Effects of inorganic seed aerosols on the particulate products of aged 1,3,5-trimethylbenzene secondary organic aerosol

Abstract Inorganic aerosols such as (NH 4 ) 2 SO 4 , NaNO 3 and CaCl 2 are commonly present in the Chinese urban atmosphere. They could significantly affect the formation and aging of ambient secondary organic aerosols (SOA), but the underlying mechanisms remain unknown. In this work we studied SOA formation from the photooxidation reaction of 1,3,5-trimethylbenzene (135-TMB) with 100 μg/m 3 of the above three types of inorganic aerosols as seeds in a laboratory chamber. We focused on the aging products of SOA particles by exposing them to high levels of oxidizing hydroxyl radicals (OH). The particulate products of SOA were measured using an aerosol laser time-of-flight mass spectrometer (ALTOFMS) and Fuzzy C-Means (FCM) were applied to organic mass spectra for clustering. In the presence of (NH 4 ) 2 SO 4 seeds, 4-methyl-1H-imidazole, 4-methyl-imidazole-2-acetaldehyde and other imidazole derivative compounds formed from reactions of NH 4 + with methylglyoxal were detected as new aged products. We also observed aromatic nitrogen-containing organic compounds as the major aged products in the presence of NaNO 3 seeds as a consequence of reaction with OH and NO 2 radicals, which were generated by UV irradiation of acidic aqueous nitrate, inducing nitration reactions with phenolic compounds. As CaCl 2 has the strongest hygroscopic properties of the three salt particles tested, the greater water content on the surface of the aerosol may facilitate the condensing of more gas-phase organic acid products to the hygroscopic CaCl 2 seeds, forming H + ions that catalyze the heterogeneous reaction of aldehydes, products of photooxidation of 135-TMB, and forming high-molecular-weight (HMW) compounds. These results provide new insight into the aromatic SOA aging mechanisms.