ASA 127th Meeting M.I.T. 1994 June 6-10

1pPA4. Thermoacoustic enhancement and control of the quality factor in a resonant photoacoustic cell for measurement of light absorption by aerosols and gases.

W. Patrick Arnott

Hans Moosmuller

Rick Purcell

Atmospheric and Energy & Environmental Engineering Centers, Desert Res. Inst., Univ. of Nevada, P.O. Box 60220, Reno, NV 89506

Jay Lightfoot

Richard Raspet

Henry E. Bass

Univ. of Mississippi, University, MS 38677

When gases or aerosols absorb energy from a modulated laser beam, surrounding air is heated, producing a time varying acoustic pressure that can be measured to quantify the absorption coefficient. Pressure is detected with a microphone in a resonator with resonant frequency that matches the laser beam modulation frequency. Induced pressure is proportional to laser irradiance, to absorption coefficient of aerosol and/or absorbing gas species, and to resonator quality factor, Q . Thermoacoustics is the interaction of heat and sound in resonators containing thin plates separated by a distance proportional to the thermal penetration depth of the gas. Q can be controlled by placing a temperature gradient along the plates in the same direction as acoustical particle velocity. This talk is on a technique for measuring light absorption by gases and aerosols that incorporates both the photoacoustics and thermoacoustics. Thermoacoustics is used to raise resonator Q for increasing the detection sensitivity of the photoacoustic spectrometer, for allowing windowless operation, and for potentially stabilizing resonator Q with respect to environmental parameter changes. Cell design was explored using a numerical model based on recently developed radial wave thermoacoustic theory. [EPA, ONR sponsorship.]