© 2019 Elsevier Ltd To investigate the reaction mechanisms during the NO3- and O3-initiated aging of aromatic secondary organic aerosol (SOA), the gases and particles formed from the OH-initiated oxidation of toluene were exposed to NO3 radicals and O3 in a laboratory chamber. Hydroxyl radicals for the oxidation of toluene were produced under light conditions through H2O2 photolysis and dark conditions through tetramethylethylene ozonolysis. Aerodyne aerosol mass spectrometer results demonstrated an increase in SOA and nitrate aerosol concentrations only after N2O5 injection, but no clear change after O3 injection. The results of proton transfer reaction-quadrupole interface-time of flight mass spectrometer showed that the signal of m/z 109.07 decreased and the signals of m/z 123.05 and m/z 154.05 increased after N2O5 injection, suggesting that gaseous cresols formed from toluene oxidation are rapidly degraded and gaseous nitrocresols and methylbenzoquinones are newly formed. The results of liquid chromatography-mass spectrometry analysis showed that nitrocresols and nitrosalicylic alcohols were the major products formed after N2O5 injection in the particle phase. To confirm the contribution of cresols on aging, NO3 oxidation of o-cresol was carried out, which resulted in the formation of similar products, i.e. gaseous nitrocresols and particulate nitrosalicylic alcohols. Yield curve analysis using two product model estimated that cresol accounted for ~62% of the increase in SOA during NO3-initiated aging of toluene SOA.