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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