Photoacoustic Detection of Radiation Absorbing Particles in Gases

Abstract

Photoacoustic detection methods have been successfully applied to monitoring trace contaminants in gases. The major focus of this study was the improvement of the response characteristics of photoacoustic detection systems which are sensitive to noise sources arising from: 1) vibration, 2) ambient acoustic noise, and 3) contamination of the optical windows of the photoacoustic devices. In this study one commercially available non-resonant spectrophone (Burleigh PAS-100) and two laboratory made spectrophones — one operating in a Helmholtz resonant mode and the other in a longitudinally resonant mode — were evaluated for their sensitivity for measuring light absorption by aerosols. The longitudinally resonant spectrophone was found to provide the greatest sensitivity of the three photoacoustic cells investigated. Using two microphones and applying noise subtraction techniques in the electronic signal processing, it was possible to reduce the noise contribution due to vibration by approximately 40 dB. Using an open-ended resonant cell in the longitudinal mode, the maximum sensitivity of 3.0×10⁻⁹ m was achieved in the laboratory experiment. With one-Hz bandwidth, typical detection limits were: 17 ppb for NO₂ in N₂ gas, and 0.39 μg/m³ for 1.0 μm diameter carbon particles.