United States
Environmental Protection
Agency
Environmental Sciences Research
Laboratory
Research Triangle Park NC 27711
Research and Development
EPA-600/S7-8M29 Oct. 1981
Project Summary
Photoacoustic Detection of
Particulate Carbon
R. R. Patty and C. A. Bennett, Jr.
A photoacoustic technique for mass
monitoring of carbonaceous aerosols
deposited on filter substrates has been
developed. A photoacoustic cell has
been designed and calibrated using
laboratory generated elemental
carbon standards. The nature of the
photoacoustic response is examined
at several modulation frequencies
using these calibration standards, and
the physical principles necessary for
an adequate interpretation of the
experimental results is presented in
detail. Practical considerations con-
cerning ambient carbon monitoring
are outlined; in particular, the pertur-
bation due to the presence of scatter-
ing particles is examined, and limited
experimental quantification of this
perturbation is reported.
This Project Summary was develop-
ed by EPA's Environmental Sciences
Research Laboratory. Research Tri-
angle Park, NC, to announce key find-
ings of the research project that is fully
documented in a separate report of the
same title (see Project Report ordering
information at back).
Introduction
Photoacoustic detection has been
successfully used in the past as a means
of measuring levels of pollutant gases in
the atmosphere. The same principle has
been used to construct a sensitive real
time photoacoustic soot monitor which
measures levels of suspended carbon
particles within an acoustically reso-
nant photoacoustic cell. Other types of
chemical species present in the atmos-
phere in paniculate form are typically
analyzed by the x-ray fluorescence of
ambient participate samples which
have been deposited on various filter
substrates by dichotomous samplers.
Unfortunately, the atomic weight of
carbon is too low to allow quantitative
results from this very sensitive nonde-
structive technique. As a consequence,
a great deal of data exists, from both
past and on-going studies, in the form of
ambient particulate samples deposited
on filter membranes and for which no
information on elemental carbon is
available. Soot represents a significant
fraction of any ambient particulate
sample, either rural or urban, and this
fraction is certain to increase as our oil
supplies diminish and more and more
demand is placed on "less clean"
energy sources. Since particulate
carbon has been related to adverse
health effects, possible climatic pertur-
bations and visibility degradation, it is
important to establish a procedure by
which accurate elemental carbon levels
in the atmosphere can be obtained so
that appropriate control strategies can
be implemented.
Perhaps the most common nonde-
structive method of elemental carbon
detection for samples collected on filter
substrates is the integrating plate
method (IPM). Although this technique
is very simple and economical, experi-
ence in our laboratory has indicated that
IPM is seriously affected by the pres-
ence of nonabsorbing scattering
particles such as ammonium sulfate.
The photoacoustic technique we have
developed is much less effected by scat-
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tering particles and is also nondestruc-
tive, simple, and economical. Even if
scattering particles were not present in
ambient samples, photoacoustic detec-
tion (PAD) would still be much more
preferable than IPM. The limit of detec-
tion of IPM lies far above that of PAD for
the following reason: IPM involves a dif-
ference of transmission measurements
on filters before and after loading. If the
light source - detection system stability
is, for example, 1%, then one could not
hope to make an IPM measurement on a
sample which is 1 % attenuating with an
accuracy of any better than + 100%. The
photoacoustic signal, on the other hand,
is linearly dependent on the light source
intensity so that a 1% fluctuation in
intensity produces a 1% fluctuation in
accuracy of any better than ± 100%. The
photoacoustic signal, on the other hand,
is linearly dependent on the light source
intensity so that a 1% fluctuation in
intensity produces a 1% fluctuation in
photoacoustic signal. Indeed, examples
of photoacoustic measurements on
samples with attenuations of 10~4-10~5
abound in the literature. It is clear, then,
that the use of PAD will lead to de-
creased sample collection times and
make possible more accurate measure-
ments in clean air environments.
The photoacoustic effect in solids is a
fascinating subject in its own right. As is
evident from our data on pure carbon
calibration standards, the very highly
absorbing (and hence very thin) carbon'
samples prepared for this study illus-
trate a structure in the photoacoustic
signal due to thermal wave interference.
Since ambient samples are typically in
the form of very thin deposits, it is
essential to have a firm understanding
of these phenomena if experimental
data obtained by PAD is to be accurately
interpreted. For example, thermal wave
interference (and the resulting struc-
ture in the photoacoustic signal) is
highly dependent upon the type of filter
substrate used; therefore, it is neces-
sary to make separate calibrations for
each type of filter material.
Results
The physical principles underlying the
production of the acoustic signal gener-
ated within the photoacoustic cell are
outlined in detail. The equation fo: the
photoacoustic signal (in one dimension)
is established via the traditional
boundary value approach and by a less
vigorous, but more illustrative, ap-
proach which better illustrates the
physics of the photoacoustic effect.
Additional computations are also in-
cluded which account for the added
photoacoustic signal due to light re-
flected from the filter surface (this effect
has been ignored in the past ). An
analysis of the IPM technique is in-
cluded as well as a discussion of the
decrease in transmission observed
when the particles are sandwiched
between the filter and the opal glass.
In order to make it possible to obtain
measurements on ambient elemental
carbon, a set of accurately weighed and
evenly deposited elemental carbon cali-
bration standards were prepared using
Teflon filters. From IPM measurements
on these pure soot calibration filters, we
determine the absorptance A = (1 - T)
where T is the fraction of incident radia-
tion transmitted by the carbon particles
contained on a filter, and the optical
thickness /3I = -ln(T) where ft is the
absorption coefficient and I is the thick-
ness of the sample. Figure 1 shows both
a plot of absorptance vs loading and a
plot of ft\ vs loading. Our measured
photoacoustic signals for the same
calibration filters used for Figure 1 are
shown in Figure 2. A comparison of the
photoacoustic with the absorptance
data suggests (a) that the photoacoustic
signal does not saturate with loading as •
quickly as does the absorptance, and (b)
that there is more scatter in the photo-
acoustic data than in the absorptance
data. We should mention that if the
same filter is repetitively inserted and
sampled, the photoacoustical signal for
each measurement agrees to within
3%; the error associated with the load-
ing measurements is contained within
the scatter present in the data shown in
Figure 1. It appears that the major
source of the scatter in the photoacous-
tic data stems from the Teflon filters
which were not thermally thick at 100
Hz. Different filters allow variable
amounts of thermal energy to be trans-,
mitted to the gas from behind the filters.
The interesting fact that the photo-
acoustic signal does not saturate as the
carbon samples become opaque can be
explained by considering the interfer-
ence of thermal waves generated within
the very thin carbon samples. Appar-
ently, the thermal waves reflecting from
the sample filter backing are shifted in
phase by 180° so that the thermal
waves at the sample-gas boundary
interfere destructively for the thinner
samples and interfere more and more
10.0
40 80 120 160
Loading (ng/cmz)
Figure 1. Plots of absorptance vs
loading and optical thick-
ness /3I vs loading.
increases, thus causing the photo-
acoustic signal to continue to rise
instead of reaching saturation as the
absorptance approaches unity.
Conclusions
Photoacoustic detection appears to be
a powerful technique for ambient
carbon monitoring. The thermal wave
mechanism associated with the produc-
tion of the acoustic signal introduces
additional structure not present when
using the optical transmission IPM
method. Although, at first glance, this
additional structure may appear to be a
complication, it is possible that the
information provided by this structure
will prove useful. It appears that the
photoacoustic technique is much less
perturbed by the presence of scattering
particles than is the IPM method; how-
ever, the effect is significant for both
methods in the range of loadings ex-
pected for ambient samples. The photo-
acoustic technique seems to provide a
better estimate of the true carbon mass
present in ambient samples than does
the IPM method. Ultimately, the effect of
scattering particles must be theoreti-
I
I
si
§
4.0
0.0
*•
120 160
40 80
Loading
Figure 2. Plot of photoacoustic re-
sponse vs loading for a
1OO Hz chopping frequency.
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cally and experimentally quantified so
that accurate carbon levels may be
deduced.
Recommendations
The photoacoustic technique prom-
ises to be an extremely accurate method
for determining the absorbing compon-
ent of atmospheric aerosols. More work
is needed to adequately establish the
role of scattering particles in relation to
particle size, etc. In the future, more
accurate photoacoustic results will be
available if suitable attention is given to
quality control of the filter substrates. It
would be exceedingly attractive to fully
automate a photoacoustic system
(possibly with a capability for simulta-
neous IPM measurements) which could
be put into routine operation.
R. R. Patty and C. A. Bennett, Jr., are with North Carolina State University,
Department of Physics, Raleigh, NC 27650.
W. A. McClenny is the EPA Project Officer (see below).
The complete report, entitled "Photoacoustic Detection of Paniculate Carbon,"
(Order No. PB 81-245 425; Cost: $8.00, 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 PRINTING OFFICE, 1981 — 757-012/7363
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