"You see a lot less diesel smoke these days," says
Arlon Hunt of Berkeley Lab's Environmental Energy Technologies Division, "because the
manufacturers have done a good job of getting rid of most of the large exhaust particles
-- the ones that are easy to see. The challenge is to measure what's not so easy to see,
and these particles may be the most dangerous of all." Hunt and his colleagues
Mary Quinby-Hunt and Ian Shepherd have recently tested a prototype optical instrument that
can characterize diesel exhaust accurately and completely within a few seconds, while the
engine is running. So far their "Scatterometer" has been used only in the
laboratory, but with simple modifications similar devices could be used to measure
individual vehicle emissions or to sample environmental air quality in the field.
"Light scattering gives the most complete possible information you can get without
interfering with a system," says Hunt, who for over 30 years has been using combined
theoretical and instrumental approaches to light scattering to study all sorts of systems,
including the interstellar medium (the stuff between the stars), the sickling of
hemoglobin in human blood, transparent aerogel ceramics, and even octopus sperm. Recently,
because of interest in the environmental hazards of small particles, Hunt and his
colleagues have turned their attention to measuring particle size and composition in
diesel exhaust.
"When you are looking at elastic scattering of light -- that is, systems where the
light that comes out is the same wavelength as the light that went in -- the easiest thing
to do is to measure opacity," says Hunt, "and until recently 'how black is it?'
is about the only quantitative question that was asked about diesel soot. But diesel
exhaust isn't black anymore. Opacity measurements tell you very little."
The technique devised by Hunt and his colleagues measures linear and circular
polarization as well as intensity. These variables are arranged in a matrix representing
the scattering of the light at all angles, from straight ahead to straight back. Only four
elements (of a possible 16 in the matrix) are needed to convey all the information about
light intensity and polarization states that can be gleaned from a suspension of spherical
aerosol particles.
Hunt's group first ran the exhaust from a one-cylinder diesel engine through a gadget
with a jaw-breaking name: the Angle-Scanning Polarization-Modulation Nephelometer, or
POLNEPH. Several features suited the POLNEPH to its task of identifying the nature, size,
and density of exhaust particles. The linear and circular polarization of a laser beam is
modulated 50 thousand times a second. There is no glass or plastic window where the beam
passes through the sample, only an open gap in the pipe that brings the exhaust through
the center of the device.
Around this central point, a long arm with a photomultiplier tube at its end rotates
through 180 degrees, recording the total intensity and polarization states of the light at
each scattering angle. A computer traces out curves as the values change with the angle of
measurement, generating four distinctive graphs.
"Unfortunately, there is no good theory that can tell us how to get from this data
back to the kind of particles that produced it," says Hunt. "However, if the
particles are spherical, or nearly so, we can calculate a large number of alternative
models based on simple Mie calculations, using different kinds and sizes of particles in
different mixtures. We compare the real data with these models, and where all four curves
fit simultaneously, we can be sure we know what we're looking at."
Particles in the exhaust of modern diesels are in fact nearly spherical under all
running conditions. "Under no-load conditions, the particles are very small,"
Hunt says, "and their refractive index suggests that they are primarily water
droplets or dilute sulfuric acid. Under full load conditions, the particles are larger and
darker, although they don't fit the model of pure soot. The particles we observe under
engine loading are probably combinations of water and soot."
Using POLNEPH, Hunt and his colleagues determined that approximations based on the
refractive index for dry soot give completely erroneous results -- as does Rayleigh
theory, even though Rayleigh equations are often used to describe scattering from diesel
engines.
While even the largest particles in the exhaust from a modern engine tend to be
spherical, older diesel engines may produce much larger, nonspherical particles for which
simple calculations are inadequate. Their presence is revealed by measuring a matrix
element other than the four used in the aerosol calculation.
With POLNEPH, Hunt's group analyzed emissions from older engines by applying a program
developed by Mary Quinby-Hunt under a grant from the National Energy Research Scientific
Computing Center (NERSC). An elaboration of the "coupled dipole approximation"
first suggested by Berkeley Lab astrophysicist Carl Pennypacker to model particles in the
interstellar medium, the program generates model particles of arbitrary size by adding
spherical nanoparticles in a random-walk pattern. Using NERSC's Cray T3E supercomputer,
Quinby-Hunt is building up a library of scattering signals from likely particulate shapes,
for determination of emission constituents.
Conventional methods of measuring diesel exhaust are unable to rapidly track changes in
particle emissions under changing conditions such as acceleration under load. The POLNEPH
setup has proved itself capable of accurate particle-size determination under slowly
changing conditions. But because better measurement speed and portability are needed, Hunt
and his colleagues designed and built a smaller, more rugged device dubbed the Diesel
Particle Scatterometer.
"The Scatterometer uses the same method of modulating the polarization of the
laser beam, and the same open viewing of the sample," explains Ian Shepherd,
"but instead of a rotating arm, it uses 12 different photomultiplier tubes that
measure different scattering angles simultaneously."
The data from the photomultipliers is converted from analogue to digital form and
processed using a dedicated personal computer. Continuous measurements can be made as
often as one to ten times a second.
"To proceed with an instrument capable of analyzing diesel exhaust from the
smallest spherical particles to the largest non-spherical particles, in real time, right
where the engine is," says Arlon Hunt, "all we need now is the funding."
The Scatterometer is the prototype of a device that can go into the garage, to the loading
dock, or on the highway to yield accurate measurements of diesel exhaust particulates.
Further information:
|