Cavity ringdown and laser-induced incandescence measurements of soot.

Currently laser-induced incandescence (LII) is widely used for the measurement of soot volume fraction. A particularly important aspect of the technique that has received less attention, however, is calibration. The applicability of cavity ringdown (CRD) for measurement of soot volume fraction f(v) is assessed, and the calibration of LII by means of CRD is demonstrated. The accuracy of CRD for f(v) determination is validated by comparison with traditional light extinction and path-integrated LII. By use of CRD, the quantification of LII for parts in 10(9) (ppb) f(v) levels is demonstrated. Results are presented that demonstrate the accuracy of CRD for a single laser pulse to be better than ?5% for measurement of ppb soot volume-fraction levels over a 1-cm path length. By use of CRD, spatially resolved LII signals from soot within methane-air diffusion flames are calibrated for ppb f(v) levels, thereby avoiding the extrapolation required of less sensitive methods in current use.