PHOTOACOUSTIC SOOT SENSOR FOR IN-SITU BLACK CARBON MONITORING

The PhotoAcoustic Soot Sensor (PASS) for in situ black carbon mass mon itoring is presented. The sensor combines a high-power laser diode (la mbda = 802 nm; P = 450 mW) and a novel spectrophone setup to achieve a portable sensor system for black carbon measurements. The acoustic re sonator with a Q-factor of approximate to 300 is operated in its 2nd a zimuthal mode at 6670 Hz. To estimate the effects of the window positi on, laser beam collimation, and different loss mechanisms on spectroph one sensitivity a model treating these effects with respect to the sig nal strength of the azimuthal modes is described. It gives a cell cons tant of 5.8 V/(W m(-1)) in good agreement with 5.9 V/(W m(-1)) obtaine d from measurements with particulate carbon. Additionally, this model permits a method for an absolute calibration of the spectrophone. To i mprove the signal-to-noise ratio, the photoacoustic signal is estimate d by a weighted least-squares fit to an averaged line profile of the e xcited normal mode instead of a direct measurement of peak height. Fin ally, the application of this data processing algorithm yields a detec tion limit of 1.5 x 10(-6) m(-1) or 0.5 mu g black carbon per m(3).