|"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.