Characterization of Flame-Generated Carbonaceous Nanoparticles by Oc/Ec Analysis, Raman and Uv-Vis Spectroscopy

Flames and combustion devices operated in rich hydrocarbon conditions produce a large variety of carbonaceous compounds from low and high molecular weight gasphase polycyclic aromatic hydrocarbons (PAHs) to solid particles. In addition to soot, which is often referred to as Black Carbon (BC) or Elemental Carbon (EC), recent works have demonstrated that nanoparticles with a more organic structure are formed in flames and combustion conditions near the onset of particle formation and are emitted into the atmosphere as primary emissions and are possible constituent of primary atmospheric organic carbon (OC). The mean size of these particles is about 2 nm, as was determined in previous works by in situ extinction and scattering measurements, on-line Differential Mobility Analysis measurements of the size distribution, and Atomic Force Microscopy (AFM) measurements on particles deposited on substrates. In this work, we investigate carbon aerosols produced in various flame conditions, across the soot formation threshold, by thermo-optical- transmission (TOT), UVvis and Raman spectroscopy in order to characterize their structural composition in correlation with their OC and EC content. The goal is to determine if and how combined TOT and optical/spectroscopic measurements allow distinguishing nanoparticles formed under "clean-like" combustion conditions from those produced in sooting flames with the perspective to improve and consolidate source apportionment. The sample collected from the non-sooting flame is mostly OC with the lowest propensity to char during the heating procedure, transforming in pyrolytic carbon (PC), and contains a low percentage of EC. Raman features are sensitive to the content of OC and PC in the particles as well as to the EC one.