Significance: Flame-formed carbon nanoparticles are observed to exhibit quantum confinement behaviors. We demonstrate that the size dependence of photoemission ionization energy in air and optical band gap of flame-generated carbon particles can be well explained by available quantum confinement theory. Carbon nanoparticles comprising mainly polycyclic aromatic hydrocarbons are widespread in nature. The results have important consequences in problems ranging from interpreting the infrared emission spectra of carbon dusts in the interstellar media, to evaluating the climate effects of atmospheric fine carbon particles, measuring soot nucleation and growth rates in flames, and developing applications of carbon nanoparticles in photovoltaic and electrochemical devices.