Recent literature kinetic studies revealed the importance of new mechanisms for polycyclic aromatic hydrocarbon (PAH) and soot inception beyond hydrogen–abstraction–acetylene–addition (HACA) and hydrogen–abstraction–vinylacetylene–addition (HAVA) mechanisms in the combustion of ethylene and other hydrocarbons. Co-flow diffusion flame is a canonical flame used to investigate the interaction between fluid dynamics and PAH chemistry. In this study, supersonic molecular beam sampling technique was utilized for the first time with synchrotron vacuum ultraviolet photoionization mass spectrometry (SVUV-PIMS) to measure laminar co-flow diffusion flame at atmospheric pressure. We report quantitative measurement of precursor radicals as well as critical intermediates and odd carbon number PAH species. A custom-designed computational code, based on OpenFOAM and Cantera, was adopted to simulate laminar co-flow diffusion flames with literature kinetic model. Chemical kinetic analyses show that addition reactions of odd carbon number species provide considerable contribution to PAH formation processes beside HACA and HAVA mechanisms. Reasonable mass growth reactions are postulated for aromatic species with odd carbon numbers, such as ethynyl-indene, fluorene, benzo-indene, which need further investigations. Reactions of resonantly stabilized radicals followed by ring expansion are shown to be critical for both odd and even carbon number aromatics, and are suggested to be included in future PAH models.