United States National Exposure

Environmental Protection Research Laboratory

Agency Research Triangle Park, NC 27711

Research and Development EPA/600/SR-97/035 July 1997

Evaluation of a Gossen, GmbH

Model PAS 1001i Photoelectric

Aerosol Sensor for Real-Time

PAH Monitoring

Project Summary

Jane C. Chuang and Mukund Ramamurthi

The objective of this study was to

evaluate the performance of a Model

PAS 1000i Photoelectric Aerosol Sensor (also referred to in this report as

the “PAH Analyzer” for real-time monitoring of polycyclic aromatic hydrocarbons (PAH) in air. Three tasks were

conducted in the study: 1) vapor tests,

2) particle tests, and 3) integrated sampler comparison tests. In the vapor

tests, benzene and PAH vapors were

introduced individually into a 17 m3

 environmental chamber as vapors, with

the chamber air concentrations monitored by a Trace Atmospheric Gas Analyzer (TAGA). 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 μm in diameter) were generated and then sampled by the PAH analyzer to determine whether the instrument responded only to aerosols containing 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 concentration levels common in indoor air (for

example, 0.1-0.3 μm diameter, 10,000-

50,000 particles/cm3

). Further step-bystep tests confirmed that the analyzer’s

response did result from the passage

of the aerosol through the ultraviolet

radiation cell. The origin of this response could not be resolved in this

study, but possible causes of the response to non-PAH aerosol were explored.

In the integrated sampler comparison study, two Battelle-developed indoor PAH samplers were collocated

with the PAH analyzer in indoor environments with and without the presence of environmental tobacco smoke

(ETS). The fine particle-bound (<2.5 μm)

PAH concentration in the air sampled

was then estimated by summing the

fine particle concentrations of individual

PAH and alkylated PAH species determined 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 analyzer concentrations were approximately 4 times higher than the fine particle PAH concentrations derived from

the integrated samplers. In the absence

of ETS, the PAH analyzer concentration was similar to the fine particlebound PAH concentration in one test,

but was only one-third of the fine particle 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 aerosol.

This Project Summary was developed

by EPA's National Exposure Research

Laboratory, Research Triangle Park, NC,
2

to announce key findings of the research project that is fully documented

in a separate report of the same title

(see Project Report ordering information at back).

Introduction

The PAH analyzer evaluated in this

study was a Model PAS 1000i Photoelectric Aerosol Sensor manufactured by

Gossen, GmbH (Erlangen, Germany) and

obtained in the U.S. with ancillary data

acquisition software from EcoChem Technologies, Inc. (West Hills, CA).

The PAH analyzer is based on the principle of photoelectric aerosol charging reported in the literature within the last decade. The presence of a mono- or

submono-layer of PAH compounds

adsorbed on the surface of aerosol particles allows UV-light induced photoemission of electrons from electron-rich PAH

molecules that are adsorbed onto a monolayer on the surface of a particle. This

phenomenon, also referred to as aerosol

photoemission (APE), is the measurement

principle used in the PAH analyzer evaluated in this study.

The aerosol sampled enters the analyzer via an electrofilter. The electrofilter

in the PAS 1000i 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 PAHadsorbed aerosols at particle sizes <1 μm.

The net APE signal is minimal at particle

sizes >1 μm 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 electrons by PAH adsorbed on the surfaces of

aerosol particles causes the particles themselves to become positively charged. The

charged aerosol is then brought through

an electrically isolated conduit to an electrometer. 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 combustion exhausts and except for residential 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 particle surface, with the highest response

associated with large π− 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 surfaceadsorbed PAH mass.

Various studies with laboratory APE systems 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 characteristics would tend to remain generally

constant for a specific source of particulate-phase PAH.

The APE signal can be expected to be

less correlated with the particle-phase PAH

concentration when the mix of PAH compounds and the aerosol surface characteristics in the air sampled are more variable. This situation is of significant interest in evaluating the PAH analyzer as a

reliable indoor screening or monitoring tool

since indoor air can conceivably contain

particulate-phase PAH originating in varying proportions from many different aerosol 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, however, not been previously conducted.

The objective of this study was to evaluate 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 response of the PAH analyzer to sodium chloride (NaCl) 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 provided here:

(1) Benzene and PAH vapor tests: Tests

of the PAH analyzer’s response to

benzene, p-dichlorobenzene, naphthalene, 1-methylnaphthalene, quinoline, 1-chloronaphthalene, and indene were conducted by introducing

the vapors individually into a 17 m3

environmental chamber and monitoring the chamber air concentrations by a Trace Atmospheric Gas

Analyzer (TAGA). The concentrations 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 dispersed 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, temperature, and particle concentration

in the chamber atmosphere, and

the response of the PAH analyzer

when sampling from the chamber.

(2) Particle tests: NaCl test aerosols in

the 0.05-0.5 μm size range were

generated using a nebulizer that

generates a droplet spray by aspirating a solution of NaCl from a

reservoir (analytical grade NaCl,

muffled at 400°C for 4 hours was

used to prepare the salt solution).
3

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 passage through a radioactive Kr-85

neutralizer (2 mCi, 5/89). The neutralized 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 measurement instruments, a TSI, Inc.

Model 3020 CNC and a PMS, Inc.

Model LAS-X Laser Aerosol Spectrometer. Size distributions of the

test aerosols were measured by

coupling a TSI Model 3040/3042

Diffusion Battery with the CNC.

Apart from the NaCl 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 specific 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 NaCl 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 system consisted of two sampling

trains. The first used a Battelle-developed indoor sampler equipped

with an open-face 47-mm quartz

fiber filter upstream of an XAD-2

resin trap and collected the combined vapor and particle-phase

PAH. The second train consisted

of the following components (listed

in the order of air flow), impactordenuder-quartz fiber filter-XAD trap,

and was designed to collect fine

particle-bound (<2.5 μm) PAH in

the sampled air stream. The sampling flow rate through the two sampling 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) evaporation. The extract was then analyzed

by electron impact (EI) GC/MS for

target PAH and alkylated PAH species.

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 absence of ETS. During each test,

the PAH analyzer was collocated

with the two sampling trains and

the response over the sampling period was continuously recorded and

stored in a data acquisition computer. 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 measured by the CNC was recorded by

a strip-chart recorder that also recorded 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 literature reports that the PAH analyzer does

not respond to vapor-phase PAH compounds. The series of seven chamber tests

were conducted at low humidity conditions (9% relative humidity at 23-25°C),

and particle concentrations in the chamber 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 subsequently at chamber humidity conditions

of ~80%, using similar vapor concentrations and temperature conditions. These

tests also showed no PAH analyzer response to vapor-phase PAH in all except

one case. In the one case where a response was observed, for indene, a substantial particle concentration of ~50,000

particles/cm3

 was measured in the chamber suggesting that the observed PAH

analyzer response was due to the adsorption of vapor-phase PAH onto particle surfaces. The particle formation observed was

traced to the polymerization of the aged

indene liquid used in the test (Merck Index). A more detailed discussion of these

tests will be reported separately.

Particle Tests. The PAH analyzer did

not show any response during the background tests, when sampling either dry,

filtered air or filtered air carrying dried,

deionized water droplets from the nebulizer. The PAH analyzer did, however,

show a response when sampling the three

different NaCl test aerosols generated at

various number concentrations in the aerosol chamber (the test aerosols had lognormal size distributions and geometric

mean diameters of ~0.04, ~0.08, and ~0.15

μm). The non-zero PAH analyzer response

increased approximately linearly with particle concentration and with increasing test

particle size. More detailed tests conducted

subsequently also established that the response observed was definitely a result of

the passage of the test aerosol through

the UV irradiation cell. The magnitude of

the response observed for NaCl particles

is generally smaller than that observed in

either typical indoor air containing low,

background amounts of PAH-coated particles or high concentrations of cigarette

smoke particles.

An analysis of the PAH content of the

muffled bulk NaCl 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 possible 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 November 1992 by Burtscher and co-workers in

ETH, Zurich (a research group closely involved with the development of the Gossen

PAS) using a laboratory APE system also

found APE response to nebulized NaCl

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 eliminated by using an evaporation-condensation NaCl aerosol generator that involves

heating the bulk NaCl to a temperature of

~700°C, suggesting that trace organic contamination may indeed be responsible for

the observed APE response from nebulized NaCI aerosols.
4

Integrated Sampler Comparison Tests.

The integrated sampler comparison tests

were used to conduct comparisons between 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 μm) PAH

concentration was estimated by summing

the fine particle concentrations of individual

PAH and alkylated PAH species determined from GC/MS analysis. PAH concentrations were calculated from the average PAH analyzer response for each corresponding test using the conversion factor 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 measured fine particle PAH concentration from

the integrated sampler in one test but was

one-third of the fine particle PAH concentration in the other test.

Conversion factors were also derived

on the basis of the fine particle PAH concentrations and the PAH analyzer’s average 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 defined either as the sum of all species, or

the sum of > 3-ring species. Within the

non-ETS category, the calculated conversion factors from one test were significantly 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 calibration functions provided by Gossen,

GmbH.

Another observation that was also evident from the non-ETS and ETS tests is that

the instrument response (in pAmp) was considerably 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 reason 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 compared to the typical, aged non-ETS indoor

aerosol. There may also be differences in

the surface characteristics of ETS aerosols 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 concentration in the chamber remained below 10 particles/cm3

 , thus minimizing the opportunity for APE from

PAH vapors adsorbed on particle

surfaces.

(2) The PAH analyzer did show a response to pure NaCl 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 NaCl used to generate the aerosol. The high-purity,

reagent-grade NaCl used was, however, muffled for 4 hours in a furnace 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

μm) 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 responses of the analyzer were converted to PAH concentrations for

these comparisons using the

manufacturer’s suggested calibration 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 response to PAH in ETS than to PAH

in non-ETS indoor aerosol.

(4) The PAH analyzer provided a realtime (< 5 sec) response that was

proportional to indoor particulatephase PAH. The response factors

relating instrument signal to actual

fine particle PAH concentration appear 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 importance and must hence be understood. A plausible explanation for

this phenomenon is that of APE

from trace species, such as organics, 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 possibility 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 conducted, and the identity of these

species could be isolated.

If the response of the PAH analyzer to non-PAH aerosols is indeed due to the trace organic species, the extent to which these species would be present in indoor

aerosol must be determined. If possible, the analyzer’s background

response when sampling different

types of indoor environments must

be estimated so that background

corrections can be made depending on the particle number concentration in the air sampled.

(2) Another issue that must be addressed 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 aerosol. 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 significant 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 performance investigated.

(3) The application of the PAH analyzer to indoor air can be addressed

once the above issues have been
5

investigated and resolved. A series

of comparison tests between integrated samplers and the PAH analyzer can be conducted to correlate

the analyzer response to PAH concentration under various indoor air

conditions. The response factors

determined from such tests would

be beneficial to the potential application of the analyzer as an indoor

air screening or monitoring tool. The

PAH analyzer could also be used

to measure PAH aerosol size distributions, both for the most common

combustion sources and in different indoor environments. We also

recommend investigating the application of the PAH analyzer for monitoring indoor particulate-phase nitro-PAH.
6

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 1001i

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

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