Molecular Evidence of Aqueous-Processed NOCs: From Mechanistic Insights to Environmental Impacts

Nitrogen-containing organic compounds (NOCs), encompassing a complex suite of oxidized and reduced organic nitrogen species, exert significant impacts on atmospheric light absorption, oxidation capacity, and global nitrogen cycling. Despite the growing recognition of NOCs as key components of atmospheric organic matter, their formation through aqueous-phase processes and potential environmental impacts have long been underestimated. This review begins by summarizing the major classes of NOC molecules, then synthesizes observational evidence on their formation in the aqueous-phase, particularly highlighting its critical role in generating nitroaromatic and N-heterocyclic compounds. Built on the observational evidence, we further discuss the related evaluation of the multi-faceted environmental impacts arising from the aqueous-phase NOC formation. The evidence demonstrates that aqueous-phase NOC chemistry exerts significant influence on atmospheric compositions, contributes up to 90% of brown carbon's radiative effects, enhances oxidative capacity and secondary organic aerosol production, and influences nitrogen speciation in wet deposition. However, most current model assessments exhibit considerable limitations in quantifying these effects, stemming primarily from oversimplified parameterizations of aqueous-phase chemistry that fail to adequately represent the full complexity of atmospheric multiphase systems. Furthermore, existing observational data sets remain insufficient, severely constraining efforts to optimize model parameters and validate simulation outputs. To address these critical knowledge gaps, we propose an integrated research framework that combines long-term monitoring of key NOC and various precursors and advanced simulations of aqueous-phase chemistry at the micrometer-scale reaction environments, which would constrain the parameterization of future models for the aqueous-phase chemistry and impacts of NOCs.