Analysis of flame-formed organic nanoparticles by UV laser photoionization measurements

Particle inception in flames is a very complex phenomenon involving: gas-phase free radical reactions, particle nucleation through polymerization and/or clustering pathways, particle growth by both heterogeneous gas-to-solid reactions and physical coagulation and/or coalescence processes, particles annealing and oxidation. To control particle formation, for both emission reduction and/or particle synthesis, the full understanding of the complex chemical-physical processes occurring in the flame reactor is required. In the recent years, the understanding of mechanisms leading to the formation of incipient organic nanoparticles in flames has attracted the interest of the combustion research community not only because of their role as soot precursors but also as possible constituent of the combustion aerosol emissions [1]. Photoelectric charging of particles is a powerful tool for the on-line characterization of submicron aerosol particles [2]. Photoionization based techniques have high sensitivity and chemical selectivity. Furthermore, photoemission studies yield information on the electronic properties of the investigated compounds, i.e. valence and conduction bands, and are very sensitive to the surface composition of aerosol particles [2]. In the present study an UV laser beam coupled to a differential mobility analyzer (DMA) has been used to measure the photoionization efficiency of carbon nanoparticles produced in flames. Ultrafine particles have been sampled on-line from well characterized laboratory laminar flames and size selected particles have been investigated in terms of their propensity to be ionized by interaction with UV photons with energy of 5.82 eV given by the fifth harmonic of a Nd:YAG laser at the wavelength of 213 nm. The size range of the investigated particles is 2-10 nm. Such particles are representative of the organic nanoparticles produced in slightly sooting flames across the soot inception threshold. The aim of this work is to gain further insights into the chemical/physical properties of the organic nanoparticles and early soot nuclei formed in combustion systems. This on-line aerosol based technique offers the advantage, over conventional off-line methods, to gain information on the pristine optical and electronic properties of the particles without interference of substrate interaction effects, condensation of molecules on the particle surface, and/or particles coagulation. Ethylene/air flames were stabilized on a McKenna burner, with a cold gas velocity of the unburned premixed gasses of 10 cm/s. Carbonaceous nanoparticles where collected by keeping constant the height above the burner (HAB), and changing the fuel/air equivalence ratio, from ?=1.73 (C/O=0.57) to ?=2.03 (C/O=0.67). Particles were sampled through a small orifice into a dilution tube probe operated with N2 as the diluent. Collected particles, were passed through an electrostatic precipitator to remove the ions in the gas stream before the interaction with the light source occurs. The remaining neutral particles entered the photoionization cell and were irradiated by the fifth harmonic of a Nd:YAG laser (?0=213 nm, h?=5.82 eV photon energy). The aerosol was then sent to the classifier of the DMA operated with a constant voltage in order to select single size particles and finally measured by electrometer. The number of neutral species was instead measured by substituting the ionization source with a commercial bipolar charger. Once both the number of neutral (N0) and photoionized particles (N+PI) were measured, the photoionization charging efficiency (CE) is obtained by dividing the number of the particle charged via photoionization by the number of neutral particles. Particle size distribution (PSDs) were measured by positioning the sam