Significant secondary formation of nitrogenous organic aerosols in an urban atmosphere revealed by bihourly measurements of bulk organic nitrogen and comprehensive molecular markers

Abstract. Nitrogenous organic aerosol (OA) has a significant impact on solar radiation, human health, and ecosystems. However, our knowledge of the total budget of aerosol organic nitrogen (ON) and its major sources, particularly the secondary formation processes, remains largely qualitative. In this study, we conducted bihourly measurements of aerosol ON and a comprehensive array of organic and inorganic source markers in urban Shanghai during the fall-winter period of 2021. ON accounted for 6–58 % of the total aerosol N, averaging 20 %. Positive factorization matrix source apportionment revealed that both primary emissions (52 %) and secondary formations (48 %) made substantial contributions to the ON mass. Dominant primary ON sources included coal combustion and vehicle emissions, accounting for 21 % each. Five significant secondary formation processes of ON were identified, namely nitrate formation (14 %), photochemical formation (10 %), nitroaromatics formation (7 %), dicarboxylic acids (DCA) formation (8 %), and oxygenated cooking OA (7 %). DCA formation-related ON likely represented reduced N-containing organic species such as imidazoles and amides. Nitrate formation processes produced OA with a very low organic carbon-to-ON ratio, suggesting a heterogeneous/aqueous formation of organic nitrates. Our field work provides first quantitative source analysis and new insights into the secondary formation processes of ON aerosols in an urban atmosphere.