Photoacoustic sensor system for the quantification of soot aerosols (abstract)
Authors: Haisch C. , Beck H. , Niessner R. 
Published: January 2003

The influence of soot particles on human health as well as global and local climate is well established by now. Hence, the need for fast and sensitive soot detection in urban and remote areas is obvious. The state of the art thermochemical detection methods for soot analysis is based on filter sampling and subsequent wet chemical analysis and combustion, which requires laborious and time consuming sample preparation. Due to the integration on a filter, a time-resolved analysis is not possible. The presented photoacoustic sensor system is optimized for a highly sensitive and fast on-line and in situ quantification of soot. Soot particles, as classical “black absorbers,” absorb electromagnetic radiation over the whole spectrum. Two similar systems are introduced. The first system is designed for the development and testing of combustion engines, mainly the next generation of diesel engines. In the next decade, legal thresholds for extremely low particle emissions are foreseen. Their implementation will be only possible if a time-resolved soot detection with sufficient sensitivity can be realized as the highest particle emissions from diesel engines are generated only for seconds during load changes. During a load change, the emitted soot concentrations can rise several orders of magnitude for only a period of few seconds. The system combines a time resolution of 1 s (sampling rate 1 Hz) with an aerosol mass sensitivity better than 10 μg m−3. Up to a maximum dimension of about 800 nm the signal is independent of the particle size. The systems consist of two photoacoustic cells, which are operated in a differential mode to avoid cross sensitivities. The cells are built as acoustical resonators to increase sensitivity. A diode laser with a wavelength of 810 nm and an output power of 1.1 W is employed for excitation. Its collimated beam passes first through the reference cell and then through the measurement cell. To avoid condensation of water, the cells are heated to 50 °C. The whole system is incorporated into a 19 in. rack, the design is rugged for field applications at an engine test site to withstand external acoustic noise, temperature fluctuations, and dust contaminations. The overall performance and figures of merit were established under laboratory conditions with soot aerosols generated by a spark discharge aerosol generator. Meanwhile, the system is routinely used for engine development at an engine test site of MAN Nürnberg (Germany). At the site, the system was calibrated by recording several stationary cycles. The time resolution was validated by on-line measurements of transient cycles. Throughout all experiments, reference analysis was performed via a thermochemical procedure, i.e., a German standard reference method (VDI guideline 2465, 1). A second system is designed for the detection of very low concentrations of soot (&lt; 200 ng m−3) with a moderate time resolution in the minute range. It is meant for atmospheric soot detection in remote areas and high altitude, which are supposed to reveal background concentrations of soot without direct contamination, e.g., by traffic and industry. At present this system is installed in an observatory on top of the highest German mountain. First results of these systems are presented and compared with data from an aethalometer, which is based on absorbance measurement.