Feasibility of cavity ringdown spectroscopy for measuring diesel exhaust

Abstract

Cavity ringdown spectroscopy (CRDS) is one way of measuring small fractional absorption for concentrations down to sub- ppm levels in an optical cavity. The applicability of CRDS for measurement of diesel exhaust is assessed. By use of CRDS, instantaneous exhaust characteristics can be immediately available for engine control. Of specific interest in this set of experiments are measurements of soot or particulate matter. The goal of these measurement would be to estimate the number density of soot particles based on line-of-sight absorption and an assumption regarding the size distribution of the particles. To test the system, the absorption coefficients of Nitrogen, Carbon Dioxide, and 2',7'-Dichlorofluorescein in a test cell at known number densities were measured instead of diesel exhaust. An Nd:YAG-laser provided light at 532 nm for the cavity ringdown measurement. Cavity ringdown waveforms were analyzed with an exponential least squares fit. An equation relating the absorption coefficients to the ringdown time in a test cell was developed including the two parallel windows in the optical cavity. This analysis is based on the assumption that the measured output of the cavity decays exponentially according to a first order expression and the Beer-Lambert law. In the case of nitrogen, the measurement result were in good agreement unit, typically used for diesel exhaust soot measurement.