The surface functional groups of organic nanoparticles formed in furfural photonucleation were identified with the help of rupture event scanning. The presence of carboxylic groups on the surface of the particles was discovered. The photonucleation kinetics was studied in a laboratory photochemical reactor using a diffusion-based aerosol spectrometer. It was demonstrated that photonucleation involves photochemically generated short-lived free radicals, detected and identified by means of spin trapping. The photonucleation mechanism is proposed.