United States Atmospheric Research and

Environmental Protection Exposure Assessment Laboratory

Agency Research Triangle Park, NC 27711

Research and Development EPA/600/SR-94/192 November 1994

E AP Project Summary

Carbonaceous aerosols, a complex

mixture of carbonate carbon, elemental

carbon, and organic carbon (organic

compounds), are of environmental importance due to their impact on visibility and the toxicity of some of the

organic compounds. A knowledge of

the relative vapor and particle concentration of semivolatile organic compounds is required to understand how

the compounds are transported in the

atmosphere and removed by deposition. Also, the deposition in the lungs

is dependent on the vapor/particle distribution. Because of the multiplicity of

compounds of varying volatility in the

organic aerosols, the sampling of these

particles presents an extremely difficult challenge. This report, based on a

literature search of measurement studies for the past 20 years, reviews the

possible artifacts that can occur that

result in positive and negative errors

owing to volatilization, sorption, or reaction 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 measurements of elemental carbon and organic

carbon without resolution of individual

organic compounds. The location, measured 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 individual compounds present, concentrations of each compound are not listed,

but the artifacts that the employed sampling method addresses are noted.

This Project Summary was developed

by EPA’s Atmospheric Research and

Exposure Assessment Laboratory, Research 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, carbonaceous aerosols have been found to account for typically 40% of the total fine

particle mass loadings in the Los Angeles

area. These particles consist of a multitude of organic compounds, both molecular and macromolecular, as well as

elemental carbon. Over 2800 compounds

have been identified in the ambient atmosphere, 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 concentration.

The potential toxicity of some of the

organic aerosols has made them the subject of environmental concern and extensive investigation. Special attention has

been devoted to the hydrocarbon fraction

owing to the mutagenic activity exhibited

by compounds such as polynuclear aromatic hydrocarbons (PAHs) and their potential carcinogenic effects. The

predominance of organic aerosols in the

fine particle size range makes them efficient transporters of any toxic compounds

into the respiratory system.

Roy L. Bennett and Leonard Stockburger

Sampling Carbonaceous

Aerosols: A Review of

Methods and Previous

Measurements
2

The carbonaceous aerosols, especially elemental carbon, are also of environmental concern because they occur

in the visibility-reducing size range

(0.1μm–1.0 μm) and may therefore have

an important impact on visibility conditions and possibly climate. Elemental

(or black) carbon is the most important

absorbing species for sunlight in the atmosphere, and its catalytic properties

may result in its contribution to important atmospheric chemical reactions.

Elemental Carbon and Organic

Carbon Aerosols

The complex mixtures of carbonaceous

aerosols are often classified as carbonate, elemental carbon (EC), and organic

carbon (OC) that consists of organic compounds. Ambient studies have shown that

carbonates account for only a negligible

fraction of the total carbon in urban aerosol. Elemental carbon has a chemical

structure similar to impure graphite and is

emitted directly from fossil fuel combustion 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 atmosphere may result from direct emissions from a source (primary OC) or may

result from condensation of low vapor pressure products of gas phase reactions of

hydrocarbons onto existing atmospheric

particles (secondary OC) or the reaction

product of primarily emitted organic compounds with atmospheric oxidants.

A method based on thermal/optical reflectance 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, temperature ramping rates, volatilization and combustion, 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-byside sampling were accounted for by differences in the analytical methods. For

OC, most methods showed greater variability in results than could be accounted

for by the differences in analytical methods.

Carbonaceous Aerosols

Sampling Artifacts

As with most ambient aerosol measurements, the most frequently employed sampling 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, sorption properties, and chemical stabilities,

the sampling process has the potential for

several sampling artifacts. These may include the following:

(1) A portion of the organic vapor phase

may sorb onto the filter surface or

on the deposited particulate matter. This is often called the “positive” artifact because the apparent

mass that would be collected and

measured is greater than the aerosol 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 results in volatilization of some of the

particulate organic compounds. This

produces a negative artifact.

(3) The collected particulate matter may

chemically react with vapor components that pass through the filter

during sampling to change the

chemical composition of the deposit.

This can result in positive or negative 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 competing artifact processes although this has

been the object of many studies.

The positive artifact has been investigated 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 atmospheric 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 decreased 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 corresponding aliphatic acid. Reaction of organic 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,

PCB, and other semivolatile organic compounds in the atmosphere requires a

knowledge of both their gas and particle

concentrations. The gas/particle ratios are

important in how the compounds are transported 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 challenge since the compounds are usually

present in low levels (nanograms per cubic meter or less), and the procedure is

subject to positive artifacts from vapor

sorption, negative artifacts from volatilization from the filter, and degradation of

compounds owing to reactions during sampling.

The potential for volatilization and sorption artifacts is related in part to the kinetics of the sorption process. The magnitude

of the artifacts is dependent on the variability of the total particle concentration,

the gas phase concentration, the temperature 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 absorbed state EVCs or the factor by which

the pure state EVCs are commonly reduced 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-
3

duced by adsorption. This was attributed

to uncertainty in the literature data rather

than the difference in actual reduction factors. Comprehensive analyses have been

presented of the partitioning of organic

compounds between vapor and solid aerosol phases. Estimates of the slope and

intercept from plots obtained from a database of gas particle distribution data vs 1/

T were found to be useful for predicting

the distribution in other urban environments. The effects of relative humidity on

gas/particle partitioning have been investigated and compared to field studies. The

log of the gas/particle partition constant

was found to have linearly decreasing dependence on the relative humidity over

the range from 40 to 85% relative humidity, based on semivolatile PAH compounds

measurements at Osaka, Japan. A thermodynamic 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 organic aerosol concentration and, if desired, the corresponding vapor phase

concentration of semivolatile compounds

have evolved from the simple arrangement of a single filter followed by a sorbent trap. As investigators have attempted

to eliminate or quantify the various artifacts that were encountered, their sampling approaches have become more

complex with various denuder arrangements, multiple combinations of filters of

different sorbency (quartz, carbon impregnated, 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 artifacts.

(2) Filter/sorbent

This approach was used for total

(vapor plus particle) concentration

but does not give correct gas/particle distributions except for nonvolatile 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 concentration passing through the second filter 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 collected particles.

(4) Dual quartz tandem filters with a

parallel Teflon/quartz tandem.

This is similar to the dual filter approach 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 positive artifact is corrected by subtracting 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 denuders, compound annular denuders

that consist of coaxial cylinders so

that the air samples pass through

the annular space, have been designed that allow the use of higher

sampling rates.

Various combinations of denuder arrangements have been employed in organic aerosol sampling. A denuder

difference procedure for specific PAH compounds 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 concentration and the degree of negative artifact

are determined by denuder difference calculation, not direct analysis of the denuder.

A compound annular denuder system

has been employed to trap vapor constituents by the denuder difference approach. Two filter/absorbent packs were

operated in parallel: one was preceded by

the denuder; the other was not. The fraction of analyte that occurred in the vapor

phase was determined by subtraction of

the denuder sampler results from the conventional sampler results.

Diffusion denuders consisting of separated layers of quartz filters have been

used under the assumption that they remove 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 conclusion that the OC on the back filter of an

undenuded filter pack is primarily a positive enhancement artifact. The approach

does not address the negative artifact.

An annular denuder-based sampler,

called the integrated organic vapor/particle 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 concentrations 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 concentration 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 semivolatile organic sampling systems consists

of three parallel samplers, two of which

contain multichannel parallel plate diffusion denuders that were carbon-impregnated filter paper to collect the gas phase.

Sampler 1 has the following sequences:

cyclone, denuder, two quartz filters, sorbent 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 artifact is a negative rather than positive artifact and previous data measurements with

filters may have underestimated the par-
4

ticulate organic carbon by more than

100%.

Survey of Carbonaceous Aerosol Measurements

Measurement of Total Organic

Aerosol Concentration

A representative listing of field studies,

where only the elemental carbon and total

organic aerosol concentrations were determined without organic compound resolution, is presented in the report with

references. Sampling techniques have

ranged from single filters to complex, parallel multicomponent sampling systems.

Measurement of Specific

Organic Compounds

A list of studies in which individual compounds or classes of compounds have

been specifically identified and measured

is also presented in the report with references. Owing to the large number of studies on pesticides, PCB, and PAH

compounds, the full report does not

present a comprehensive listing of all measurements that have been made. Also,

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 determined positive or negative artifacts

have varied considerably. This is

probably due, in part, to differences

in the organic molecular composition (hence in the relative volatilities and sorptivities) and in the

concentrations that occurred at the

different sampling locations.

(3) Multiple component sampling systems employing vapor phase separation 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 application of their multiple component system to the determination of

total OC aerosol for mixtures of atmospheric organic compounds presents a formidable challenge.

(4) The use of paired, parallel samplers (denuder difference method)

is subject to the propagation of errors through subtraction, which degrades the precision. More

development work with control experiments needs to be done on a

wide range of organic compounds

and concentrations before the sampling approach becomes credible

for extended field studies.

United States

Environmental Protection Agency

Center for Environmental Research Information

Cincinnati, OH 45268

Official Business

Penalty for Private Use

$300

EPA/600/SR-94/192

The EPA authors, Roy L. 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

BULK RATE

POSTAGE & FEES PAID

EPA

PERMIT No. G-35

the sum of the concentrations of the resolved compounds in most of the studies

does not account for more than a few

percent of the total organic mass. Because many compounds in these specific

organic classes exist as both vapor and

particles their measurement involves the

same sampling and artifact problems encountered with total organic aerosol measurements.

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. Examination of the peer-reviewed literature for

organic aerosol sampling studies during

the last 20 years yields the following observations:

(1) A large percentage of the reported

measurements are subject to artifact errors. In a few cases the investigations 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-