Photoacoustic soot sensor for in-situ black carbon monitoring

The PhotoAcoustic Soot Sensor (PASS) for in situ black carbon mass monitoring is presented. The sensor combines a high-power laser diode (λ = 802 nm; P = 450 mW) and a novel spectrophone setup to achieve a portable sensor system for black carbon measurements. The acoustic resonator with a Q-factor of ≈ 300 is operated in its 2nd azimuthal mode at 6670 Hz. To estimate the effects of the window position, laser beam collimation, and different loss mechanisms on spectrophone sensitivity a model treating these effects with respect to the signal strength of the azimuthal modes is described. It gives a cell constant of 5.8 V/(W m^-1) in good agreement with 5.9 V/(W m^-1) obtained from measurements with particulate carbon. Additionally, this model permits a method for an absolute calibration of the spectrophone. To improve the signal-to-noise ratio, the photoacoustic signal is estimated by a weighted least-squares fit to an averaged line profile of the excited normal mode instead of a direct measurement of peak height. Finally, the application of this data processing algorithm yields a detection limit of 1.5 × 10^-6 m^-1 or 0.5 μg black carbon per m³.