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Combustion Aerosols: Factors Governing Their Size
and Composition and Implications to Human Health
a
b
c
d
Joseph J. Helble , Matthew S. DeVito , Chang-Yu Wu , Francis L. Smith & D.
e
Marrack
a
Department of Chemical Engineering , University of Connecticut
b
CONSOL ENERGY Inc.
c
Department of Environmental Engineering Sciences , University of Florida
d
Kellogg, Brown & Root, Inc.
e
Fort Bend Medical Clinic
Published online: 27 Dec 2011.
To cite this article: Joseph J. Helble , Matthew S. DeVito , Chang-Yu Wu , Francis L. Smith & D. Marrack (2000)
Combustion Aerosols: Factors Governing Their Size and Composition and Implications to Human Health, Journal of the
Air & Waste Management Association, 50:9, 1619-1622, DOI: 10.1080/10473289.2000.10464194
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2
000 CRITICAL REVIEW DISCUSSION
ISSN 1047-3289 J. Air & Waste Manage. Assoc. 50:1619-1622
Copyright 2000 Air & Waste Management Association
Combustion Aerosols: Factors Governing Their Size and
Composition and Implications to Human Health
INVITED DISCUSSIONS
Joseph J. Helble
One aspect that is briefly addressed in their review is
the overlooked importance of particle morphology. A
particle of 1-µm aerodynamic diameter will have much
lower surface area than will a fractal-like particle of equiva-
lent size. Transition metals, which are often surface en-
riched in combustion-derived particles, have been
suggested as one plausible damage-inducing species; if
true, then the surface area and, hence, morphology of
the PM is of considerable importance. Human airway
deposition studies generally assume spherical particles,
yet recent data from our laboratory indicate that 40–50%
of the particles in a rural ambient air sample are non-
spherical, ranging from elliptical and angular particles
with an aspect ratio of 1.6 or greater to fractal-like soot
and metal-derived aggregates consisting of much smaller
10–40 nm primary particles. As the authors suggest, this
adds to the challenge of identifying the primary compo-
nents of PM2.5 responsible for the observed health effects.
The authors, in presenting a comprehensive summary
of particle formation, particle emissions, and particle-
associated health effects, have laid out the many remain-
ing research challenges associated with this important
problem. It is up to the research community to act upon
these challenges and unravel the complex issues associ-
ated with this potential hazard to public health.
Department of Chemical Engineering
University of Connecticut
Nearly a decade of epidemiologic research has indicated
that exposure to elevated concentrations of ambient par-
ticulate matter less than 2.5 µm in size (PM ) can ad-
2
.5
versely impact human health. Despite this, plausible
mechanistic pathways remain poorly understood. This
limited understanding stems in part from the complexity
of the PM itself. Although epidemiologic studies often treat
PM2.5 as a single entity, in reality, it is broadly distributed
in particle size, particle morphology, and particle compo-
sition. All of these factors may plausibly influence the
interaction of the particles with the human airway.
In the Critical Review by Lighty, Veranth, and Sarofim,
the authors provide a comprehensive review of these com-
plexities associated with PM . In their paper, PM for-
2
.5
2.5
mation in combustion systems, measurement methods
for PM , and exposure studies associated with PM are
2
.5
2.5
all discussed in an interrelated fashion. The synthesis of
these three topics in one comprehensive review makes
this paper a valuable resource for researchers in the field.
The emphasis on following particles through their entire
history, from formation to emissions and ultimately depo-
sition and health effects, also makes this review particu-
larly useful for researchers in one community seeking to
understand the governing phenomena in another.
One particularly valuable aspect of the review is the
discussion of particle formation in combustion systems
from a fundamental perspective, followed by more de-
tailed discussions addressing phenomena specific to indi-
vidual fuels or combustion devices. This discussion is
useful for its identification of the factors that remain
poorly characterized and in need of additional study. For
example, the authors present a stark contrast between the
relatively well-characterized emissions of coal-fired util-
ity plants and the poorly characterized emissions of do-
mestic biomass combustion. Their comments suggest that
coupling epidemiologic studies in regions burning diverse
fuels with detailed particle size and composition distribu-
tion measurements would be quite useful in understand-
ing the importance of particle size and composition on
PM -induced health effects.
Matthew S. DeVito
CONSOL ENERGY Inc.
The research team has written a comprehensive review of
the existing data associated with combustion aerosols. With
the recent revision of the National Ambient Air Quality Stan-
dards (NAAQS), this is both an important and timely sub-
ject. The authors covered a broad range of topics associated
with combustion aerosols, and the resulting document is
an excellent starting point for focused, future research.
Combustion aerosol formation and emissions involve
a number of complex mechanisms. The review provides
an in-depth study of particle formation and the factors
that influence both particle size and composition. This is
the one technical area where there have been consider-
able research and agreement. As in many cases, reviews
of this nature often uncover additional questions. The
authors correctly identify areas where more research is
needed.
2.5
Volume 50 September 2000
Journal of the Air & Waste Management Association 1619

The major research areas include:
Chang-Yu Wu
•
•
•
•
improved understanding of combustion funda-
mentals,
Department of Environmental
Engineering Sciences
University of Florida
improved understanding of fine particulate and
aerosols control,
improved sampling and particle characterization
methods, and
The authors have done an excellent job on this review.
Various aspects of combustion aerosols, from the sources
and control to the health impact, are covered to provide
a clear and integrated picture of this subject. Of special
importance is the manifestation of the direction for fu-
ture research. Cross-disciplinary research that involves
both engineers and health professionals is certainly a must.
There are several supplemental issues that I would
like to address here. First, as shown in the review, com-
bustion aerosols are generally in the submicrometer size
range, with their primary particles in the size range of
nanometers. With the enthusiastic pursuit of the
nanotechnology today, numerous types of nanoparticles
of large quantities are being manufactured and applied.
For example, nanosized titania has been commonly ap-
plied to windows, cosmetics, and air purification. The
introduction of these nanosized particles into our daily
life, which results from attrition and subsequent sus-
pension, may pose significant impact on the environ-
ment (e.g., enhancing photocatalysis in the ambient
air) and human health. Their effects are expected to be
much more acute than those of the submicron particles
due to their size effect. In addition, certain materials
that are nontoxic in the larger size range have been
shown to cause adverse effects when they are in the
nanosized range. However, the impact of these manu-
factured products has not been quantified yet. This is
certainly an area that needs to be extensively and ur-
gently researched.
improved understanding of the health effects.
The establishment of additional regulations and more
stringent ambient particulate levels in the absence of solid
health-effect data is distressing to the fossil-fuel industry
and the energy consumer. The association between in-
creased ambient particulate concentrations and adverse
health effects without the identification of a toxicological
mechanism clearly suggests that more research is needed
prior to establishing PM2.5 regulations. The authors correctly
point out that the correlation of fine particulates with ad-
verse health effects does not prove causality. Particle mass,
or more specifically, particle concentration, is more likely
a surrogate for harmful airborne species. What roles do
natural source materials, uncontrolled by human activity,
play? How do PM and weather conditions correlate and
thus affect human health? Can correlated events be sepa-
rated to determine causality? We must answer these ques-
tions to effectively address the health issues.
Future research in these areas is being driven by the
proposed revision to the ambient air quality standard. A
number of organizations, including the U.S. Environmen-
tal Protection Agency, U.S. Department of Energy, EPRI,
and industry, are conducting ambient particulate (PM )
2.5
measurement and characterization programs. The objec-
tive of these programs is to develop a database of fine par-
ticulate ambient concentrations and speciation. Another
objective of these studies is to identify the impact of out-
door air quality on indoor air quality and the influence of
indoor air quality and personal exposure on human health.
CONSOL is participating in a study that specifically ad-
dresses this issue. In response to the need for improved
mass concentration and speciation data, a new generation
of ambient samplers has been developed and improved
chemical speciation techniques are being utilized. We may
soon have an improved understanding of “how much” and
The second issue is related to the control of heavy
metals from combustion sources. While many of the met-
als are toxic to the environment and human beings, they
are also valuable resources with high market values. For
example, the price of ferrovanadium is $12.75 per kg,
much higher than the $0.10 per metric ton for Fe. Recy-
cling these materials is certainly meritorious, both eco-
nomically and environmentally. Currently, ~2000 metric
tons of vanadium is recovered from petroleum residues
annually, which accounts for 45% of U.S. consumption.
The development of new techniques that can effectively
retrieve various precious metals from combustion systems
will provide attractive incentives for industry and should
be encouraged.
“
what types” of particles are in the air. However, this infor-
mation can only be applied into effective regulations when
coupled with accurate health information.
In summary, all environmental regulations carry some
economic costs. This fact is recognized in this review. The
authors of this review are applauded for the complete-
ness and accuracy of this summary of the body of knowl-
edge associated with combustion aerosols. The
information provided in this review should serve to focus
future research that will bridge the data gaps to assure
that future regulations are based on sound science.
Francis L. Smith
Kellogg, Brown & Root, Inc.
The material presented has convinced me that a substantive
body of scientific knowledge exists concerning both the
character of combustion aerosols and their implications to
1620 Journal of the Air & Waste Management Association
Volume 50 September 2000

human health. So, after having established what these
aerosols are, and what hazards they present to the popu-
lation, I think that the next logical question is: Shouldn’t
we now also investigate in parallel the technical solutions
for the control of this problem, rather than just studying,
in relative isolation, the science of the sources, nature, and
consequences of the problem? I propose that the former
path is by far the wiser, and that it makes excellent sense
to begin searching now for effective technical means for
air pollution control (APC) of these submicron combus-
tion aerosols.
recovered in a CHE by using the otherwise wasted energy
to heat a cold fluid, typically cold water.
The most commonly used CHE commercially avail-
able uses low alloy construction, with the surfaces exposed
1,2
to condensing flue gas being Teflon-coated. The company
2
manufacturing this technology, CHX, markets it for heat-
economizing, but it is obvious from their marketing video
that particulates can also be captured and removed with
the condensate. The video shows a sample of the low-pH
condensate, which is black like India ink. This observation
led Consolidated Edison Co. of New York to investigate
the use of the CHE in an integrated design, for the purpose
of combined heat recovery and pollution control, includ-
The history of similar problems (low-level radiation,
water pollution, air pollution, hazardous waste disposal,
etc.) shows that the lapse of a significant period of time
1
ing particulate pollution control. Although the results were
(
measured in decades) is to be expected between the iden-
measured as total particulate loading in LB/million BTU, it
was reported that “1/4 condensation also occurs on sub-
micron-size particulate, enhancing their removal” by “caus-
ing particle growth through agglomeration, and ultimately
removing the fine particulates.” Also reported were signifi-
cant Hg removal efficiencies, ~50%.
tification of such problems and the effective implemen-
tation of remediation, nationwide. This produces the
curious result that the scientists who initially identify and
quantify the problem might anticipate that their grand-
children may well enter adulthood, without effective pro-
tection from the consequences of the problem, before it
has been fixed.
The second technology is the so-called Cloud Cham-
3
ber Scrubber (CCS). This technology was developed to
An extensive delay between discovery and cure is not
particularly desirable, appropriate, or even necessary.
Rather, the search for effective APC solutions for these
combustion aerosols should be pursued, as a companion
and parallel research endeavor, for the specific objective
of reducing this delay.
simulate conditions within clouds for the purpose of clean-
ing gases while using very modest amounts of both water
and electricity. It is specifically marketed as an APC tech-
4
nology for “all generators of particulate under 1 micron.”
Very fine droplets of water are generated and electrically
charged. It was reported that “as they (droplets and par-
ticulates) approach 10 microns of separation, electrical
attraction causes the particulate to enter the droplet” and
“the CCS has been shown to be 99%+ efficient for the
collection of all particulate from 0.1 to 3.0 microns.”
These two technologies are not presented here be-
cause they are the solutions to the APC of the submicron
combustion aerosols. Rather, they are presented as exist-
ing, commercially available, working technologies that
might be adapted for this use. The advantage is that they
are available today, as working systems. They can be
quickly pilot-tested on actual source streams to determine
their effectiveness. If these results are promising, the tech-
nologies can then be further investigated, scientifically,
for the purpose of determining the practicality of adapt-
ing them for the effective APC of these submicron com-
bustion aerosols.
What has been presented here is not a technical
means for the APC of combustion aerosols. Rather, a rec-
ommendation has been made for the search for such a
means, employing technological “gatekeeping,” to iden-
tify and then attempt to adapt existing technology for
this role. Two examples were briefly described. By inves-
tigating in parallel both the scientific fundamentals of
the nature of the problem of these aerosols and the tech-
nical means for their control, the lapse of time between
Natural processes continuously introduce submicron
particulates into the atmosphere, and consequently, other
natural processes that remove them must exist in bal-
ance. Therefore, it seems rational that effective APC tech-
nologies might well utilize some of the same phenomena
involved in these latter, particulate “removal” processes.
Some of these processes occur within clouds, where nucle-
ation, condensation, and electrostatic forces are particu-
larly active. In fact, at least two such commercial
technologies already exist, utilizing such phenomena,
and each has been investigated to some extent for use in
the APC of similar submicron particulates. Better yet, pilot
scale equipment for each technology exist that would
permit the evaluation of the effectiveness of these tech-
nologies for the APC of these aerosols.
The first technology is the so-called condensing heat
exchanger (CHE). In the form described here, the CHE
was developed to recover waste heat from boilers. Fuel
combustion typically produces acidic flue gas. Boilers are
designed to avoid corrosion from acidic flue gas by being
operated above that temperature at which any conden-
sate can form, the so-called dew point. This standard op-
erating practice results in boiler flue gases being released
to the atmosphere at temperatures typically between 300
and 400 ºF. The consequent lost energy can be partially
Volume 50 September 2000
Journal of the Air & Waste Management Association 1621

the discovery and the effective cure for this problem could
be significantly reduced.
clinical condition identified may overlap:
(1) watery/itching eyes—association unidentified;
(
2) reduced lung diffusion capacity—associated with
exposure to carbonaceous PM2.5 and acid aero-
sols (sulfuric, sulfurous, nitric, or others);
REFERENCES
1
.
Collins, S. Utility Seeks to Integrate Heat Recovery, Flue-Gas Treat-
ment. Power, May 1993, pp 65-68.
2
3
4
.
.
.
Condensing Heat Exchanger Corp. (CHX), Warnerville, NY
Tri-Mer Corporation, Owosso, MI, 48867
http://www.tri-mer.com/whatsnew.html
(3) cancers, possibly birth defects—specific pollut-
ant association in doubt, but probably nitro-
PAHs;
D. Marrack
Fort Bend Medical Clinic
(4) child lung-growth impairment—associated air
pollutant is undetermined; and
PM2.5 and smaller PM biology is different from that of
other larger PM and gas-phase molecules that follow the
partition coefficient between the gas and lung cell sur-
face aqueous secretion—a reversible process. The chair
of the Critical Review committee pointed out that drugs
can be given by aerosol inhalation in sizes ~0.1 µm (e.g.,
insulin) with nearly 90% uptake. Consider only the con-
sequences of inhalation of ambient PM of less than
(5) heart failure—shown in a multitude of epidemio-
logic studies. Associated with PM -bearing tran-
2
.5
sition metals, extractable by dilute acids.
In rats with mild lung inflammation, heavy oil-de-
rived PM inhalation can cause cardiac dysrythmia and
2
.5
1
death. As Lighty pointed out, these carry more V and Fe
than PM from other sources do. Smog-related dysrythmia
has been recorded in humans.
2
-µm diameter, the “garbage bags of pollution” and the
Molecular sequences phagocytose metal-bearing PM.
The heart is mediated by cytokines, including tumor ne-
crosis factor and others that may be involved in the bio-
logical chain reactions connecting the organs. TMF is a
cardiac muscle depressant decoupling energy generation
and contraction. This is not a clearly defined molecular
sequence, and there may be more than one connecting
mechanism, but it provides a basis for elucidation of the
mechanisms. Defining the triggers for these health ef-
fects is important in order that selective control mecha-
nisms can be developed and implemented in place of
villains of air pollution, which are very effectively taken
up by the alveolar cells, and often engulfed by macro-
phages in the presence of surfactants, with some possi-
bly going directly into circulation and arriving in the
heart. There is no return channel for these particles and
their absorbed baggage of chemicals; they are delivered
into the inner workings of the cells with the potential to
release all the chemicals they carry. “Aged PM” may carry
chemicals that are significantly different from those origi-
nally absorbed due to chemical reactions on their sur-
faces, which are probably also accelerated due to catalytic
effects.
across-the-board reductive control of PM2.5
.
At least six groups of health effects associated with
air pollution can be clinically recognized, and in some,
the pollutants that seem most closely associated with the
REFERENCES
1
.
Marrack, D. Particulate Pollution—A Biological Dilemma. Presented
at A&WMA Annual Conference, San Diego, CA, June 1998; Paper 98-
MA 1202.
1622 Journal of the Air & Waste Management Association
Volume 50 September 2000