High-resolution mass spectrometry characterization of secondary organic aerosol formation: Comparative effects of NH₃ on biotic and abiotic precursors

This study explores the role of ammonia (NH₃) in the formation of nitrogen-containing secondary organic aerosol (SOA) using smog chamber experiments. Reactive organic gases, including toluene, m-xylene, ethylbenzene, α-pinene, and isoprene, were analyzed under NH₃-present and NH₃-absent conditions. High resolution mass spectrometry revealed that toluene, m-xylene, ethylbenzene and isoprene exhibited higher signal-to-noise (S/N) ratios and an increased abundance of nitrogen-containing oxygenated compounds in the presence of NH₃. Conversely, α-PIN demonstrated high S/N ratios in both conditions, though CHON formation increased with NH₃, emphasizing its role in nitrogen incorporation into SOA. The study also identified low-volatility, highly oxidized compounds with double-bond equivalent values exceeding 5, along with oligomeric structures that contribute to particulate matter growth. Mechanistic analysis showed that CHON compounds are formed through photooxidation pathways involving hydroxyl radical reactions, oxidation, nitration, and hydrolysis, consistent with mass spectrometry data. These findings underscore the critical role of NH₃ in modifying SOA composition and highlight its potential implications for atmospheric particulate matter formation and air quality.