area about 20 km ease of downtown
Houston, but with a stationary source (petroleum refineries) nearby. The higher concentrations of the
relatively volatile PAH may be due to the contribution of both the petroleum refinery source and local
mobile source emissions. On the other hand, the higher concentrations of most 4- to 7-ring ?AH at
the Boston site may be attributed to the heavier mobile source emissions and fuel combustion.
At the Boston site, higher average ambient concentrations of most 4- to 7-ring PAH were
observed in the winter, and similar concentration levels were observed in the spring and fall. Lowest
concentrations of most 4- to 7-ring PAH were found in the summer. However, higher average
ambient concentrations of 2- to 3-ring PAH were observed in the summer; similar concentration
levels were found in fall and winter and the lowest concentration levels were in spring.
At the Houston site, a similar seasonal concentration trend was observed for most of the target
compounds. It should be noted that the winter average temperature at the Houston site is about 10°C
higher than that at the Boston site, and thus a less pronounced seasonal variation in the volatile PAH
levels in Houston might be expected. The relatively higher concentrations of volatile PAH observed
in the summer at the Houston site may be due to stationary source emissions, as mentioned above. In
general, at both sites day-to-day variations of ambient concentrations of most target compounds were
observed to range from 10 to 100% within each season.
Naphthalene, which ranged in concentration from 140 to 1,160 ng/m3 in the non-denuder
samples, was often found in higher amounts in the denuder samples, indicating a contamination
problem. Analysis of the silicone grease used to coat the denuders showed high blank values for
naphthalene. Out gassing of naphthalene from the coated denuders apparently occurred during samp-
ling, and was particularly significant during warm weather. Consequently, no phase distribution
could be determined for naphthalene. Similar problems were sometimes found for several other of
the more volatile PAHs, making it difficult or impossible to interpret phase distribution data.
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TABLE II. Total (vapor + particle) concentrations of airborne PAHs in two cities*'*
Compound0"
Naphthalene
Acenaphlhene
Fluore?ic
Fluorenonc
Phenanlhrenc
Anthracene
Fluoranlhene
Pyrenc
CycIopenU|c.d]pyrene
Benzja|anthracene
Chrysenc
Benzofluoran thanes
Benzo(e]pyrene
Bcnzoja (pyrcne
Indeno( 1 .2.3-c.d)pyrene
Dibenzofa , h Janthracene
Benzo{g. h,i]perylene
Coronene
Vapor Pressure
kPA al 25°C«>
1.1 x 10 2
ca. 10J
ca. 1CT4
ca. 10-*
2.2 x 10*
3.3 x 10*
6.5 x ID'7
3.1 x I041
ca. Iff7
I.Si 10'
l.7x 10*
ca. ia'
7.3 x 10 I0
7.3 x 10 I0
ca. If/"
ca. Itf"
l.3x 10'"
2x 10-"
Summer
606
24.1
36.2
10.2
101
3.0
16.7
8.4
0.3
0.9
1.0
l.t
0.4
0.4
0.5
0.3
0.6
0.3
(a) Data arc from PS-t sampler without denudcr at STP
(b) Listed in order of elution on DB-5 GC column.
(c) Vapor pressure values are either from published data
Mean
Concentration ng/m1
Boston
Fall
569
16.9
18.0
6.5
52.1
2.9
11.8
10.7
0.5
0.8
1.3
1.6
0.8
0.8
0.7
0.2
1.3
0.8
Winter
453
5.7
10.0
8.2
30.2
1.6
9.4
8.3
1.1
2.0
2.1
2.9
1.0
1.5
2.0
0.4
3.2
2.7
conditions.
or calculated using
Spring
255
4.3
5.5
2.7
18.2
1.0
6.8
4.7
0.6
0.9
1.0
2.1
0.7
1.2
1.2
0.3
1.6
1.0
Summer
515
22.8
24.2
5.4
50.1
1.4
10.2
7.9
0.2
0.6
0.6
0.5
0.2
0.2
0.2
0.2
0.3
0.2
the Clapeyron-ClauMUs
Houston
Fall
44 1
15.8
19.9
6.4
45.9
1.8
10.0
11.8
0.2
0.7
1.0
0.4
0.3
0.2
0.2
0.2
0.4
0.3
equation.
Winter
671
12.8
18.0
7.2
37.9
2.2
6.7
6.4
0.2
0.4
0.7
1.0
0.4
0.4
0.8
0.2
1.4
0.9
Spring
153
5.5
5.0
1.3
11.2
0.5
3.8
2.3
O.I
O.I
0.2
0.2
0.1
0.2
0.2
0.2
0.3
0.4
C
1
00
s
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IU-ILB.09
particularly during summer months, and particularly under the warm conditions typical of the
Houston site. Efforts were made in the course of the study to minimize the transfer of PAH con-
tamination from the denuder, but some contamination was unavoidable. As a result, some phase
distribution dau known 10 be affected by the denuder outgassing were deleted from the daia set.
Figure 1 shows the percentages estimated lo be in the vapor-phase at both sites for II of the
target compounds. At the Boston site, the lowest percentages of vapor -phase PAH were observed
from data collected in winter. This finding is in agreement with the ambient temperature data
indicating that lower proportions of vapor-phase PAHs are present at lower temperatures. However,
this seasonal difference in phase-distribution of PAH concentrations was not as pronounced in the data
from the Houston site. This is due in part to smaller differences in ambient temperatures in Houston.
Another reason for the relatively lower fractions of fluorene, phenanthrene, and anthracene in
the vapor phase in Houston during winter months may be the release of contaminants from (he
denuder silicons grease into the sampling cartridge. In general, the major proportions of the 3-ring
PAHs are in the vapor phase. Approximately 10 to 50% of the 4-ring PAHs are present in the vapor-
phase. depending upon the ambient temperature. The nonvolatile (6- and "7-ring) PAHs are present
predominantly in the paniculate phase, as expected.
Several of the targeted PAHs are not shown in Figure 1. Naphthalene and acenaphthene were
omitted because no good denuder data could be obtained (see above). The benzofluoranthene isomers
showed 3 to 10% vapor phase composition (range 0-57%), but could not be completely separated by
GC. Of the four Least volatile PAHs, dibenzo [a ,h] anthracene, which exhibited the largest vapor
component (mean 1%, range 0-56%), was chosen for p\oKJrtg. The mean vapor phase fraction was
0% for tndeno[lt2,3-c,dlpyrene, berao[g,h,i]perylene, and coronene in Houston, while in Boston the
mean vapor fractions were 3 to 7%. The concentrations of the 6- and 7-ring PAHs were generally
below I ng/m3; therefore, estimations of vapor fractions by denuder difference calculations are
rt cc large errors .
When separate analyses of panicle tiller and vapcrtrap collections by trad! row] samplers are
performed, 75-100% of 3- and 4-rhg PAHs are generally found in the vapor trap, while SO 10 100*
of 5- (o 7-ring PAHs are found on the fiJfer.1"* The quantities found in the vapor trap reflect the
combination of the fraction originally in the vapor phase (V) plus the portion of particle-adsorbed
PAH that vaporized from the filter after collection, A (i.e., the volatilization artifact). Since only
particles pass through the denuder to be collected on the filter, the quantity of a given PAH found in
che bacfc-up vapor trap of a denuder-equippsd s&npter may be laken as the approximate value of A
for a traditional sampler. Unfortunately, due to budget constraints, only in four cases could separate
analyses of the filter and XAD-2 trap be carried out to provide volatilization artifact estimates for this
study.
Table III lists the values of A and associated data for those instances in which reliable mea-
surements could be made. Median values of A ranged from 17 to 44% for the more volatile group of
PAHs (vapor pressures ICT* to 10* kPa) and 2 to 15% for the less volatile group (v.p. 10'7 to 1CT*
kPa). No detectable quantities of die PAH with vapor pressures k>wer than IP* JcPa were found in
the XAD-2 traps of the denuder sampler, indicating A values of approximately zero. The relatively
low A values for fluorenone may reflect stronger adsorption to the paniculate matter and/or filter
medium due to the presence of the ketone group.
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FL FN PHE ANT FLAN PYR CPPY BAA CHRY BEP BAP DBA
FIGURE I. Proportions or airborne PAHs in vapor phase in Boston and Housion. Vertical bars represent average
percent vapor phase for all sampling days during the indicated season. Range of deviation from the average is
indicated by superimposed vertical scales. Fl, fluorene; FN, fluorenone; Phe, phenanthene; Ant, anthracene;
Flan, fluoranthene; Pyr, pyrene; CPPy, cyclopenta[c,d]pyrcne; BaA, benz[a)anthracene; Chry, chrysene;
BeP, benzo[e]pyrene; BaP, benzo[a]pyrene; DBA, dibenzo[a,h]anthracene. Data for the first four PAHs could
not be calculated accurately except for the winter season due to contamination from the denuder.
(Note: Asterisk indicates that no vapor phase component was detected in any sampte.)
po
S
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IU-11B.09
TABLE III. Medians of empirically determined volatilization artifacts and
other parameters for late winter and spring(i>
Concentration.
Compound ng/mj
Fluorene
Fluorenone
Phenanlhrese
Anthracene
Fluoranthene
Pyrene
Cyclopenu[c,d]pyrene
Benzfa janchracene
Chrysene
4.5
1.5
10.0
0.6
5.0
3.1
0.2
0.4
0.5
Percent
of Total
in Vapor
Trap*1
95
84
90
83
82
80
29
27
33
Volatilization
Artifact (A) as
percent of Total'"
Median
44
17
32
44
30
24
15
9
9
Range
11-68
8-67
12-40
19-17
13-65
3-72
0-36
9-34
0-17
Percent
Originally
in Vapor
Phase""
(V)
52
71
59
40
51
56
16
13
23
(a) Data compiled from boih Boston and Houston sites; sampling dates February 19-20
and April 2-3, 1991; median temperature 1TC; range, 7 10 19°C.
(b) Determined by separately analyzing filter and vapor trap of PS-1 basic sampler.
(c) Determined by separately analyzing filter and vapor trap of denuder sampler and
calculating artifact per reference 5.
(d) Calculated from difference in total (vapor + panicle) concentrations determined by
basic and denuder sampler.
The sum of the fraction V originally in the air as vapor and A closely approximates the fraction
X^j of the total sample collected in the vapor trap of the non-denuder sampler, as expected. The
results of this study are in generally good agreement with those obtained previously in Columbus,1
although the A values were somewhat lower in the present study, especially for the less volatile group
of PAHs. At least part of these differences in magnitude is undoubtedly due to the higher average
temperatures experienced in the previous study (14°C versus 11°C). As expected, the magnitude of
A observed in this study increased with the mean ambient temperature at the time of sampling. The
mean values of A were 18% at 7°C and 66% at 19°C for the more volatile group of PAHs, com-
pared to 3% and 30%, respectively for the less volatile group.
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IU-1 IB.09
CONCLUSIONS
This was the first field evaluation of the HVCAD In which (he denuder samplers were set up
and operated by personnel other than the researchers who developed the device. Except for some
initial handling problems encountered in Houston, field performance was quite acceptable, as exem-
plified by good overall agreement with previous studies. Contamination problems with the silicone
grease used to coat the denuders were serious, however, making it necessary to discard much of the
data on the most volatile PAHs. These problems were not identified in the previous studies because
naphthalene, acenaphthene, fluorene, and fluorenone were not determined in the pievious study.
These contamination problems also are partially attributed to changes In the manufacture of the
grease. Efforts are under way to correct most of these problems through the use of purer silicone
grease preparations. An interference in (he determination of acenaphihene was encountered here that
appears to result from the release of a semivolatile silicon-containing compound which yields an ion
of mass/charge 154, and which coelutes with acenaphihene, resulting in an overestimate of
acenaphihene in denuded samples. This problem might be overcome using different GC temperature
conditions to separate the interferant peak fron acenaphthene, however, this approach might also
substantially lengthen the analysis time.
The findings of this study confirm our earlier, more limited work, and demonstrate that
volatilization artifacts cnn be very substantial when traditional sampling methods are used. While the
denuder difference method can directly measure phase distributions, the costs of multiple analyses and
operating two sampling systems are burdensome. However, studies such as these are necessary to
obtain a more substantial data base which can be used for the development of models to predict phase
distributions from traditional sampling data.
Several factors affect phase distribution of a compound; (I) equilibrium vapor pressure, (2)
chemical functionality, (3) total suspended paniculate matter concentration, (4) concentration of the
compound, (5) concentrations of coexisting compounds, (6) ambient air temperature, and (7) relative
humidity, Ths higher the vapor pressure, the greater the proportion of a compound expected to be in
the vapor phase. The presence of polar functional groups (e.g. keto, nitro, carboxyl) will result in
stronger binding to panicles and reduce the proportion of a given compound found in the vapor phase
relative to a less polar compound with similar vapor pressure. Higher TSP concentrations will pro-
vide more opportunity for compounds to be adsorbed by panicles, and hence reduce the vapor com-
ponent. Conversely, the higher the atmospheric concentration of a given compound, the closer to
saturation of the panicle phase and hence 'he greater the vapor phase fraction. Likewise, the
presence of other compounds in the air parcel which can compete for TSP adsorption sites may enrich
the vapor component Since vapor pressures increase with temperature, the vapor fraction will, of
course, be larger in warmer weather (and the volatilization artifact may also be greater). Competition
by water for sites of adsorption on panicles may result in enhanced vapor phases at high relative
humidities.
There is still much to be learned before routine monitoring and modeling of the phase distri-
butions of PAHs and other airborne compounds can be reliably made. Clearly, simpler sampling sys-
tems are needed not only to provide more economical measurements but also to supply the data bases
required for modeling to predict phase distribution.
10
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ACKNOWLEDGMENTS
We thank Sydney Gordon. Joachim Pleil, Gary Evans, and Jeffrey Childers for technical
assistance; David Davis for sample preparation; and Vanessa Katona for data analyses.
DISCLAIMER
This paper has been reviewed in accordance with the U.S. Environmental Protection Agency's
peer and administrative review policies and approved for presentation and publication. Mention of
trade names or commercial products does not constitute endorsement or recommendation for use.
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