This paper tests the proposed mechanisms in the literature [1] where the fragmentation was suggested to be as either the result of aggregate break up due to preferential burning in the bridge sites or the result of primary particle fragmentation due to internal burning. In order to evaluate aggregate break up, high-resolution transmission electron microscopy (HR-TEM) images were analyzed to compare the soot nanostructure of the bridge sites with the particle sites. Results demonstrated that the bridge sites were formed by less-ordered nanostructure indicating a higher oxidation potential than particle sites. In addition, the feasibility of internal burning in the single particles was tested through an analysis of intraparticle diffusion. In this case, the actual oxygen diffusion pathway as well as effective spacing between the crystalline structures was estimated by the image analysis technique. By using the obtained data in the effectiveness factor calculation, it was found that the possibility of internal burning by 02 molecules increased for particles less than 10 nm as compared to larger particles. In addition, increasing temperature decreased the potential for internal burning. (C) 2015 The Combustion Institute. Published by Elsevier Inc. All rights reserved.