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

Abstract

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.]