Characterization of Combustion-Generated Carbonaceous Nanoparticles by Size-Dependent Ultraviolet Laser Photoionization

Abstract

Photoelectric charging of particles is a powerful tool for online characterization of submicrometer aerosol particles. Indeed photoionization based techniques have high sensitivity and chemical selectivity. Moreover, they yield information on electronic properties of the material and are sensitive to the state of the surface. In the present study the photoionization charging efficiency, i.e., the ratio between the generated positive ions and the corresponding neutral ones, for different classes of flame-generated carbonaceous nanoparticles was measured. The fifth harmonics of a Nd:YAG laser, 213 nm (5.82 eV), was used as an ionization source for the combustion generated nanoparticles, 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., sizes ranging from 1 to 10 nm, show a photoionization charging efficiency clearly dependent on the flame conditions. In particular, we observed that the richer the flame is, i.e., the higher the equivalent ratio is, the higher the photon charging efficiency is. We hypothesized that such an increase in the photoionization propensity of the carbonaceous nanoparticles from richer flame condition is associated to the presence within the particles of larger aromatic moieties. The results clearly show that photoionization is a powerful diagnostic tool for the physical-chemical characterization of combustion aerosol, and it may lead to further insights into the soot formation mechanism.

Flame-Formed Carbon Nanoparticles: Morphology, Interaction Forces, and Hamaker Constant from AFM

“…The flames were stabilized on a water-cooled McKenna burner, and the cold gas velocity was kept constant at 10 cm/s. Flame equivalence ratio, F, was changed in order to move from flame conditions with unimodal particle size distributions with average Table 1 reports the size ranges of the first and second mode of the size distribution measured by online scanning mobility particle sizer in previous works (Echavarria et al 2009;Sgro et al 2011;Commodo et al 2013). …”

“…Modeling and experimental activities have demonstrated that fuel-rich flame conditions produce particles with a wide size range roughly grouped in two classes of nanoparticles on the basis of the bimodal shape of the size distribution function (Bockhorn 1994;Wang 2001;Sgro et al 2003;D'Anna 2009;Echavarria et al 2009;Commodo et al 2013). In addition to their sizes, these two classes of particles differentiate by chemical structure, morphology, and optical and spectroscopy behaviors.…”

Photoionization Study of Soot Precursor Nanoparticles in Laminar Premixed Ethylene/Ethanol Flames

“…By contrast, UV photons, hν = 4−6 eV, are typically sufficient to photo-ionize nanoparticles and combustion aerosols (Burtscher, 1992), so that they can be used to implement atmospheric pressure aerosol-based techniques. In a our recent study (Commodo et al, 2013), we implemented an experimental method based on aerosol photoionization of size-selected flame-formed nanoparticles, by means of an UV laser source (5.82 eV photon energy), and an electrostatic classifier (DMA) with sensitivity to nanoparticles as small as 1 nm. The proposed method showed to be successful in differentiating inception particles collected from ethylene flames at different equivalent ratio.…”

Nanoparticle Filtration Characteristics of Advanced Metal Foam Media for a Spark Ignition Direct Injection Engine in Steady Engine Operating Conditions and Vehicle Test Modes

“…In phase 2 and phase 3 of the FTP-75 mode, as the engine was fully warmed-up, the number of particles with diameters greater than 48 nm decreased to less than 40% of the total number of particles, and those of sub-23 nm particles increased to greater than 35%. Therefore, for the GDI engine, particles are assumed to be generated from the unburned hydrocarbons and the wall-film when the engine is in the cold condition, and these nanoparticles originating from hydrocarbons are relatively large [8,10,13,[15][16][17][18][19]22,26,28,[33][34][35]39,43]. In contrast, almost all of sub-23 nm nucleation particles were strongly reduced in the GDI-GPF vehicle, and its proportion relative to the total number of nanoparticles was not affected by the engine operating states.…”

“…PM pollutants are currently of considerable interest because medical findings have indicated that exposure to ultrafine aerosol particles (<100 nm diameter) from internal combustion engines cause significant health risks [7,8,[21][22][23][24]. Furthermore, previous studies have indicated that GDI engines produce considerably hazardous nucleation mode particles, which are typically a complex mixture of solid and more volatile particles [8,17,18,[25][26][27][28]. Based on the high market growth of GDI vehicles, the European Union (EU) has regulated the particle number (PN) standard for GDI vehicles, which considers non-volatile particles with a diameter above 23 nm, in the Euro-6 vehicle emissions standards [18,20,21,24].…”