Analysis of Large PAH as components and precursors of nano/ultrafine carbon particulate matter

Adverse health effects of ultrafine and nanoparticle air pollution are considered greater than the health effects associated with coarse (PM10) and fine particulate matter (PM2.5) in consideration of their high contribution to the particle number as opposed to their very low contribution to the total PM mass. New air quality standards and testing methods are thus going to be established based on the measurement of airborne particle number down to the very small size featuring nano/ultrafine particles. The predominant contribution of organic carbon, to the nano-ultrafine particles, mainly coming from combustion sources, has been established (1-2), however the chemical complexity of organic carbon, mostly constituted of aromatic species, requires sophisticated techniques for detecting and identifying their chemical composition, and eventually testing their biological activity. Large polycyclic aromatic hydrocarbons (L-PAH), defined as aromatic species with aromatic core of at least 24 carbon atoms (3), are the common thread linking small volatile PAH and carbon nanoparticles formation occurring during some phases of the combustion process or in the case of failure of the combustion system. Indeed, L-PAH constitute a class of species which can be separated from the carbon particulate matter through extraction/chromatography and are also individuated as main structural moieties linked each other by single bond and bridging rings in the bulk of carbon particles. This suggests their possible role as precursors of nano/ultrafine particles formation. In previous work structural insights on the L-PAH fraction of carbon particulate matter have been obtained by size exclusion chromatography (SEC) coupled with UV-Visible absorption. Specifically, the optical band gap and the UV peak position of L-PAH components have been evaluated by means of a spectral reconstruction procedure (4). In this work the characteristics of L-PAH produced from fuel-rich combustion, namely sooting premixed flames, have been studied by their preliminary SEC separation from the particulate matter and detailed analysis with other diagnostic, mainly spectroscopic, tools. The study of the properties of L-PAH formed in sooting premixed flames has been made easy from referring to pitch samples, almost totally composed of L-PAH in a quite similar mass range. Structural analysis of L-PAH constituting the pitch and isolated from the flame-formed PM has been focused on the carbon network structure in terms of sp2 and sp3 sites and size and stacking of the aromatic units. The L-PAH structure and composition have been inferred by looking at specific absorption and emission spectral features measured in the UV-visible range as the optical band gap and the position of the maximum absorption and fluorescence. References 1.J.H. Seinfeld, S.N. Pandis, Atmospheric Chemistry and Physics: From Air Pollution to Climate Change, John Wiley & Sons, Inc., NY, 1998. 2.A D'Anna, Combustion-formed nanoparticles Proceedings of the Combustion Institute, 32 (2009) 593-613. 3.Fetzer JC (2000) Large (C>=24) Polycyclic aromatic hydrocarbons: Chemistry and Analysis. Wiley-Interscience, New York. 4.Russo, C., Stanzione, F., Ciajolo, A.,Tregrossi, A. (2013): Study on the contribution of different molecular weight species to the absorption UV-Visible spectra of flame-formed carbon species". Proceedings of the Combustion Institute 34 3661-3668, (2013).