Characterizing the Source and Formation of Gaseous Nitrated Phenolic Compounds in Urban Shanghai

Abstract Nitrated phenolic compounds (NPs) are important components of brown carbon and contribute to the formation of HONO and OH radicals, yet their gas‐phase abundance and formation mechanisms in polluted urban environments remain poorly understood. In this study, continuous measurements of gaseous NPs were conducted in urban Shanghai during the summer of 2021 using a nitrate‐based chemical ionization time‐of‐flight mass spectrometer. Sixteen NP species, including eight mono‐NPs and eight di‐NPs, were identified and quantified, with total concentrations ranging from 3.5 to 151.8 pptv. Pronounced diurnal variations were observed, with mono‐NPs peaking during daytime and most di‐NPs accumulating at night, indicating distinct formation pathways. Budget analysis based on box model simulations demonstrated that secondary formation dominated the observed NP levels. Incorporating styrene as an additional precursor substantially improved the simulation of C 6 H 5 NO 3 , highlighting the importance of a previously underrepresented multi‐step oxidation pathway in NP formation. The default mechanism underestimated C 7 H 7 NO 3 under low‐NO and high toluene/NO ratio conditions, suggesting altered toluene oxidation chemistry across different pollution regimes. Di‐NP levels were attributed to secondary formation from mono‐NPs and regional transport. Photolysis was identified as the primary sink for gaseous NPs, producing HONO at an average rate of 6.2 ± 7.6 pptv/hr and accounting for approximately 13% of the unexplained daytime HONO source in urban Shanghai. These results underscore the critical role of secondary chemistry in determining NP levels and reveal the complexity and ambiguity of their reaction mechanisms, calling for further in‐depth investigations.