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Spatially resolved measurement of the soot particle size distribution function (PSDF) was made in a laminar premixed ethylene-argon-oxygen flame (φ = 2.07) using a scanning mobility particle sizer. The emphasis of the study was to follow the evolution of the PSDF from the onset of particle inception to particle mass growth. At the onset of soot inception, the PSDF was found to follow a power-law dependence on particle diameter. The PSDF becomes bimodal at larger height above the burner surface, and remains bimodal throughout the flame. Numerical simulation using a kinetic model proposed previously and a stochastic approach to solve aerosol dynamics equations again showed a bimodal PSDF. Further analysis revealed that bimodality is intrinsic to an aerosol process involving particle-particle coagulation and particle nucleation dominated by monomer dimerization.