Real-time in situ measurements of atmospheric optical absorption in the visible via photoacoustic spectroscopy. 1: Evaluation of photoacoustic cells.

Four configurations of resonant photoacoustic cells were evaluated for maximum signal sensitivity to light absorption by the sample, with minimal noise and background. Theoretical and experimental data are discussed. Azimuthal and radial resonant modes were compared for one cell. Of the four, the best cell was a brass cylinder, 2.5-cm radius and 9.5-cm length, which was operated in the azimuthal mode. An argon-ion laser (lambda = 514.5 nm) was the light source. A continuous sample flow through the cell, required for real-time in situ atmospheric measurements, gave an acceptable noise level and time for signal response at ~500 cc/min. Linearity of the photoacoustic signal was checked in the range applicable to atmospheric absorption. At a signal-to-noise ratio (SNR) equal to 1, a light absorption detection limit of 4.7 x 10(-6) m(-1) could be achieved.