EPA-AA-SDSB-80-19
Technical Report
An Investigation of Photoacoustic Spectroscopy as a
Technique for Measuring Diesel Particulate Emissions
by
Daniel P. Reiser
September, 1980
NOTICE
Technical Reports do not necessarily represent final EPA decisions
or positions. They are intended to present technical analysis of
issues using data which are currently available. The purpose in
the release of such reports is to facilitate the exchange of
technical information and to inform the public of technical devel-
opments which may form the basis for a final EPA decision, position
or regulatory action.
Standards Development and Support Branch
Emission Control Technology Division
Office of Mobile Source Air Pollution Control
Office of Air, Noise and Radiation
U.S. Environmental Protection Agency
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I. Introduction and Background
EPA is in the process of proposing a particulate emission
standard for heavy-duty diesel vehicles. With this mandated
requirement to regulate particulate emissions, EPA must propose a
test procedure for the measurement of particulate emissions from
diesel engines. A draft recommended practice for measuring partic-
ulate emissions from heavy-duty diesel engines has already been
developed and described in a previous report.^/ In the draft
recommended practice, the necessary additions and changes to the
current Federal Test Procedure (FTP) were discussed and generally
involved 1) the use of a dilution tunnel coupled with a constant
mass sampler and 2) the use of filter media to collect the partic-
ulate matter over both the cold and hot start portions of the
test. The latter of these two requirements (i.e., filter measure-
ments) is of special interest here.
The method of measuring emitted particulate matter with filter
media is believed to yield accurate results. Filter-based measure-
ments are currently required for the certification of light-duty
diesel vehicles ,2j and have also been accepted as the method of
measurement in EPA's current program to determine heavy-duty diesel
emissions over the transient cycle._3/ One major drawback in using
filter media to measure particulate emissions is that the time
needed to obtain results, or data processing time, is very slow,
at least one hour after the end of the test.^/ In addition, there
are also the disadvantages of being unable to analyze mass par-
ticulate emissions instantaneously as a function of the mode of
engine operation, and of being unable to rapidly and effectively
monitor the effects of minor engine adjustment with the engine
operating. Also, filter measurements are subject to a number of
errors during the filter stabilization period, such as changes in
humidity, faulty handling procedures, etc.
The technique of photoacoustic spectroscopy may be an attrac-
tive alternative to filter measurements in that it would bring
about instantaneous measurement and a short data processing time
and thus help eliminate some of the problems mentioned above.
Photoacoustic spectroscopy involves the use of a laser beam to heat
particles in a cell. If a rapidly modulated laser beam is used,
the particles heat and cool quickly, which in turn heat and cool
the surrounding air resulting in pressure waves which are picked up
by a spectrophone, which is a sensitive microphone with associated
electronics. The response time is between 0.5 and 1.0 seconds4/_5_/
while the data processing time is less than one minute.6/
Photoacoustic spectroscopy is currently being used by Ford,
General Motors, and possibly others for monitoring light-duty
diesel particulate emissions. This report will investigate
these present applications. This report will also discuss the
feasibility of this technique as an alternative to filter measure-
ments for certification testing of light-duty diesel vehicles and
heavy-duty diesel engines.
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II. Present Operation of Photoacoustic Spectroscopy
The photoacoustical effect can be used to measure the light
absorption of airborne particulate matter in a suitable cell.
In theory, the spectrophone response should be proportional to the
amount of light absorbed which is in turn assumed to be propor-
tional to the mass concentration of particles in the exhaust.
Measurement of particulate emissions from diesel exhaust
has been attempted with photoacoustic spectroscopy .J5/7/8/ The
spectrophone measures only the portion of particulate which
is elemental carbon, as elemental carbon is the only component
of the particulate which absorbs light to a high enough degree .J7/
This presents a problem with diesel particulate because it is
composed of both elemental carbon and organic compounds that
are attached to the carbon. If the elemental carbon to total
mass ratio is known, then the airborne particulate concentra-
tions can be determined. However, the fraction of attached
organic material ranges from 10 to 70 percent of the total mass
particulate,^/ depending on the type of engine and the operating
conditions. Thus, the elemental carbon to total mass ratio
must be approximated so that spectrophone correlations can be
made.
Ford and General Motors appear to be doing the most research
on the use of photoacoustic spectroscopy for measuring diesel
particulate emissions. At present, photoacoustic spectroscopy
is used by both Ford and GM for an "instant" readout of partic-
ulate emissions so that the process of modifying engine design
as a function of particulate emissions can be done more rapidly
and effectively than that possible with filter measurement.
To better understand the present role of photoacoustic spectro-
scopy, the Ford and GM set-up and their most recent studies on
monitoring of diesel particulate emissions will be examined in
detail below.
A. Ford 5/7/i/
Ford uses an argon (Ar+) laser of 1.5 watts with a wavelength
of 514.5 nanometers (nm) as the source of light. The laser beam is
split into two beams. One beam serves as a reference and is
detected by a photodiode (a device which converts a light beam into
a usable electrical signal). The second beam is directed into a
spectrophone cavity which intensifies the sound waves produced by
the heated particulate in the cavity. The second beam is then
detected by another photodiode. The output signals from the
microphone are detected by amplifiers and sent to an appropriate
instrument (ratiometer) to obtain the normalized spectrophone
signal. Response time is approximately one second, while the data
processing time is less than one minute.
Ford estimates the cost of their set-up to be about $10,000
for the laser, and $2,500 for the remaining equipment.10/
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Ford's latest documented work of photoacoustic spectroscopy
measurements involves a study of diesel particulate from a 1979 2.3
liter Opel diesel which could be run with two different rear axle
ratios (3.89 or 2.70) and with or without exhaust gas recirculation
(EGR).J/ In this experiment, the vehicle exhaust is diluted and
mixed in a dilution tube that is about 30 feet long and 1.5 feet in
diameter,llj with samples being taken for both spectrophone and
conventional filter analysis. The results of several tests per-
formed on this Opel diesel show that the average spectrophone
response (in MV) is proportional to the particulate mass loading
(within reasonable accuracy) only under conditions where the ratio
of elemental carbon to organic compound is fairly constant.
For example, one set of Ford's data shows that when the Opel
diesel is run without EGR, the correlation between spectrophone
response and particulate mass loading is accurate within ^_5 per-
cent. YLJ The organic fraction in this case is believed to be
constant at about 55 percent of total particulate mass.J_3_/ Another
set of Ford data on the Opel diesel, this time with EGR, also shows
good correlation between spectrophone response and particulate mass
loading, again within an accuracy of about +5 percent. Again, the
organic fraction is believed to be constant, this time about 35-40
percent of total particulate. However, this latter correlation has
a slope (change in spectrophone response per change in particulate
mass concentration) that is about 20 percent higher than the
slope for the data without EGR.J_3_/ Although good correlation
exists for each case, a combined plot of these two sets of data
yields a poor correlation between spectrophone response and partic-
ulate mass loading with an accuracy of roughly ^50 percent.13/
Thus, the Ford data indicate that the correlation of spectrophone
response and particulate mass loading is poor for the combined
results of the Opel diesel with and without EGR. Also, these test
data show that the elemental carbon to organic ratio 1) must be
known for an accurate correlation between spectrophone response and
particulate mass loading to be developed, and 2) may vary greatly
from one engine (or vehicle) design or operation to another.
In addition to the problem of determining the elemental carbon
to organic ratio, another potential limitation with the photo-
acoustic spectroscopy system, as discussed by Ford, is that optical
absorption per particle mass may not be independent of particle
size. For carbon particles it has been shown that the absorption
of light is constant per unit of mass only for particles with
diameters much smaller than the incident wavelength of light (known
as Rayleigh particles) .J_4_/ It is true that a laser beam of ex-
tremely long wavelength could bypass these small particles com-
pletely, with no absorption taking place. However, this would
occur only at particulate concentrations much lower than those
expected to be measured from light-duty diesels, even with partic-
ulate emission control devices. In the Ford set-up, the particle
diameter may not be sufficiently less than the argon laser beam's
wavelength of 514.5 nm (0.5 micrometer) to avoid this problem.
This problem is also compounded by the fact that the particle
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size distribution may change under various vehicle operating
conditions.
Ford has tried laser beams of longer wavelength in an effort
to solve this problem involving particle size. For an argon laser,
a wavelength of 514.5 nm is the longest possible wavelength.ll/
Lasers other than argon with longer wavelengths may be used, but
such lasers have difficulty producing the power output necessary
to obtain sufficient spectrophone response (approximately 1
watt). For example, Ford tried using a krypton ion laser because
of its longer wavelength, but the power of this laser was insuffi-
cient. _!_!_/ Ford also investigated the use of a carbon dioxide
laser, which happens to be a laser with a wavelength in the infra-
red region that is powerful enough for spectrophone response.
However, Ford found that the response to the carbon dioxide laser
had to be corrected for gas phase absorption by both carbon dioxide
and water, the concentrations of which both vary considerably in
diesel exhaust. Also, absorption of the carbon dioxide laser may
occur for some organic compounds. Thus, from Ford's published
work, systems with lasers of longer wavelength appear to have as
many of their own problems as the argon laser. (See discussion on
GM set-up below.)
Thus, while Ford uses the photoacoustic spectroscopy method
as an effective means to detect the relative effects of engine
design on particulate emissions when the organic fraction is
constant, they have not demonstrated at the moment that this
technique can be used to measure absolute levels of particulate
emissions with the accuracy necessary for certification work; an
accuracy which is currently achievable with the filter technique.
B. General Motors (GM) 8/15/
According to GM's latest published report, photoacoustic
measurements are made using a dual cell method in series. Both
particulate and exhaust gases enter this apparatus at the second
cell. The particulate is filtered after leaving the second cell,
so that the first cell contains gases only. The particulate
absorption signal can then be determined differentially. For
diesel particulate measurements a carbon dioxide laser of 10.6
micrometer wave length is directed into the first cell and then
into the second cell, operating at a power of 3.4 watts. (GM has
used an argon laser of 514.5 nm wavelength for acetylene smoke
measurements. )_1_5_/ With this high modulation frequency, background
noise is neglible. In the second cell, the infrared absorption due
to particulate plus gases (C02, H20, etc.) is measured, while in
the first cell, the absorption of gaseous species only are mea-
sured. The particulate absorption is computed by electronically
subtracting the response of the two photoacoustic cells. The
spectrophones are placed in each cell midway between the open ends
of the cells, to provide good coupling of both the laser and the
microphone to the resonant mode. The response time is about 0.5
seconds.
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GM estimates the cost of the whole unit to be about $35,000.
16/ This breaks down to $12,000 for the C02 laser, $4,000 -
$5,000 for the optical bench, $10,000 - $12,000 for the electronics
and amplifier, and about $4,000 for each of the two photoacoustic
cells.
GM has used this system to measure diesel exhaust particulate
concentrations from a 5.7 liter diesel engine. This engine was
operated at various steady-state conditions and apparently was run
without any modifications during the test program. Results indi-
cate that for this diesel engine only, the photoacoustic signal is
proportional to the diesel particulate mass concentration, within
an overall accuracy of +^.5 percent. GM believes that by their
measuring of particulate emissions in a cell sufficiently removed
from the combustion source, the optical effects due to variation in
the elemental carbon to organic compound ratio have been minimized
for this test. However, they admit that data are too scattered
(as shown on a plot of particle mass concentration vs. optoacoustic
signal) to appreciably determine that the optoacoustic signal was
not affected by variation in particle composition.
GM concludes from this study that the photoacoustic effect is
a convenient and sensitive method for measuring mass emissions.
However, GM states that there are problems involved with the
photoacoustic spectroscopy method which may lead to inaccurate
results. First, as mentioned above, the chemical composition of
the particulate may vary and affect optical properties for a given
mass concentration. Second, changes in particle size may still
affect optoacoustical calibration in the visible light region
somewhat because the wavelength is not sufficiently larger than
particle size. This may be solved by using an infrared source
(such as a carbon dioxide laser) of sufficiently long wavelength
which should give measurements that are independent of particle
size variations, if the particles are spheres. However, infrared
light response can be affected by the shape of a particle if it is
not spherical, which occurs frequently with diesel particulate
emissions, where long chains of small particles tend to form.17/
In conclusion, GM uses photoacoustic spectroscopy to observe
the effects of engine operating conditions on diesel particulate
emissions. GM does not presently have data which compare photo-
acoustic spectroscopy to filter measurements over the Federal Test
Procedure. At the moment they do not intend to use this technique
as an alternative to conventional filter measurements for measuring
exact amounts of particulate emissions, such as would be required
in the Federal Test Procedure.
III. Future Applications of Photoacoustic Spectroscopy
Photoacoustic spectroscopy has been proven to be effective for
relating changes in particulate emissions to changes in engine
design and operating conditions of light-duty diesel vehicles,
particularly if the organic fraction of the particulate is con-
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stant. This effectiveness is due to quick response (about 0.5 to
1.0 seconds) and data processing times (less than 1 minute) in-
herent in the system which allow it to show variations of emissions
with driving conditions such as acceleration, steady highway
speeds, and idles. However, the photoacoustic spectroscopy system
can not be depended on for exact measurement of particulate emis-
sions, as the overall accuracy of the correlation between spectro-
phone response and particulate mass loading is estimated to be
about _+15 percent for the engine tested with no modifications by
GM, and about ^_50 percent for the Opel diesel engine tested with
modifications (EGR) by Ford.
Because of this limited accuracy, photoacoustic spectroscopy
should not be considered for certification application at this
time. Also, photoacoustic spectroscopy has only been studied using
a few light-duty diesel vehicles by Ford and General Motors.
Given the problems already seen with this technique these few
studies would not be adequate to show that the accuracy of the
photoacoustical technique would not be affected to an even greater
degree by different engines or vehicles. In other words, before
photoacoustic spectroscopy could be considered for certification
purposes the technique must correlate with the filter measurement
technique for all types of light-duty diesel vehicles and heavy-
duty diesel engines. Until data show otherwise, photoacoustic
spectroscopy should not be allowed as an alternate certification
technique for measuring particulate emissions.
Conclusion
Photoacoustic spectroscopy is currently used to monitor the
effects of engine design and operating conditions on particulate
emissions. Accurate measurements of the correlation between
spectrophone response and particulate mass loading have not been
obtained for particulate emissions because the absorption of the
laser beam used in this technique seems to be affected by 1)
particle size and shape and 2) particle composition. Thus, photo-
acoustic spectroscopy should be rejected at the moment as a tech-
nique for measuring particulate emissions during certification.
Progress in photoacoustic spectroscopy should be monitored in the
future as improvements may justify it as an alternative to the
filter measurement test procedure.
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References
JY Danielson, Eugene, "Draft Recommended Practice for Measurement
of Gaseous and Particulate Emissions from Heavy-Duty Diesel
Engines Under Transient Conditions," SDSB, ECTD, EPA, April,
1979.
2J "Standard for Emission of Particulate Regulation for Diesel-
Fueled Light-Duty Vehicles and Light-Duty Trucks," FR Vol. 45,
No. 45, March 5, 1980, pp. 14496-14525.
_3_/ Southwest Research Institute Diesel Baseline Emissions
Summary, EPA, June 1, 1980.
47 "Optoacoustics Measure Diesel Particulates," Automotive
Industries, September, 1979, p. 31.
5/ Japar, S. M., D. K. Killinger, and J. Moore, "The Use of
Photoacoustic Spectroscopy to Characterize and Monitor
Soot in Combustion Processes," presented at Symposium of
Lasers in Combustion Chemistry, ACS Meeting, Washington,
D. C., September, 1979.
6J Truex, T.J. and J. E. Anderson, "Mass Monitoring of Carbon-
aceous Aerosals with a Spectrophone," Atmospheric Environment,
Vol. 13, pp. 507-509, September 26, 1978.
Tj Japar, S. M. and Ann Guneo Szkarlat, "Measurement of Diesel
Vehicle Exhaust Particulate Using Photoacoustic Spectroscopy,"
Engineering and Research Staff, Ford Motor Co.
8/ Faxvog, Fred R. and David M. Roessler, "Optoacoustic Measure-
ments o'f Diesel Particulate Emissions," Journal of Applied
Physics, Vol. 50, No. 12, December, 1979, pp. 7880-7882.
_9_/ Japar, S.M. , and O.K. Killinger, "Photoacoustic and Absorption
Spectrum of Airborne Carbon Particulate Using a Tunable Dye
Laser," Chemical Physics Letters, Vol. 66, No. 1, Sept. 15,
1979, pp. 207-209.
10/ Telephone conversation with Steven Japar, Engineering and
Research Staff, Ford Motor Co., July 3, 1980.
ll/ Telephone conversation with Steven Japar, Engineering and
Research Staff, Ford Motor Co., August 18, 1980.
12/ Unpublished data from Ford Motor Company, Engineering and
Research Staff.
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13/ Telephone conversation with Steven Japar, Engineering and
Research Staff, Ford Motor Co., August 21, 1980.
14/ Faxvog, Fred R. and David M. Roessler, "Carbon Aerosol Visi-
bility vs. Particle Size Distribution," Applied Optics, Vol.
17, No. 18, August 15, 1978, pp. 2612-2616.
15/ Faxvog, F.R. and D.M. Roessler, "Optoacoustic Measurement of
Optical Absorption in Acetylene Smoke," Optical Society of
America, Vol. 69, No. 12, Dec. 1979, pp. 1699-1704.
16/ Telephone conversation with Dr. Fred Faxvog, Physics Depart-
ment, General Motor Research Laboratories, July 3, 1980.
_1_7_/ Lipkea, William L., John H. Johnson, and Carl T. Vick, "The
Physical Chemical Character of Diesel Particulate Emissions -
Measurement Techniques and Fundamental Considerations," SAE
780108, 1978.
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