United States
Environmental Protection
Agency
National Exposure
Research Laboratory
Research Triangle Park, NC 27711
Research and Development
EPA/600/SR-97/035 July 1997
Project Summary
Evaluation of a Gossen, GmbH
Model PAS 1001i Photoelectric
Aerosol Sensor for Real-Time
PAH Monitoring
Jane C. Chuang and Mukund Ramamurthi
The objective of this study was to
evaluate the performance of a Model
PAS 10001 Photoelectric Aerosol Sen-
sor (also referred to in this report as
the "PAH Analyzer" for real-time moni-
toring of polycyclic aromatic hydrocar-
bons (PAH) in air. Three tasks were
conducted in the study: 1) vapor tests,
2) particle tests, and 3) integrated sam-
pler comparison tests. In the vapor
tests, benzene and PAH vapors were
introduced individually into a 17 m3 en-
vironmental chamber as vapors, with
the chamber air concentrations moni-
tored by a Trace Atmospheric Gas Ana-
lyzer (TAG A). The concentrations of the
spiked chemicals in the air ranged from
200 ppb for 1-methylnaphthalene to 500
ppb for benzene. The PAH analyzer did
not yield any response when sampling
these chemical vapors.
In the particle tests, sodium chloride
aerosols of various sub-micron sizes
(0.05-0.5 um in diameter) were gener-
ated and then sampled by the PAH ana-
lyzer to determine whether the instru-
ment responded only to aerosols con-
taining PAH. The results showed that
the PAS did respond to NaCl aerosol
that was anticipated to contain no PAH.
This response depended on the size
and number concentration, and was a
small fraction of the typical response
obtained at particle sizes and concen-
tration levels common in indoor air (for
example, 0.1-0.3 um diameter, 10,000-
50,000 particles/cm3). Further step-by-
step tests confirmed that the analyzer's
response did result from the passage
of the aerosol through the ultraviolet
radiation cell. The origin of this re-
sponse could not be resolved in this
study, but possible causes of the re-
sponse to non-PAH aerosol were ex-
plored.
In the integrated sampler compari-
son study, two Battelle-developed in-
door PAH samplers were collocated
with the PAH analyzer in indoor envi-
ronments with and without the pres-
ence of environmental tobacco smoke
(ETS). The fine particle-bound (<2.5 UJTI)
PAH concentration in the air sampled
was then estimated by summing the
fine particle concentrations of individual
PAH and alkylated PAH species deter-
mined by gas chromatography-mass
spectrometric (GC/MS) analysis of the
filters and XAD-2 sorbent traps from
the integrated samplers. The response
of the PAH analyzer (in pAmp) over the
corresponding sampling periods was
also averaged and then converted to a
PAH concentration using the
manufacturer's suggested conversion
factor of 3000 ng/m3/pAmp.
In the presence of ETS, the PAH ana-
lyzer concentrations were approxi-
mately 4 times higher than the fine par-
ticle PAH concentrations derived from
the integrated samplers. In the absence
of ETS, the PAH analyzer concentra-
tion was similar to the fine particle-
bound PAH concentration in one test,
but was only one-third of the fine par-
ticle PAH in the other test. It was also
observed from the tests conducted that
the PAH analyzer yielded a larger unit
response to PAH in the ETS aerosol
than to PAH in the non-ETS indoor aero-
sol.
This Project Summary was developed
by EPA's National Exposure Research
Laboratory, Research Triangle Park, NC,
Printed on Recycled Paper
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to announce key findings of the re-
search project that is fully documented
In a separate report of the same title
(see Project Report ordering informa-
tion at back).
Introduction
The PAH analyzer evaluated in this
study was a Model PAS 1000i Photoelec-
tric Aerosol Sensor manufactured by
Gossen, GmbH (Erlangen, Germany) and
obtained in the U.S. with ancillary data
acquisition software from EcoChem Tech-
nologies, Inc. (West Hills, CA).
The PAH analyzer is based on the prin-
ciple of photoelectric aerosol charging re-
ported in the literature within the last de-
cade. The presence of a mono- or
submono-layer of PAH compounds
adsorbed on the surface of aerosol par-
ticles allows UV-light induced photoemis-
sion of electrons from electron-rich PAH
molecules that are adsorbed onto a mono-
layer on the surface of a particle. This
phenomenon, also referred to as aerosol
photoemission (APE), is the measurement
principle used in the PAH analyzer evalu-
ated in this study.
The aerosol sampled enters the ana-
lyzer via an electrofilter. The electrofilter
in the PAS 1000! has a nominal operating
voltage potential of =465 V, and removes
all the ions and a fraction of the charged
particles that may be present in the air
being sampled. The aerosol exiting the
electrofilter enters the UV irradiation cell
where it is irradiated and APE occurs. In
principle, APE results from the PAH-
adsorbed aerosols at particle sizes <1 urn.
The net APE signal is minimal at particle
sizes >1 urn in diameter; at these sizes
recapture of the photo-emitted electrons
by the particles is believed to occur too
rapidly to permit the charged particles to
be detected. The photoemission of elec-
trons by PAH adsorbed on the surfaces of
aerosol particles causes the particles them-
selves to become positively charged. The
charged aerosol is then brought through
an electrically isolated conduit to an elec-
trometer. The current measured by the
electrometer is then a sensitive indicator
of the degree of APE occurring in the UV
irradiation cell.
Previous evaluation studies reported in
the literature have reported that APE arises
only from PAH adsorbed on aerosols. At
the UV irradiation energy used of ~4.2 eV
no other adsorbate was found in these
studies to result in an APE signal in these
laboratory APE systems. In addition, the
APE signal was found to be proportional
to the amount of adsorbed PAH, up to the
limit of a monolayer on the surface of a
particle. Researchers have suggested,
however, that in typical residential com-
bustion exhausts and except for residen-
tial wood combustion, the particle number
concentrations emitted are sufficiently high
in relation to the PAH mass emitted that
only low PAH surface coverages are likely
to result. Under these circumstances, the
net APE response could continue to be
proportional to the total adsorbed PAH.
In addition to these characteristics, the
APE signal from the analyzer varies with
the PAH compound adsorbed on the par-
ticle surface, with the highest response
associated with large n- electron systems.
Of the PAH compounds tested, coronene
was the most photoelectrically active. The
nature of the surface on which the PAH
compounds are adsorbed also affects the
APE response. For each type of particle
surface, however, the response generally
increases linearly with increasing surface-
adsorbed PAH mass.
Various studies with laboratory APE sys-
tems have shown that for specific sources
of PAH aerosols, such as cigarette smoke,
oil stoves, spark-ignition engines, etc., the
APE signal is reasonably correlated with
the concentration of particle-phase PAH
compounds measured by wet-chemical
techniques. This result is consistent with
the two characteristic response features
described above, since the mix of PAH
compounds and the aerosol surface char-
acteristics would tend to remain generally
constant for a specific source of particu-
late-phase PAH.
The APE signal can be expected to be
less correlated with the particle-phase PAH
concentration when the mix of PAH com-
pounds and the aerosol surface charac-
teristics in the air sampled are more vari-
able. This situation is of significant inter-
est in evaluating the PAH analyzer as a
reliable indoor screening or monitoring tool
since indoor air can conceivably contain
particulate-phase PAH originating in vary-
ing proportions from many different aero-
sol sources such as ETS, woodstoves,
burners, fireplaces, combustion-derived
aerosols in outdoor air, etc. An evaluation
of the relationship between the PAH
analyzer's response and measured PAH
in different indoor environments has, how-
ever, not been previously conducted.
The objective of this study was to evalu-
ate the overall performance of the PAH
analyzer in monitoring PAH indoors, with
the following three tasks:
(1) Vapor tests to verify that the PAH
analyzer does not respond to PAH
vapors, as reported previously in
the literature.
(2) Particle tests to determine the re-
sponse of the PAH analyzer to so-
dium chloride (NaCI) test aerosols
and to determine whether the PAH
analyzer has the desired selective
response for only PAH containing
aerosols.
(3) Integrated sampler comparison
tests to compare the indoor PAH
concentrations determined from
conventional integrated sampling
and subsequent GC/MS analyses
with the average responses over
corresponding sampling periods
from the collocated PAH analyzer.
Procedure
A brief description of the procedures
used to conduct the three tasks is pro-
vided here:
(1) Benzene and PAH vapor tests: Tests
of the PAH analyzer's response to
benzene, p-dichlorobenzene, naph-
thalene, 1-methylnaphthalene, quino-
line, 1-chloronaphthalene, and in-
dene were conducted by introducing
the vapors individually into a 17 m3
environmental chamber and moni-
toring the chamber air concentra-
tions by a Trace Atmospheric Gas
Analyzer (TAGA). The concentra-
tions of the spiked chemicals in the
chamber air ranged from 200 ppb
for 1-methylnaphthalene to 500 ppb
for benzene. An aliquot of each of
the chemicals was injected through
a heated injection port where it was
vaporized, and subsequently dis-
persed into the chamber. Prior to
injection, the chamber was flushed
with AADCO zero air. During the
experiment, the chamber was
sampled by the TAGA, the PAH
analyzer, and by a TSI Model 3020
condensation nucleus counter
(CNC) for monitoring the particle
concentration in the chamber. The
measured concentration of the
spiked vapor was recorded for each
test, together with the humidity, tem-
perature, and particle concentration
in the chamber atmosphere, and
the response of the PAH analyzer
when sampling from the chamber.
(2) Particle tests: NaCI test aerosols in
the 0.05-0.5 nm size range were
generated using a nebulizer that
generates a droplet spray by aspi-
rating a solution of NaCI from a
reservoir (analytical grade NaCI,
muffled at 400°C for 4 hours was
used to prepare the salt solution).
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The nebulized droplets were dried
either by heating in a coiled copper
tube maintained at ~150°C or by
dilution with dry air. The solid salt
particles were then brought to a
state of charge equilibrium by pas-
sage through a radioactive Kr-85
neutralizer (2 mCi, 5/89). The neu-
tralized aerosol was mixed with dry,
filtered air and introduced into a
Plexiglas aerosol mixing chamber
from which it was sampled by the
PAH analyzer and two aerosol mea-
surement instruments, a TSI, Inc.
Model 3020 CNC and a PMS, Inc.
Model LAS-X Laser Aerosol Spec-
trometer. Size distributions of the
test aerosols were measured by
coupling a TSI Model 3040/3042
Diffusion Battery with the CNC.
Apart from the NaCI aerosol tests,
deionized water without dissolved
salt was nebulized, dried, and
sampled from the aerosol chamber
to establish background conditions.
Carrier air alone was also sampled
from the aerosol chamber in other
background tests. A second series
of detailed PAH analyzer tests were
also conducted in which two spe-
cific components of the analyzer,
namely the electrofilter and the UV
lamp were turned off selectively
without disabling the electrometer.
These detailed tests of the response
of the PAH analyzer to NaCI test
aerosols were conducted using a
660-1 stainless steel chamber. The
large chamber allowed the test
aerosols to be aged for several
hours to simulate a typical indoor
aerosol before being sampled by
the PAH analyzer.
(3) Integrated sampler comparison
tests: The integrated sampler sys-
tem consisted of two sampling
trains. The first used a Battelle-de-
veloped indoor sampler equipped
with an open-face 47-mm quartz
fiber filter upstream of an XAD-2
resin trap and collected the com-
bined vapor and particle-phase
PAH. The second train consisted
of the following components (listed
in the order of air flow), impactor-
denuder-quartz fiber filter-XAD trap,
and was designed to collect fine
particle-bound (<2.5 ^m) PAH in
the sampled air stream. The sam-
pling flow rate through the two sam-
pling trains was controlled at 20.0
± 0.2 Ipm using a pump and a
metering valve and was monitored
continuously by an in-line mass flow
meter.
The filter and XAD-2 trap from each
integrated sampler were combined
and extracted with dichloromethane
(DCM) by the Soxhlet technique.
The DCM extract was concentrated
by Kuderna-Danish (K-D) evapora-
tion. The extract was then analyzed
by electron impact (El) GC/MS for
target PAH and alkylated PAH spe-
cies.
Four sets of indoor integrated
samples were collected during the
study: two tests were performed in
the presence of ETS and two tests
were conducted without ETS. The
sampling periods employed in the
tests varied between 8 and 24 hours
depending on the presence or ab-
sence of ETS. During each test,
the PAH analyzer was collocated
with the two sampling trains and
the response over the sampling pe-
riod was continuously recorded and
stored in a data acquisition com-
puter. During all tests, a TSI Model
3020 CNC was used to monitor the
particle concentrations in the indoor
environment being sampled. The
particle number concentration mea-
sured by the CNC was recorded by
a strip-chart recorder that also re-
corded the PAH analyzer response
during the sampling intervals.
Results
Benzene and PAH Vapor Tests. The
PAH analyzer did not yield a response
when sampling chamber air containing
each of the various chemical vapors. This
finding agrees with the published litera-
ture reports that the PAH analyzer does
not respond to vapor-phase PAH com-
pounds. The series of seven chamber tests
were conducted at low humidity condi-
tions (9% relative humidity at 23-25°C),
and particle concentrations in the cham-
ber remained below 1-2 particles/cm3 in
all tests, i.e., there was minimal possibility
for water vapor-induced hydrolysis and/or
condensation processes that might result
in particles with surface-adsorbed PAH.
Additional vapor tests were conducted sub-
sequently at chamber humidity conditions
of ~80%, using similar vapor concentra-
tions and temperature conditions. These
tests also showed no PAH analyzer re-
sponse to vapor-phase PAH in all except
one case. In the one case where a re-
sponse was observed, for indene, a sub-
stantial particle concentration of ~50,000
particles/cm3 was measured in the cham-
ber suggesting that the observed PAH
analyzer response was due to the adsorp-
tion of vapor-phase PAH onto particle sur-
faces. The particle formation observed was
traced to the polymerization of the aged
indene liquid used in the test (Merck In-
dex). A more detailed discussion of these
tests will be reported separately.
Particle Tests. The PAH analyzer did
not show any response during the back-
ground tests, when sampling either dry,
filtered air or filtered air carrying dried,
deionized water droplets from the nebu-
lizer. The PAH analyzer did, however,
show a response when sampling the three
different NaCI test aerosols generated at
various number concentrations in the aero-
sol chamber (the test aerosols had log-
normal size distributions and geometric
mean diameters of ~0.04, -0.08, and -0.15
|im). The non-zero PAH analyzer response
increased approximately linearly with par-
ticle concentration and with increasing test
particle size. More detailed tests conducted
subsequently also established that the re-
sponse observed was definitely a result of
the passage of the test aerosol through
the UV irradiation cell. The magnitude of
the response observed for NaCI particles
is generally smaller than that observed in
either typical indoor air containing low,
background amounts of PAH-coated par-
ticles or high concentrations of cigarette
smoke particles.
An analysis of the PAH content of the
muffled bulk NaCI used in preparing the
salt solutions was conducted using DCM
extraction and GC/MS analysis. PAH was
not detected in the bulk salt extract, but
trace amounts of aliphatic hydrocarbons
and phthalates were detected. It is pos-
sible that the observed signal is related to
other unknown organic species, and this
possibility could be examined further in a
future study.
While the results obtained in this study
appear to contradict literature reports by
other researchers, recent tests in Novem-
ber 1992 by Burtscher and co-workers in
ETH, Zurich (a research group closely in-
volved with the development of the Gossen
PAS) using a laboratory APE system also
found APE response to nebulized NaCI
tests aerosols. These responses were
found to be of a similar magnitude as .
those found in this study. Further,
Burtscher et al. found that APE is elimi-
nated by using an evaporation-condensa-
tion NaCI aerosol generator that involves
heating the bulk NaCI to a temperature of
~700°C, suggesting that trace organic con-
tamination may indeed be responsible for
the observed APE response from nebu-
lized NaCI aerosols.
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Integrated Sampler Comparison Tests.
The integrated sampler comparison tests
were used to conduct comparisons be-
tween the vapor+particle-phase PAH and
fine particle-bound PAH concentrations
from the integrated samplers and the PAH
analyzer's average response (in pAmp)
over the corresponding sampling interval.
The fine particle-bound (< 2.5 \im) PAH
concentration was estimated by summing
the fine particle concentrations of individual
PAH and alkylated PAH species deter-
mined from GC/MS analysis. PAH con-
centrations were calculated from the aver-
age PAH analyzer response for each cor-
responding test using the conversion fac-
tor of 3,000 ng/m3/pAmp provided by
Gossen, GmbH.
The average PAH concentrations from
the PAH analyzer were about 4 times
higher than the measured fine particle PAH
concentrations for the two tests conducted
in the presence of ETS. In the absence of
ETS, the average PAH concentration from
the PAH analyzer was similar to the mea-
sured fine particle PAH concentration from
the integrated sampler in one test but was
one-third of the fine particle PAH concen-
tration in the other test.
Conversion factors were also derived
on the basis of the fine particle PAH con-
centrations and the PAH analyzer's aver-
age response in each of the four tests.
The calculated conversion factors were
similar for the two tests within the ETS
category, when fine particle PAH was de-
fined either as the sum of all species, or
the sum of ^ 3-ring species. Within the
non-ETS category, the calculated conver-
sion factors from one test were signifi-
cantly higher than the conversion factors
calculated from the data collected in the
other test. The conversion factors derived
from the tests were generally within (with
the exception of the one non-ETS test)
the range of the various instrument cali-
bration functions provided by Gossen,
GmbH.
Another observation that was also evi-
dent from the non-ETS and ETS tests is that
the instrument response (in pAmp) was con-
siderably higher when sampling cigarette
smoke aerosols than it was to the general
indoor aerosol. This result is consistent with
reports in the literature that APE is the highest
(per unit mass) from PAH such as coronene,
benzo(g,h,i)perylene, benzo(e)pyrene, and
benzo(a)pyrene, all of which are species
present in ETS aerosol. Another possible rea-
son for this higher response rate for ETS
aerosol is that a greater fraction of the PAH in
these freshly generated aerosols may be
present in a surface-adsorbed state com-
pared to the typical, aged non-ETS indoor
aerosol. There may also be differences in
the surface characteristics of ETS aero-
sols that result in a higher response rate
compared with non-ETS indoor aerosol.
Conclusions and
Recommendations
The following conclusions can be drawn
from this study:
(1) The PAH analyzer does not yield a
response when sampling air from a
test chamber atmosphere spiked
with benzene and PAH vapors. In
these tests, the aerosol concentra-
tion in the chamber remained be-
low 10 particles/cm3, thus minimiz-
ing the opportunity for APE from
PAH vapors adsorbed on particle
surfaces.
(2) The PAH analyzer did show a re-
sponse to pure NaCI aerosol. The
exact mechanism for this response
is presently uncertain; one possible
origin of the observed response is
APE from trace organics (other than
PAH) in the bulk NaCI used to gen-
erate the aerosol. The high-purity,
reagent-grade NaCI used was, how-
ever, muffled for 4 hours in a fur-
nace at 400°C prior to use in the
experiments.
(3) In the presence of ETS, the PAH
concentrations calculated from the
PAH analyzer's average responses
were approximately four times (4X)
higher than the fine particle (< 2.5
(im) PAH concentrations derived
from integrated sampler-GC/MS
measurements. In the absence of
ETS, the calculated concentration
from the PAH analyzer's average
response was similar to the fine
particle PAH concentration in one
test but was only one-third of the
fine particle PAH concentration in
the other test. The average re-
sponses of the analyzer were con-
verted to PAH concentrations for
these comparisons using the
manufacturer's suggested calibra-
tion constant of 3,000 ng/m3 per
pAmp of analyzer signal. It was also
observed from the tests that the
analyzer yielded a larger unit re-
sponse to PAH in ETS than to PAH
in non-ETS indoor aerosol.
(4) The PAH analyzer provided a real-
time (< 5 sec) response that was
proportional to indoor particulate-
phase PAH. The response factors
relating instrument signal to actual
fine particle PAH concentration ap-
pear to vary with the nature of the
indoor aerosol being sampled.
Based on these results, the following
major recommendations were developed:
(1) The origin of the observed response
of the PAH analyzer to non-PAH
aerosols is of considerable impor-
tance and must hence be under-
stood. A plausible explanation for
this phenomenon is that of APE
from trace species, such as organ-
ics, other than PAH. However, the
identity of these species is not clear
at the present time and hence it is
difficult to speculate on the possi-
bility of APE from these or similar
organic species adsorbed on indoor
aerosols. Several tests to determine
if the response is indeed due to
trace organic species could be con-
ducted, and the identity of these
species could be isolated.
If the response of the PAH ana-
lyzer to non-PAH aerosols is in-
deed due to the trace organic spe-
cies, the extent to which these spe-
cies would be present in indoor
aerosol must be determined. If pos-
sible, the analyzer's background
response when sampling different
types of indoor environments must
be estimated so that background
corrections can be made depend-
ing on the particle number concen-
tration in the air sampled.
(2) Another issue that must be ad-
dressed is the need and function of
the low-voltage electrofilter used in
the PAH analyzer. In the current
operating mode, the electrofilter is
removing gaseous ions as well as
a small fraction of the charged aero-
sol. However, removing a portion
of the charged fraction of the
sampled aerosol may result in the
removal of a possibly significant
portion of the particulate PAH in
the air. This effect may be signifi-
cant for combustion aerosols that
may be highly charged immediately
after production. On the other hand,
sampling an initially positively or
negatively charged aerosol could
interfere with the measurement of
APE from adsorbed PAH. The need
for the electrofilter must thus be
studied and the impact of removing
the electrofilter on instrument per-
formance investigated.
(3) The application of the PAH ana--
lyzer to indoor air can be addressed
once the above issues have been
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investigated and resolved. A series determined from such tests would combustion sources and in differ-
of comparison tests between inte- be beneficial to the potential appli- ent indoor environments. We also
grated samplers and the PAH ana- cation of the analyzer as an indoor recommend investigating the appli-
lyzer can be conducted to correlate air screening or monitoring tool. The cation of the PAH analyzer for moni-
the analyzer response to PAH con- PAH analyzer could also be used taring indoor particulate-phase ni-
centration under various indoor air to measure PAH aerosol size distri- tro-PAH.
conditions. The response factors butions, both for the most common
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Jane C. Chuang and Mukund Ramamurthi are with Battelle, Columbus, OH 43201-
2693.
Nancy K. Wilson is the EPA Project Officer (see below).
The complete report, entitled "Evaluation of a Gossen, GmbH Model PAS 100H
Photoelectric Aerosol Sensor for Real-Time PAH Monitoring," (Order No. PB97-
147938; Cost: $21.50, subject to change) will be available only from:
National Technical Information Service •
5285 Port Royal Road
Springfield, VA 22161
Telephone: 703-487-4650
The EPA Project Officer can be contacted at:
National Exposure Research Laboratory
U.S. Environmental Protection Agency
Research Triangle Park, NC 27711
United States
Environmental Protection Agency
Center for Environmental Research Information
Cincinnati, OH 45268
Official Business
Penalty for Private Use $300
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