This project aims to design and realization of an optical microsystem for the measurement of the size distribution and number of particulates, i.e. Particle Number (PN), in vehicle exhaust. Available particle detectors used for meeting emission standards measure total Particulate Matter of fine particles with a diameter of 2.5 um and smaller (PM2.5) within a defined air volume. Consequently, sensors used in both environmental studies on the health impact of particles and in-vehicle systems for emission management have been based on PM2.5 measurement.
The typical size distribution of particles generated by a diesel engine shows a large number of particles of less than 100 nm diameter. However, because of the proportionality of mass with the cube of the particles’ dimeter, the response of PM2.5 sensor is dominated by the larger particles (D>1 um). Furthermore, the PM2.5 primarily relies on the efficient capture of the particles on the sensor surface. The approach proposed is based on elastic optical scattering spectroscopy of airborne nanoparticles; more specifically the angular distribution of scattered light as a function of wavelength. Applying the general theory on radar cross-section at these diameters for illumination of the particulate-containing exhaust gas with 800 nm light results in maximum Rayleigh absorption for 150 nm diameter particles and less than 1% of that for 40 nm diameter particles. Due to a shift from the Rayleigh regime at small particle toward the Mie regime for the larger particles, the measurement of the size distribution of particles in air with diameters in the range covers much of the nucleation mode and the accumulation mode; between 40 nm and 150 nm for directional illumination at wavelength between 250 nm and 800 nm. The system implementation is based on micro-spectrometers in a compact system that could be integrated in every vehicle for continuous PN monitoring.