Characterization of brown carbon constituents of benzene secondary organic aerosol aged with ammonia

Nitrogen-containing organic compounds (NOC) formed from secondary organic aerosols (SOA) age via reaction with reduced nitrogen species are a vital class of brown carbon compounds. NOC compounds from ammonia (NH₃) gas-aging of benzene SOA were investigated in present study, and the experiments were performed by irradiating benzene/CH₃ONO/NO/NH₃ air mixtures in a home-made smog chamber. The particulate NOC products of aged benzene SOA in the presence of NH₃ were measured by UV-Vis spectrophotometer, attenuated total reflectance-Fourier transform infrared (ATR-FTIR), and aerosol laser time-of-flight mass spectrometer (ALTOFMS) coupled with Fuzzy C-Means (FCM) clustering algorithm, respectively. Experimental results demonstrated that NH₃ has significant promotion effect on benzene SOA formation. Organic ammonium salts, such as ammonium glyoxylate, ammonium 6-oxo-2,4-hexadienoiclate, which are formed from NH₃ reactions with gaseous organic acids were detected as the major particulate NOC products of NH₃-aged benzene SOA. 1H–imidazole, 1H–imidazole-2-carbaldehyde and other imidazole products via the heterogeneous reactions between NH₃ and dialdehydes of benzene SOA were successfully detected as important brown carbon constituents. The formation of imidazole products suggests that some ambient particles contained organonitrogen compounds may be come from this mechanism. The results of this study may provide valuable information for discussing NH₃ deposition and SOA aging mechanisms.