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To investigate the reaction mechanisms during the NO 3 - and O 3 -initiated aging of aromatic secondary organic aerosol (SOA), the gases and particles formed from the OH-initiated oxidation of toluene were exposed to NO 3 radicals and O 3 in a laboratory chamber. Hydroxyl radicals for the oxidation of toluene were produced under light conditions through H 2 O 2 photolysis and dark conditions through tetramethylethylene ozonolysis. Aerodyne aerosol mass spectrometer results demonstrated an increase in SOA and nitrate aerosol concentrations only after N 2 O 5 injection, but no clear change after O 3 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 N 2 O 5 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 N 2 O 5 injection in the particle phase. To confirm the contribution of cresols on aging, NO 3 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 NO 3 -initiated aging of toluene SOA. Image 1 • SOA formed from OH oxidation of toluene was exposed to NO 3 radicals and O 3. • SOA increased by 22% only after NO 3 radical addition. • Cresols were rapidly degraded, nitrocresols and nitrosalicylic alcohols were formed. • Cresols accounted for a 57–67% increase in SOA upon exposure to NO 3 radicals. [ABSTRACT FROM AUTHOR]