United States
Environmental Protection
Agency
Atmospheric Research and
Exposure Assessment Laboratory
Research Triangle Park, NC 27711
Research and Development
EPA/600/SR-94/192 November 1994
EPA Project Summary
Sampling Carbonaceous
Aerosols: A Review of
Methods and Previous
Measurements
Roy L. Bennett and Leonard Stockburger
Carbonaceous aerosols, a complex
mixture of carbonate carbon, elemental
carbon, and organic carbon (organic
compounds), are of environmental im-
portance due to their impact on visibil-
ity and the toxicity of some of the
organic compounds. A knowledge of
the relative vapor and particle concen-
tration of semivolatile organic com-
pounds is required to understand how
the compounds are transported in the
atmosphere and removed by deposi-
tion. Also, the deposition in the lungs
is dependent on the vapor/particle dis-
tribution. Because of the multiplicity of
compounds of varying volatility in the
organic aerosols, the sampling of these
particles presents an extremely diffi-
cult challenge. This report, based on a
literature search of measurement stud-
ies for the past 20 years, reviews the
possible artifacts that can occur that
result in positive and negative errors
owing to volatilization, sorption, or re-
action during the sampling processes.
The sampling approaches that have
been used range from single filters up
to complex parallel multiple component
systems that employ diffusion devices
to separate particle and vapors. The
artifacts that were addressed by each
approach are included. Representative,
but not exhaustive, lists of previous
carbonaceous aerosol measurements
are presented based on the literature
review. The first list includes measure-
ments of elemental carbon and organic
carbon without resolution of individual
organic compounds. The location, mea-
sured concentrations, and comments
on the artifacts addressed are presented
for each study. The second list is a
representative group of measurements
of specific organic compounds or
classes of organic compounds. Here,
because of the large number of indi-
vidual compounds present, concentra-
tions of each compound are not listed,
but the artifacts that the employed sam-
pling method addresses are noted.
This Project Summary was developed
by EPA's Atmospheric Research and
Exposure Assessment Laboratory, Re-
search Triangle Park, NC, 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
Carbonaceous materials are usually a
major component of ambient fine particle
mass concentration. For example, carbon-
aceous aerosols have been found to ac-
count for typically 40% of the total fine
particle mass loadings in the Los Angeles
area. These particles consist of a multi-
tude of organic compounds, both molecu-
lar and macromolecular, as well as
elemental carbon. Over 2800 compounds
have been identified in the ambient atmo-
sphere, but in those studies that have
quantified the individual organic compound
concentrations, usually the sum of the
measured compounds account for only a
few percent of the total organic mass con-
centration.
The potential toxicity of some of the
organic aerosols has made them the sub-
ject of environmental concern and exten-
sive investigation. Special attention has
been devoted to the hydrocarbon fraction
owing to the mutagenic activity exhibited
by compounds such as polynuclear aro-
matic hydrocarbons (PAHs) and their po-
tential carcinogenic effects. The
predominance of organic aerosols in the
fine particle size range makes them effi-
cient transporters of any toxic compounds
into the respiratory system.
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The carbonaceous aerosols, espe-
cially elemental carbon, are also of en-
vironmental concern because they occur
in the visibility-reducing size range
(0.1|j,m-1.0 |im) and may therefore have
an important impact on visibility condi-
tions and possibly climate. Elemental
(or black) carbon is the most important
absorbing species for sunlight in the at-
mosphere, and its catalytic properties
may result in its contribution to impor-
tant atmospheric chemical reactions.
Elemental Carbon and Organic
Carbon Aerosols
The complex mixtures of carbonaceous
aerosols are often classified as carbon-
ate, elemental carbon (EC), and organic
carbon (OC) that consists of organic com-
pounds. Ambient studies have shown that
carbonates account for only a negligible
fraction of the total carbon in urban aero-
sol. Elemental carbon has a chemical
structure similar to impure graphite and is
emitted directly from fossil fuel combus-
tion sources. Optical methods have been
used to measure EC, as it is considered
the primary component of atmospheric
aerosol that strongly absorbs light.
Organic aerosol compounds in the at-
mosphere may result from direct emis-
sions from a source (primary OC) or may
result from condensation of low vapor pres-
sure products of gas phase reactions of
hydrocarbons onto existing atmospheric
particles (secondary OC) or the reaction
product of primarily emitted organic com-
pounds with atmospheric oxidants.
A method based on thermal/optical re-
flectance procedures has been used to
measure the EC and OC components of
ambient aerosols. Several variations of
the procedure have been used that differ
in respect to the analysis time, tempera-
ture ramping rates, volatilization and com-
bustion, and calibration procedures.
Variations in the measured EC and OC
results of a variety of samples in an
interlaboratory comparison study have
been reported. In general, the agreement
between laboratories for total carbon was
within 20% for all samples, but there were
large interlaboratory differences in the OC/
EC ratio for all samples. Differences
among sampling methods were compared
with differences in analytical methods. For
EC, the observed differences in side-by-
side sampling were accounted for by dif-
ferences in the analytical methods. For
OC, most methods showed greater vari-
ability in results than could be accounted
for by the differences in analytical meth-
ods.
Carbonaceous Aerosols
Sampling Artifacts
As with most ambient aerosol measure-
ments, the most frequently employed sam-
pling technique for organic aerosols is
collection on filters, usually quartz fiber
filters. Because the organic aerosols are
a complex mixture of many compounds
with a wide range of vapor pressure, sorp-
tion properties, and chemical stabilities,
the sampling process has the potential for
several sampling artifacts. These may in-
clude the following:
(1) A portion of the organic vapor phase
may sorb onto the filter surface or
on the deposited particulate mat-
ter. This is often called the "posi-
tive" artifact because the apparent
mass that would be collected and
measured is greater than the aero-
sol mass that existed in the volume
of the atmosphere sampled.
(2) The sampling process may change
the gas-solid distribution of the
sample being collected, which re-
sults in volatilization of some of the
particulate organic compounds. This
produces a negative artifact.
(3) The collected particulate matter may
chemically react with vapor compo-
nents that pass through the filter
during sampling to change the
chemical composition of the deposit.
This can result in positive or nega-
tive artifacts, especially if individual
chemical compounds or chemical
functionalities are being measured.
These sampling artifacts complicate the
organic aerosol measurements, and it is
difficult to separate the different compet-
ing artifact processes although this has
been the object of many studies.
The positive artifact has been investi-
gated extensively. For example, samples
collected on a high-volume sampler quartz
filter indicated a positive bias of about
15%. With 24-h samples, positive errors
in organic carbon of 15-20% were found,
but the degree to which this was offset by
loss of volatile OC is not known. A typical
glass fiber filter has a surface area that is
much smaller than that of collected atmo-
spheric particulate matter, so that sorption
of vapor on the particles is believed to
outweigh sorption on filters. Diminished
levels of OC recovered from filters at higher
sampling velocities were attributed to de-
creased sorption rather than increased
volatilization. The negative artifact has also
been investigated. Theoretical calculations
have indicated that the evaporation losses
are probably most important when filters
with high pressure drops are encountered.
Investigators who used a multiple parallel
collector with a diffusion denuder system
concluded that samples from a remote
location exhibited negative artifacts as
large as 40 to 80%.
Changes in sample integrity have been
indicated by studies on the degradation or
reaction of PAH. A portion of particulate
phase aldehyde has been shown to be
oxidized during sampling to the corre-
sponding aliphatic acid. Reaction of or-
ganic acids in atmospheric samples with
basic components of glass fiber filter may
occur.
Gas/Particle Distributions of
Semivolatile Organic
Compounds
A comprehensive understanding of the
environmental impact of pesticides, PAH,
PCS, and other semivolatile organic com-
pounds in the atmosphere requires a
knowledge of both their gas and particle
concentrations. The gas/particle ratios are
important in how the compounds are trans-
ported in the atmosphere and removed by
deposition. Particles are quite effectively
removed by rain events. Health effects
also depend on the gas/particle ratios as
they are related to how the compounds
deposit in the lungs and exert potential
toxic effects.
The earliest systems for measuring the
gas/particle distributions were a particle
filter (glass fiber, quartz fiber, or Teflon)
followed by a solid sorbent trap such as
polyurethane foam (PUF), Tenax, or
XAD-2. The measurement is a severe chal-
lenge since the compounds are usually
present in low levels (nanograms per cu-
bic meter or less), and the procedure is
subject to positive artifacts from vapor
sorption, negative artifacts from volatiliza-
tion from the filter, and degradation of
compounds owing to reactions during sam-
pling.
The potential for volatilization and sorp-
tion artifacts is related in part to the kinet-
ics of the sorption process. The magnitude
of the artifacts is dependent on the vari-
ability of the total particle concentration,
the gas phase concentration, the tempera-
ture during the sampling, and the
mass-transfer reaction time scales. The
equilibrium vapor pressure concentration
(EVC) of atmospheric compounds and the
significance of the efficiency of collection
of the compounds on filters have been
considered. Absolute values of the ab-
sorbed state EVCs or the factor by which
the pure state EVCs are commonly re-
duced by sorption have been deduced
from measurements of losses from actual
air samples. A critical evaluation of the
literature gave a wide range of factors
(2-500) by which the pure state was re-
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duced by adsorption. This was attributed
to uncertainty in the literature data rather
than the difference in actual reduction fac-
tors. Comprehensive analyses have been
presented of the partitioning of organic
compounds between vapor and solid aero-
sol phases. Estimates of the slope and
intercept from plots obtained from a data-
base of gas particle distribution data vs 1/
T were found to be useful for predicting
the distribution in other urban environ-
ments. The effects of relative humidity on
gas/particle partitioning have been inves-
tigated and compared to field studies. The
log of the gas/particle partition constant
was found to have linearly decreasing de-
pendence on the relative humidity over
the range from 40 to 85% relative humid-
ity, based on semivolatile PAH compounds
measurements at Osaka, Japan. A ther-
modynamic model has been developed to
predict the partition between vapor and
liquid phases based on two parameters:
initial vapor mole concentration ratio and
the ratio of Henry's Law constant to initial
vapor partial pressure. As opposed to
soluble organic compounds, the partially
soluble organics tend to partition toward
the vapor phase owing to the large Henry's
Law constants.
Measurement Approaches for
Sampling Carbonaceous
Aerosols
Sampling methods for determining or-
ganic aerosol concentration and, if de-
sired, the corresponding vapor phase
concentration of semivolatile compounds
have evolved from the simple arrange-
ment of a single filter followed by a sor-
bent trap. As investigators have attempted
to eliminate or quantify the various arti-
facts that were encountered, their sam-
pling approaches have become more
complex with various denuder arrange-
ments, multiple combinations of filters of
different sorbency (quartz, carbon impreg-
nated, Teflon) and various sorbent traps.
Sample arrangements that have been used
included the following:
(1) Filter only
This approach assumed there were
neither sorption nor desorption arti-
facts.
(2) Filter/sorbent
This approach was used for total
(vapor plus particle) concentration
but does not give correct gas/par-
ticle distributions except for non-
volatile organics that are due to
artifacts.
(3) Multiple filters in series
Two quartz filters were employed
back to back with the amount on
the second filter used to correct the
positive artifact (vapor adsorption)
on the first. The vapor concentra-
tion passing through the second fil-
ter is assumed to be essentially the
same as the first, which may not
be true if the vapor is strongly
sorbed by the filter. Also, it does
not account for the negative artifact
or the amount adsorbed on the col-
lected particles.
(4) Dual quartz tandem filters with a
parallel Teflon/quartz tandem.
This is similar to the dual filter ap-
proach above but had two parallel
sets of tandem filters: the first is a
quartz filter with a quartz after-filter
and the second is a Teflon filter
with a quartz after-filter. The posi-
tive artifact is corrected by subtract-
ing from the quartz of the first set
the amount on quartz after-filter of
the second set. The rationale is
that the Teflon would absorb much
less of the vapor than quartz so the
exposure of the quartz filter after
the Teflon would be closer to the
atmospheric vapor concentration.
(5) Denuders
Diffusion denuders are devices with
channels operated at laminar flow
conditions in which vapors of the
atmospheric sample are separated
from the particles owing to the much
greater diffusion coefficients of the
vapors. High performance denud-
ers, compound annular denuders
that consist of coaxial cylinders so
that the air samples pass through
the annular space, have been de-
signed that allow the use of higher
sampling rates.
Various combinations of denuder ar-
rangements have been employed in or-
ganic aerosol sampling. A denuder
difference procedure for specific PAH com-
pounds has been developed in which two
parallel sampling arrangements are used.
On one side was the sequence: denuder,
quartz fiber filter, sorbent trap. On the
other was quartz fiber filter, sorbent trap
(without the denuder). For specific PAH
compounds the method measures true
particle phase, while the vapor concentra-
tion and the degree of negative artifact
are determined by denuder difference cal-
culation, not direct analysis of the de-
nuder.
A compound annular denuder system
has been employed to trap vapor con-
stituents by the denuder difference ap-
proach. Two filter/absorbent packs were
operated in parallel: one was preceded by
the denuder; the other was not. The frac-
tion of analyte that occurred in the vapor
phase was determined by subtraction of
the denuder sampler results from the con-
ventional sampler results.
Diffusion denuders consisting of sepa-
rated layers of quartz filters have been
used under the assumption that they re-
move only the gaseous organics that are
normally retained on the quartz filter. This
denuder was followed by a dual quartz
filter pack. Comparison of the results with
a parallel dual quartz pack led to the con-
clusion that the OC on the back filter of an
undenuded filter pack is primarily a posi-
tive enhancement artifact. The approach
does not address the negative artifact.
An annular denuder-based sampler,
called the integrated organic vapor/par-
ticle sampler (IOVPS), has been used for
the direct determination of gas and vapor
semivolatile PAH compounds in tobacco
smoke. Finely ground XAD-4 was used
for the denuder wall coating since it has
been shown to be an efficient sorbent for
PAH and nicotine. The technique provides
a means for determining phase concen-
trations for specific compounds where their
collection, recovery, and analysis have
been adequately demonstrated, but it is
not yet applicable to total organic vapor/
aerosol phase distribution and concentra-
tion measurement. Similarly, a diffusion
denuder sampler made from 120 short
parallel gas chromatographic (GC) tubes
coated with stationary phase has been
employed to measure PAH. The collected
gas on the denuder was analyzed directly
by desorption onto a GC column without
solvent dilution. The denuder was followed
by a quartz fiber filter and a PUF plug.
Particles are collected on the filter.
Vapor-phase molecules that break through
the denuder as well as molecules that
volatilize off the particles are collected on
the PUF.
One of the most elaborate of the semi-
volatile organic sampling systems consists
of three parallel samplers, two of which
contain multichannel parallel plate diffu-
sion denuders that were carbon-impreg-
nated filter paper to collect the gas phase.
Sampler 1 has the following sequences:
cyclone, denuder, two quartz filters, sor-
bent filter (the same carbon-impregnated
material used in the denuder plates) and,
finally, a quartz filter. Sampler 2 is the
same as Sampler 1 except the denuder is
placed after the two quartz filters instead
of before. Sampler 3 is only a filter pack;
that is, the same as Samplers 1 and 2 but
without the denuder. Conclusions drawn
by the investigators from sampling with
the system at Canyonland National Park
are that the conventional quartz filter arti-
fact is a negative rather than positive arti-
fact and previous data measurements with
filters may have underestimated the par-
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ticulate organic carbon by more than
100%.
Survey of Carbonaceous Aero-
sol Measurements
Measurement of Total Organic
Aerosol Concentration
A representative listing of field studies,
where only the elemental carbon and total
organic aerosol concentrations were de-
termined without organic compound reso-
lution, is presented in the report with
references. Sampling techniques have
ranged from single filters to complex, par-
allel multicomponent sampling systems.
Measurement of Specific
Organic Compounds
A list of studies in which individual com-
pounds or classes of compounds have
been specifically identified and measured
is also presented in the report with refer-
ences. Owing to the large number of stud-
ies on pesticides, PCS, and PAH
compounds, the full report does not
present a comprehensive listing of all mea-
surements that have been made. Also,
the sum of the concentrations of the re-
solved compounds in most of the studies
does not account for more than a few
percent of the total organic mass. Be-
cause many compounds in these specific
organic classes exist as both vapor and
particles their measurement involves the
same sampling and artifact problems en-
countered with total organic aerosol mea-
surements.
Conclusions
The sampling of organic aerosols in the
atmosphere is an extremely difficult task
owing to the very complex composition of
its organic molecular constituents. Exami-
nation of the peer-reviewed literature for
organic aerosol sampling studies during
the last 20 years yields the following ob-
servations:
(1) A large percentage of the reported
measurements are subject to arti-
fact errors. In a few cases the in-
vestigations ignore the artifacts, in
some they are acknowledged but
unaddressed, and in other studies
attempts are made to estimate the
magnitude of the artifacts by vari-
The EPA authors, RoyL. Bennett (also the Project Officer, see below) and Leonard
Stockburger, are with the Atmospheric Research and Exposure Assessment
Laboratory, Research Triangle Park, NC 27711.
The complete report, entitled "Sampling Carbonaceous Aerosols: A Review of
Methods and Previous Measurements," (Order No. PB95-129060; Cost: $17.50,
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
Atmospheric Research and Exposure Assessment Laboratory
U. S. Environmental Protection Agency
Research Triangle Park, NC 27711
ous sampling designs, many of
which yielded results that were later
challenged.
(2) In studies where the investigators
attempted to measure the sampling
error, the magnitudes of the deter-
mined positive or negative artifacts
have varied considerably. This is
probably due, in part, to differences
in the organic molecular composi-
tion (hence in the relative volatili-
ties and sorptivities) and in the
concentrations that occurred at the
different sampling locations.
(3) Multiple component sampling sys-
tems employing vapor phase sepa-
ration devices such as denuders
require a thorough evaluation to
demonstrate efficient removal of the
vapor at the operating conditions.
This has been done for specific
PAH compounds. However, the ap-
plication of their multiple compo-
nent system to the determination of
total OC aerosol for mixtures of at-
mospheric organic compounds pre-
sents a formidable challenge.
(4) The use of paired, parallel sam-
plers (denuder difference method)
is subject to the propagation of er-
rors through subtraction, which de-
grades the precision. More
development work with control ex-
periments needs to be done on a
wide range of organic compounds
and concentrations before the sam-
pling approach becomes credible
for extended field studies.
United States
Environmental Protection Agency
Center for Environmental Research Information
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