The high spectral brilliance of x-rays produced at the Basic Energy Sciences Synchrotron Radiation Center of Argonne's Advanced Photon Source allows us to perform small-angle x-ray scattering (SAXS) measurements of the distributions of soot particles in flames. SAXS provides an in situ probe of the size and distribution of particles in the region between 1 and 100 nm. Detailed measurements on a propylene/ air diffusion flame allow us to extract a spatially dependent background, which occurs in gas-phase combustion systems, and to perform Abel inversions, which provide the radial dependence of the scattering intensity. A bimodal distribution of soot particles is needed to describe our results. The radial behavior of the two modes of this distribution implies that the chemistry and fluid dynamics are strongly coupled in this simple diffusion flame. The larger particles of this distribution correspond to the previously observed primary particles, which have a relatively complex radial dependence. Midway between the fuel source and the widest part of the flame, the primary particles have a mean radius of 6 nm or less and their concentration is symmetrically distributed about the flame front. At the widest part of the flame, two distinct distributions of primary particles are observed. Near the center of the flame, the particles have a mean radius of 10 nm and a polydispersity of 0.3 and beyond a transition region they have a mean radius of 21 nm and a polydispersity of 0.2. The smaller particles, which require additional experiments before they can be identified, correspond to soot nuclei, polycyclic aromatic hydrocarbon species such as naphthalene, and/or disordered carbons with graphitic basal planes.
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