The Characteristics of Soot Formed in Premixed Flames by Different Fuels Carmela Russo a (lina.russo@virgilio.it), Fernando Stanzione b (f.stanzione@irc.cnr.it), Rosalba Barbella b (barbella@irc.cnr.it), Antonio Tregrossi b (treg@irc.cnr.it), Anna Ciajolo b (ciajolo@irc.cnr.it) a Dipartimento di Ingegneria Chimica, Università Federico II, P.le V. Tecchio, 80 - 80125, Napoli, ITALY b Istituto di Ricerche sulla Combustione - C.N.R, P.le V. Tecchio, 80 - 80125, Napoli, ITALY The internal structure of soot sampled from fuel-rich methane and benzene premixed flames burning in different temperature conditions was investigated. UV-visible spectroscopy was used for its sensitivity to the carbon network in terms of sp 2 and sp 3 sites and of extension of sp 2 aromatic moieties. The UV-visible absorption maximum and the optical band gap have shown to be affected by the fuel characteristics rather than by flame temperature. The important role of fuel aromaticity on soot internal structure from particle inception throughout the soot formation region was demonstrated. Introduction Carbonaceous aerosols, largely produced from the combustion of fossil fuels and widely distributed in the atmosphere, affect the solar input to the earth–atmosphere system by scattering and absorbing radiation and by providing sites for the condensation of clouds. Thus, the optical properties as well as the number size distribution of combustion- formed carbonaceous particles determine their role in climate change governing the radiative forcing and their study is important to foresee and model how aerosols affect the environment. Previous works have yielded a wide range of values for the absorptive properties of light-absorbing carbon (Bond and Bergstrom, 2006), that is generally termed soot in combustion science and by the Intergovernmental Panel on Climate Change (IPCC, 1996). Absorptive properties of carbon materials as soot are known to mainly depend on the molecular form. In particular the size of sp 2 -bonded clusters, constituting the aromatic units of soot, affects the absorption spectra. The size of sp 2 - bonded clusters, as well as the number of sp 2 and sp 3 sites contained in soot, are induced by diverse combustion conditions and different fuels (Tregrossi et al., 2007; Apicella et al., 2004). It derives that the effect of combustion parameters on the absorptive properties of soot at the exhaust has to be studied to provide the input parameters for aerosol modelling also in view of further modifications of soot in the atmosphere. UV- visible spectroscopy can be used as probe of the sp 2 clusters mainly ruling the electronic properties of carbon materials (Jager et al., 1999). In this work the position of the UV- visible absorption maximum and the spectral shape of soot have been measured to