In this article we present an experimental and modeling study of Organic Carbon (OC) particles, with a typical size range of 1-10nm, and soot particles, with sizes in the range 10-100nm, formed in premixed methane flames at atmospheric pressure. An optical diagnostic method based on the interaction of an ultraviolet source, corresponding to the fifth harmonic of a Nd:YAG laser, with combustion-generated aerosols was used in order to measure the concentration profiles of both OC and soot particles with high spatial resolution. The high energy of the selected harmonic induces fluorescence from aromatic chromophoric groups and also allows soot particles to heat up and emit incandescent radiation. Optical measurements data are compared with numerical modeling simulations. The experimental data, in agreement with the model results, show that methane burned in slightly rich conditions forms prevalently organic particulate with mean size lower than 10nm, while soot formation is observable only in very rich conditions.