THE EFFECT OF ETHANOL ON SOOT PRECURSORS NANOPARTICLES BY PHOTOIONIZATION-DMA

The effect of ethanol addition in ethylene laminar premixed flames, on the characteristics of soot's precursor nanoparticles, was investigated by aerosol photoionization technique. Indeed photoelectric charging of particles is a powerful tool for online characterization of fine and ultrafine aerosol particles because of its high sensitivity and chemical selectivity. Photoionization charging efficiency, i.e. the ratio between the generated positive ions and the corresponding neutral ones, was measured for flame-generated carbonaceous nanoparticles. Nucleation mode nanoparticles were sampled by means of a dilution probe, to prevent particle coagulation, size-selected and analyzed on-line. The fifth harmonic of a Nd:YAG laser, 213 nm (5.82 eV), was used as an ionization source, whereas a differential mobility analyzer (DMA) coupled to a Faraday cup electrometer was used for particle classification and detection. Carbonaceous nanoparticles in the nucleation mode, i.e. with sizes ranging from 1 to 10 nm, show a photoionization charging efficiency clearly dependent on the percentage of ethanol used as dopant in the ethylene flame. In particular, we observed that the more ethanol was added as fuel, the lower the particle photo-charging efficiency was. This result clearly indicates a modification in the nanoparticle chemical structure as the amount of ethanol is increased. These experimental evidences may be explained by the decrease of aromaticity of the particles and/or by the presence of oxygen bonds within the nanoparticles. Further work is necessary to confirm the validity of these two hypotheses with the use of complementary techniques. However, the results reported herein confirm particle photoionization as a powerful analytical diagnostic tool, providing fingerprints and characterization of combustion aerosol.