Photoacoustic insights on diesel exhaust particles

Photoacoustic measurements at General Motors Research Laboratories on diesel exhaust particulate emissions are reviewed. Conventional optical techniques provide real-time qualitative monitoring of particle mass concentration, but typical data exhibit considerable scatter, suggesting that the assumption of a simple correlation between optical response and particle mass concentration is unsound. Photoacoustic measurements at long wavelengths not only provide excellent sensitivity but, in principle, also avoid problems associated with variations in particular size. However, variations in shape (via clustering) and particle chemistry, resulting from varying engine operating conditions, can drastically affect the optical attenuation per unit particle mass, particularly for very small particles.