Abstract
The presence of very fine carbonaceous particles in the ambient causes concern either for their toxic properties or for their influence on atmospheric chemistry and physics. Previous work has shown that carbonaceous nanoparticles with a typical size of about 2–3 nm are formed in the reaction zone of rich premixed sooting flames and may act as soot precursors. This paper ascertains the presence of nanoparticles in rich laminar C 2 H 4 /air premixed flames with C/O ratio ranging over a wide interval below the soot limit. Ultraviolet (UV)-visible absorption starting from 200-nm, laser-induced fluorescence (LIF), and laser light scattering (LLS) methods were used for the in situ measurements and for the characterization of the material collected from the flames as hydrosol in water traps. It was found that the absorption spectra from 200 to 300 nm were due both to hot CO 2 , whose absorption cross sections are orders of magnitude higher than those found at room temperature, and to carbonaceous high molecular mass structures. The absorption spectra of those structures, obtained by subtracting the CO 2 contribution, and the fluorescence spectra show the same behavior as those measured on the hydrosol suspension. The monochromatic absorption coefficient in the UV due to high molecular mass structures have their onset at C/O=0.4, whereas the fluorescence excited at λ 0 =266 nm appears around C/O=0.5, where also light scattering coefficients, in the UV and visible, in excess with respect to those due to burned gases, start to be measurable. These results are interpreted in terms of a carbonaceous network of one-and two-ring aromatic functionalities, connected by aliphatic bonds, with typical size around 2 nm. The high-temperature formation of these carbonaceous nanoparticles below soot formation limit is considered an important route for explaining the presence of organic carbon aerosols in the atmosphere.
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