Detection of extremely fine carbonaceous particles in the exhausts of diesel and spark-ignited internal combustion engines, by means of broad-band extinction and scattering spectroscopy in the ultraviolet band 190-400 NM

Abstract

Recent studies have shown that organic matter found in fine aerosol or sampled in flames can be only partially speciated, the major part being unidentified. Instrumental limitations of chemical analysis at very high molecular masses and of particle detectors at very low sizes leave unexplored the nanometric size range, where organic molecular clusters might accumulate.

This work reports on the detection of organic extremely fine particles in the exhausts of both diesel and spark-ignited engines, by means of broad-band extinction and scattering spectroscopy in the ultraviolet 190–400 nm band.

The detection techniques rely on a light source, resulting from the laser-induced optical breakdown of air, which features “blackbody” ultraviolet-visible emission, duration of few tens of nanoseconds and tighly confined spot volume.

Samples of internal combustion (IC) engines' emissions have been analyzed in two forms:(a) ordinarily air-diluted exhausts, for extinction measurements and (b) solution/suspension of condensed combustion water, which proved to increase the trapped species concentrations to levels suitable for spectral scattering measurements.

Extinction and scattering spectral data have led to characterize the scatters in terms of: (1) their complex index of refraction in the ultraviolet band 190–450 nm: (2) their average size, in the order of few nonometers and (3) their volume fraction f_v (hundreds of ppm) in the water-trapped exhausts.

The spectral shapes of the extinction coefficient α(λ) in the ultraviolet band have been interpreted in the framework of the solid-state physics, by relating the spatial structures of organic molecular clusters to the value E_g of the optical gap, derived experimentally by the Tauc relationship.

Resulting optical gaps are very low (E_g=0.2 eV) for air-diluted diesel exhausts, involving the presence of soot, as expected, whereas, in all the other cases explored, E_g spans over values greater than 3 eV, associated with carbon-containing nanoparticles.