Light scattering, UV-visible absorption and UV laser excited fluorescence measurements were performed in rich laminar premixed flames in order to assess the contribution of soot and other organic aerosols to the particulate produced in flames. UV absorbing nanoparticles with a typical size of about 6 nm were detected in situ in the early region of the flame, before soot inception. These particles are almost transparent to the visible radiation. A new sampling system based on water spraying allowed the stabilisation of these particles in aqueous suspension. The UV spectral characteristics of these structures were in excellent agreement with those obtained in flame. A particulate sampling, in the soot forming regime allowed the distinction between the absorption due to soot particles to that due to the nanoparticles still present in the burned gas region. The UV-visible absorption spectrum due to these aerosols has been interpreted in terms of Tauc optical band gap thus allowing a better qualification of both classes of aerosols which is of interest also in the identification of carbon containing particles in the atmosphere.