Measurement of Diesel Vehicle Exhaust Particulate Using Photoacoustic Spectroscopy

Photoacoustic and filter measurements related to aerosol light absorption during the Northern Front Range Air Quality Study (Colorado 1996/1997)

“…The disadvantages of the integrating plate type methods can be eliminated by measuring the aerosol light absorption in situ, that is, with the particles suspended in their ambient state. This has been done by using resonant photoacoustic detection [e.g., Adams, 1988;Foot, 1979;Japar and Szkarlat, 1981;Roessler, 1984;Terhune and Anderson, 1977;Truex and Anderson, 1979], where light from an intensity modulated source is absorbed by aerosol, resulting in periodic heating and subsequent expansion of the surrounding air. This expansion results in a sound wave at the modulation frequency which can be sensitively detected with a microphone [e.g., Pao, 1977].…”

Carbon in fly ash analysis using photoacoustic spectroscopy

“…The remaining material, greater than 95% by mass, is carbonaceous in nature, a strong absorber of radiation [4].…”

“…CO; lasers (X = 10.6 |am) have been used so that the soot particles satisfy Rayleigh scattering criteria (D«A,), but at this wavelength gaseous combustion products also absorb the infrared radiation and produce a significant photoacoustic signal. A two cell photoacoustic device was needed to subtract out the combustion gas contribution to the signal [4,7,37]. With the dual-cell setup care must be taken that the gaseous components going through each cell are equivalent; one flow with soot particulates and the other flow with soot particulates filtered out [4,7,37].…”

“…PAS research with argon-ion lasers (X, = 0.55 |am) showed that light absorption by soot particulates could be approximated as Rayleigh scattering [4,5]. The advantages of working at this wavelength are two fold.…”

“…PAS work done with soot particles [4,5] suggests that PAS would also be suited to the challenge of on-line determination of carbon loading because the PAS signal is based solely on the absorption of radiation and not on the energy lost from the incident beam due to absorption and scattering. Although elutriated carbon from coal combustion differs from diesel soot in both particles size and morphology, the technique has virtues that should allow the transition from diesel exhaust to fly ash.…”