Listening in on Air Pollution

There's no doubt that air pollution assaults the senses. You can smell it in the belch of diesel smoke from a passing truck, and you can see it in the murky haze that settles in our mountain valleys. Did you know that, if you've got the right machine, you can hear it, too?

Drs. Pat Arnott, left and Hans Moosmüller show the inner workings of one of their photoacoustic aerosol measuring instruments prior to shipping it off to the Max Planck Institute in Germany for further air quality research in Europe.

A group of DRI researchers, with Dr. Pat Arnott, Dr. Hans Moosmüller, and Dr. Fred Rogers at its core, has developed an instrument to improve the study of black carbon aerosols. Black carbon aerosol is a component of air pollution mainly emitted by vehicles, especially by those with diesel engines. The device is a photoacoustic instrument that employs a laser beam, an acoustic resonator (something like the pipe of a church organ), and a highly sensitive microphone to determine how much light these aerosols absorb.

While this may sound esoteric, aerosol has a direct influence on the Earth's radiation balance because it scatters and absorbs light. It is crucial to understanding what drives, and perhaps alters, the global climate. Aerosol light absorption is due mostly to black carbon particles which, unlike the better-understood sulfate aerosols that scatter sunlight, actually absorb radiative energy and convert it to heat, warming the atmosphere around them. Researchers are discovering that, due to this light absorption and localized warming, high black carbon concentrations can inhibit cloud formation and significantly reduce visibility.
While measuring the light scattering properties of an aerosol is relatively simple for researchers, measuring absorption of light has been problematic. The most commonly used technique works by pulling air through a filter and then doing a simple light transmission measurement of the aerosols deposited on the filter. Problems with this method range from the need to change filters diligently to uncertain accuracy. DRI's photoacoustic instrument measures black carbon light absorption in-situ, that is, without any intervening filters, to observe the aerosol closer to its natural state. In the simplest terms, it works by passing aerosol-containing air through a laser beam. The aerosols absorb the laser's light energy and convert it to thermal energy, or heat. This heat flows quickly to the surrounding air, which expands its pressure. With the acoustic resonator, that pressure disturbance, or acoustic signal, can be amplified and detected by a microphone. This technique creates a way to measure the light absorption of the aerosols.

While the instrument in its current form is unique, the theory behind it isn't new. "Alexander Graham Bell understood this concept and envisioned it as having communications applications. But, it was an idea before its time," explains Arnott. Enter 20th century technology. "With advances in lasers, improvements in microphones, and the ability to create very small components, it was suddenly a much more practical idea."

Of course, technology is wasted without talent, and DRI had the right pool of people to set the idea in motion. Both Moosmüller and Arnott are quick to credit the late Dr. Bill Pierson, a pioneering DRI air quality researcher and senior research manager, with helping them understand the possibilities. "Bill had worked on photoacoustic instruments at Ford Motors before coming to DRI," explains Moosmüller. "He got me interested, and when Pat joined DRI and Bill became aware of his acoustic background, he got us together. We have been collaborating on this and other projects ever since." And, he says, each researcher brought his own expertise to the project.

Drs. Moosmüller and Arnott work in the Carter Family Optics and Acoustics Laboratory at DRI.

"Pat took the lead using his extensive knowledge and experience in acoustics. Fred contributed his experience in aerosol sampling and properties, and I dealt more with questions of optical design and spectroscopy." As Arnott continues, the list of DRI contributors quickly expands. "John Walker's a machinist who helped develop the prototype. Rick Purcell helped miniaturize some components so this could be a practical field instrument. And Peter Barber, then head of DRI's Division of Atmospheric Sciences, was very supportive and helped us get some of the funding to put together the initial prototype." Outside of DRI, the researchers cited Dr. Reinhard Bruch of the University of Nevada, Reno, Physics Department, who introduced them to the "Sage," Dr. Jin Crystalaser, of the Reno company that manufacturers the tiny butpowerful laser used in the instrument.

The instrument itself is a brilliant blending of the highly technical and the highly practical. One look at the $25 plumbing parts (which serve as acoustic filters) nestled near the $7,000 palm-sized laser and the $2,000 quarter-sized microphone, and you quickly appreciate the pragmatism that went into this state-of-the-art creation. "Hey," shrugs Arnott, "you spend the money when you have to. But why spend it when you don't have to?"

Apparently there are plenty of other researchers willing to spend the money to get their hands on the latest version of the instrument. One was recently shipped to the University of Utah, and the group is hard at work assembling another to be sent to Germany's highly respected Max Planck Institute for Tropospheric Chemistry.

Closer to home, the instrument has already made its way into several DRI projects. The first was the Northern Front Range Air Quality Study in Colorado, conducted from 1996 to 1997. This study was the first field test of the instrument, and served not just as a proving ground for its effectiveness, but also as a chance to adjust various aspects of the design to make it work smoothly in the field. "We learned a lot," says Arnott. "We were very happy with how well it worked, and we discovered a few ways to make it work even better." The instrument has also been used to directly measure the black carbon content of vehicle exhaust at the University of California, Riverside, and to investigate the effects of aerosols on visibility at Big Bend National Park in Texas.

Photoacoustic field test: Dr. Pat Arnott tracks the operation of the new DRI photoacoustic device as it measures the black carbon aerosol emissions from a diesel truck at Hill Air Force Base. The new instrument gives air quality monitors the ability to get onsite, real-time measurements of black carbon aerosol emissions. Photo by Kerry Kelly, University of Utah.

Most recently, the instrument was operated at the U.S. Department of Energy's Atmospheric Radiation Measurement (ARM) Site in central Oklahoma. ARM is a network of monitoring stations designed to collect data that will help improve climate models.

Arnott says that this array of successful applications illustrates one of the instrument's big advantages: it is effective in a broad range of pollution settings. "It was able to detect the aerosols in the relatively pristine air of Big Bend and also handle a very heavy load of aerosol concentration from the tailpipe of a running vehicle," Arnott reports. "If you were to use an aerosol filter in that situation it would be filled up almost immediately." The team is also working with graduate student Khadeejah Hamasha on a variation of the instrument that can measure the size of the aerosols, an important consideration in estimating long-rangetransport in the atmosphere, as well as any possible effects on human health.

With this versatility and range of uses, it seems likely that DRI's photoacoustic instrument is an innovation that's here to stay. Good thing, says Arnott, because black carbon aerosols aren't going anywhere either. " It's not likely that we will overcome our dependence on fire and combustion for transportation, warmth, etc., anytime soon. We might as well provide decent means of quantifying their effects."

As researchers become increasingly concerned with discovering and counteracting the less positive by-products of our 21st century lifestyle, DRI's photoacoustic instrument will be available to help-whether they're designing cleaner vehicles or creating more accurate climate prediction models. "It's really an example of developing technology and developing need colliding," says Arnott. "We think the photoacoustic instrument is ready for the job."

Jackie Allen

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