AbstractSecondary organic aerosol (SOA) from the photooxidation of aromatic compounds is a very complex mixture containing products with a different chemical nature that are dependent on aging processes. In this study, we focus on the chemical characterization of major products that are formed from the OH–initiated oxidation of 1,3,5–trimethylbenzene and subsequent aging through OH–initiated reactions in the presence of NOx.The chemical composition of aged particles were measured in real–time by an aerosol laser time of flight mass spectrometer (ALTOFMS) coupled with Fuzzy C–Means (FCM) clustering algorithm. Experimental results demonstrated that methyl glyoxylic acid, 2–methyl–4–oxo–2–pentenoic acid, 3,5–dimethylbenzoic acid, 2–methyl–2,3–dihydroxyl–4–oxo–pentanoic acid, dimethyl–nitrophenol, 3,4–dimethyl–2–hydroxy–3–oxo–pentandioic acid, 2,4–dimethyl–2,3–dihydroxy–6–oxo–4–heptenoic acid, 2,4–dimethyl–4–hydroxy–2,3–epoxy– heptylic acid, 2,4–dimethyl–2,3,4–trihydroxy–5,6–dioxo–heptylic acid, and oligomer components were the predominant products in the aging particles. The possible reaction mechanisms leading to these aged products were also discussed and proposed.