The chemical composition of secondary organic aerosol (SOA) formed from the photolysis of 1-nitronaphthalene in a series of simulation chamber experiments has been investigated using an aerosol time-of-flight mass spectrometer (ATOFMS). The resulting SOA is characterized by the presence of a dimer (286 Da) proposed to be formed through the self-reaction of naphthoxy radicals along with the expected product nitronaphthol. The molecular formulas of the SOA products were confirmed by collecting filter samples and analyzing the extracts using ultrahigh resolution mass spectrometry. Further evidence for the radical self-reaction mechanism was obtained by photolyzing 1-nitronaphthalene in the presence of excess nitrobenzene, where it was shown that the resulting SOA contained a product consistent with the cross-reaction of phenoxy and naphthoxy radicals. The naphthoxy dimer was formed from the photolysis of 1-nitronaphthalene under a variety of different experimental conditions including the presence of excess butyl ether as an OH scavenger and the presence of ambient air and particles. However, formation of the dimer was suppressed when 1-nitronaphthalene was photolyzed in the presence of excess NO and nitronaphthol was instead found to be the dominant particle-phase product indicating that the yield of the dimer is dependent upon the concentration of pre-existing NO(x). The results of this work suggest that photolysis of 1-nitronaphthalene represents a previously unidentified pathway to SOA formation in the troposphere. Analogous mechanisms may also be important for other nitrated polycyclic aromatic hydrocarbons.