United States
Environmental Protection
Agency
Environmental Sciences Research ^
Laboratory
Research Triangle Park NC 27711 *^
Research and Development
EPA-600/S3-80-095 Mar. 1981
Project Summary
Aerosol Formation from Diesel
Exhaust and SO2
A Chamber Study
Auto exhaust emissions are one of
the most omnipresent, man-made
pollution sources. Especially within
the past decade, a significant quantity
of research has been devoted to the
impact of the emissions of gasoline-
fueled autos upon the health of man
and the environment, the information
on smog, and the composite levels of
various pollutants. These investiga-
tions have provided data upon which
both federal and state control strate-
gies are based. During this period, the
use of light duty diesel-fueled autos in
the United States was insignificant.
Total on-highway diesel fuel usage
was about 10% by volume of the
amount of gasoline used (Office of
Technology Assessment, 1977).
With the recent escalation in the
price of fuel for autos, there is an
economic justification for light duty
diesel-fueled autos. Several foreign
auto makers and at least one domestic
auto maker offer such vehicles. It is
speculated that light duty diesels
could comprise 25% of the new car
fleet by 1985. The potential for such a
dramatic increase of light duty diesel-
fueled autos as well as development of
medium duty diesels, necessitates
that investigations anticipate the
impact of such levels of diesel emis-
sions upon urban pollution levels (both
paniculate and gaseous), photochem-
ical and catalytic oxidation processes
responsible for significant rates of
gas-to-particle conversion in the
urban atmosphere, and the health of
the urban population.
As a result of the significant dif-
ferences between diesel and gas-
fueled auto emissions and the poten-
tial for widespread use of diesels in the
near future, smog chamber studies of
a simulated diesel exhaust/urban
system were performed by Calspan
Corporation under Contract No. 68-
02-2987 from the U.S. Environmen-
tal Protection Agency. The program,
which was performed in Calspan's
600 m3 indoor smog chamber, was
aimed at assessing the role of diesel
emissions in future urban environ-
ments. The experiments provided
semi-quantitative data on aerosol
formation and growth, chemical
kinetics, and the evolution of muta-
genicity in the diesel exhaust/SO2/
hydrocarbon system.
This Project Summary was devel-
oped by EPA's Environmental
Sciences Research 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).
Experimental Results
In recent years, a significant level of
effort has been devoted to the study of
the chemical and physical processes
responsible for smog formation in urban
environments. Experimental data pro-
vided by field and laboratory measure-
ments have allowed many of the more
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important parameters to be identified
and modeled. As a result, current photo-
chemical kinetic models generally
provide at least the qualitative trends
observed experimentally.
One of the primary differences be-
tween the emissions of light duty
diesels and gasoline-fueled, catalyst-
equipped autos is the great quantity of
diesel primary particulates (soot). One
of the purposes of the program was to
investigate the influence of the primary
particulates of the diesel emissions
upon the rate of gas-to-particle conver-
sion. If the primary particulates do
significantly influence the chemical
reactions of the system, existing models
will not provide an accurate assessment
of the system.
Condensible organics known to be
mutagenic are also associated with the
primary particulates of the diesel emis-
sions. Another aspect of this program
was to investigate the fate of these
organics under various simulated
conditions to determine if the mutagen-
icity of the emissions is likely to be
modified with exposure to an urban
environment.
This experimental study of the diesel
emission/hydrocarbon system was
performed to assess the physical and
chemical nature of the system. Particu-
lar interest was vested in the primary
particulates of the diesel emissions and
their influence upon the system.
Overview
Dilute diesel emission systems were
studied in a 600 m3 indoor smog
chamber under various conditions likely
to be encountered in an urban environ-
ment. The major emphases of the study
were aerosol formation and growth,
chemical kinetics, and the mutagenic
activity of the diesel aerosol. Important
parameters with respect to visibility
degradation and the formation of new
aerosol material included relative
humidity, S02 concentration, HC/NOx
ratio, irradiation interval, and the
presence of primary diesel aerosol
(primarily soot).
Aerosol formation and growth in UV-
irradiated diesel exhaust/SOz/propy-
lene experiments were primarily attri-
buted to sulfate aerosol formation. The
rate of SO2 oxidation to form sulfate
aerosol was 5% hr"1 maximum during
the Oa production period. After the peak
ozone production period, the first 3 to 6
hours of a 24-hour experiment, the
ozone concentration remained relative-
ly constant and the SO2 oxidation rate
decreased to approximately 1% hr"1.
These observations could be tentatively
attributed to the S02/hydroxyl radical
reaction during the initial hours of the
experiments. The hydroxyl radical
concentration is high during the first
few hours of ozone production and the
gas phase SOa/hydroxyl reaction pre-
dominates.
The effect of the primary diesel
aerosol on new particle formation was
investigated by conducting experiments
with and without prefiltered diesel
exhaust. The presence of diesel
aerosols precluded the formation of
new aerosol particles; instead, the
existing aerosols grow in diameter by
the condensation of nuclei on them. The
presence of the primary diesel aerosols
appeared to increase the rate of S02
oxidation and the amount of sulfate
formed via unknown heterogeneous or
catalytic reactions. Increases in aerosol
sulfate of from 25 to 75 percent were
attributed to the presence of the primary
diesel aerosol in contrast to what was
attributed to the SO2/hydroxyl radical
gas phase reaction.
In most irradiated experiments, the
production of organic and nitrate
aerosols represented minimal contribu-
tions to aerosol formation and growth.
Nitrate aerosols (<1000 //g/m3) did
represent a substantial fraction of the
total aerosol growth in an experiment in
which NH3 was added to the system.
The mutagenic activity of the diesel
aerosol was observed to be affected by
simulated atmospheric exposure.
Containment of dilute diesel emissions
under non-irradiated conditions
resulted in an increase in the mutagenic
activity of the diesel aerosol. While irra-
diation alone appeared to have no effect
upon the mutagenic activity,
photochemically reactive systems
exhibited decreases in mutagenic
activity of the diesel aerosols with
irradiation. At least a portion of the
observed decrease in mutagenicity was
attributed to the presence of O3 (a
product of the photochemistry).
Conclusions
For UV-irradiated tests, the activity of
the diesel emission/SOz/propylene
system (with respect to aerosol forma-
tion and growth) had three distinct
levels which correlated well with the
rate of change of ozone concentration
with time. Although new particle forma
tion was frequently associated with th
initiation of irradiation, the rate c
increase in aerosol volume remaine
low until substantial amounts of nitre
gen dioxide were formed by the oxida
tion of nitric oxide. The ozon
concentration was very low during thi
period (<10 ppb). Following an increas
in the concentration of nitrogen dioxide
the ozone concentration began t
increase. This period of increasing
ozone concentration represented thi
most active portion of the experimen
for aerosol growth. Sincethere general
ly were sufficient particles to serve a
sites for condensation of the nev
matter, few new particles were formed
The final level of system activit
occurred after the ozone peak, whicl
generally coincided with the depletioi
of propylene (the predominant reactivi
hydrocarbon species). Subsequent t<
the ozone peak, there was a gradual bu
persistent decay in the ozone concen
tration. Little further aerosol growth oi
visibility degradation was associatec
with this period.
Aerosol formation and growth in the
diesel emission/SO2/propylene system
was primarily due to sulfate formation.
Prior to the period of rapidly increasing
ozone (during the decay of nitric oxide),
S02 oxidation rates (Rso2)on the order oj
0.1 % hr"1 were observed. S02 oxidatidl
rates were determined from measured
increases in aerosol sulfate with time
and the concentration of available S02.
During the period of increasing ozone
concentration, Rso2 generally
approached 5% hr"1. Subsequent to the
formation of the ozone peak, Rso2
decreased to a low value (<1 % hr"1) and
occasionally was undetectable due to
losses of sulfate aerosols in excess of
the rate of formation of sulfate. Such
aerosol losses were due to sedimenta-
tion and diffusion to the chamber walls.
The fact that sulfate formation was
greatly reduced, even though ozone and
SO2 continued to coexist, provided
evidence that the SOz-ozone reaction
was not the primary path for sulfate
formation during the period of high Rso2.
Comparisons of aerosol sulfur as deter-
mined by 1C (soluble sulfate) and XRF
(total elemental sulfur) demonstrated
good agreement, indicating that the
concentration of sulfonated organics
was small with respect to total sulfates.
The primary particulates of the diesel
emissions were principally composed of
elemental carbon and various
condensible organic compounds.
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Analyses of the primary particulates of
diluted diesel emissions revealed an
elemental to organic carbon ratio of
about 5 to 1 . One of the major effects of
the primary particulates was to increase
the mass mediandiameter(MMD)of the
aerosol (e.g., 0.3 to 0.4 /urn) through a
redistribution of mass versus particle
size. As a result, the presence of primary
particulates yielded an increased effec-
tiveness of the aerosol to scatter light.
The increased MMD might be indicative
of an increasing fraction of material
deposited deep within the human lungs
during respiration, as a larger portion of
the individual aerosol particles falls
within the respirable size range (0.5 to 3
fjm diameter).
Greater quantities of aerosol sulfate
were consistently produced by irradi-
ated systems containing the primary
diesel aerosol than by comparable
systems which did not contain the
primary diesel aerosol. While the actual
quantity of increased sulfate was
difficult to assess (due to few duplicate
experiments), increases in aerosol
sulfate of from 25 to 70 percent were
attributed to the primary particulates.
This apparent increase in sulfate
formation may have been due to hetero-
geneous reactions on the surface of the
carbonaceous particles or may have
been associated with the high molecu-
lar weight organics known to be carried
by the primary aerosols of the diesel
exhaust. No significant sulfate forma-
tion was observed in the non-irradiated
diesel exhaust/SOa/hydrocarbon
system at low humidity. However, at
high humidities (—92%) SO2 oxidation
fates as high as 1.5% hr"1 were
observed under dark conditions. To
minimize error in the measurement of
calibrated at the beginning of each
experiment using carrier air with same
RH as that to be encountered in the
experiment.
In irradiated diesel emission/SO2/
hydrocarbon systems, aerosol nitrate
concentrations of about 1 pg/m"
(approximately the blank level of the
filter.) and gaseous ammonia concen-
trations of less than 10 ppb were
typically observed. With the
introduction of ozone at low RH (<80%)
into the system, the concentration of
aerosol nitrate appeared to increase
temporarily. At high humidity (94%),
significant aerosol nitrate was formed
(~30 pg/m3) and persisted with the
introduction of ozone. Aerosol nitrate
represented a substantial fraction of
total aerosol growth in only one
experiment (where 10 ppm of NH3 had
been added to the chamber). The
reaction of NH3 with gaseous nitric acid
to produce the more stable ammonium
nitrate apparently was responsible for
the presence of a significant
concentration of aerosol nitrate (in
excess of 1000 fjg/m3) in this particu-
lar experiment.
Recommendations
This program was wide-ranging in
scope and as a result, survey-type in
nature. The goals of the program were
to identify potential problem areas
associated with diesel emissions in an
urban atmosphere to isolate the specific
parameters responsible.
The most significant trends in the
tests of the diesel emission/SOa/
hydrocarbon system were associated
with primary particulates. The primary
particulates were apparently respon-
sible for an increase in the amount of
sulfate formed during irradiated tests
and during high humidity conditions in
the absence of irradiation. Further
experiments should be performed to
determine the expected increase in
sulfate in contrast with the loading of
primary particulates. Experiments
should also be performed to identify the
component of the primary particulates
responsible for the increased sulfate
formation (i.e., the carbon or the
organics).
The experiments performed in this
program concentrated on simple
systems (i.e., the addition of propylene
and butane to diluted diesel emissions)
to simulate the diesel emission/urban
atmosphere. This system was chosen
initially to aid in modeling the observed
photochemistry. Only a limited number
of experiments were performed with a
more representative mixture of diesel
and gasoline-fueled engine emissions.
While the photochemical reactivities of
the simple and more complex systems
are similar, there are definite differ-
ences in specific aspects of the system.
To assure optimum relevance of experi-
mental data, future experiments should
be performed with the more representa-
tive diesel/gasoline-fueled engine
emission system.
The potential for nitrates to contribute
substantially to aerosol formation and
growth in the presence of high NHs
concentrations has been demonstrated.
Further investigations are needed to
determine the role of NH3 and liquid
water in promoting the formation of
aerosol nitrate from an apparently
abundant source of gaseous nitrates.
This program began to explore the
possible effects of age and atmospheric
exposure upon the mutagenicity of
diesel aerosols. There are many unan-
swered questions pertaining to the
mutagenicity of diesel aerosols and
aerosols in general. The questions
concern: the chemical species respon-
sible for the mutagenicity of diesel
emissions; the effects of irradiation
upon specific mutagenic compounds;
the overall lifetime of mutagenic
materials in the atmosphere; mechan-
isms for the elimination of mutagenic
compounds in the atmosphere;
potential mechanisms by which
mutagenic compounds may be formed
by atmospheric reactions; and, the
mutagenicity of gaseous constituents of
diesel emissions. With the potential for
an increasing atmospheric burden of
mutagenic aerosols to adversely affect
the public health, basic studies should
be performed to determine the lifetimes
and possible formation mechanisms of
mutagenic compounds in the
atmosphere.
This Project Summary was authored by Staff of the Center for Environmental
Research Information, USEPA, Cincinnati. OH 45268.
H. M. Barnes is the EPA Project Officer (see below).
The complete report, entitled "Aerosol Formation from Diesel Exhaust andS02,"
(Order No. PB81-121 147; Cost: $11.00, 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:
Environmental Sciences Research Laboratory
U.S. Environmental Protection Agency
Research Triangle Park, NC 27711
> U.S. GOVERNMENT FWNTINO OFFICE: Ml -757-012/7009
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United States
Environmental Protection
Agency
Center for Environmental Research
Information
Cincinnati OH 45268
Postage and
Fees Paid
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Agency
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