Aerosol organic nitrogen (AON) is an important fraction of marine organic aerosols and atmospheric N deposition over the open ocean. However, it is unclear how much of the AON is of external origin or internally recycled from the ocean. Here, we quantified AON over the Northwest Pacific Ocean using a newly developed Aerosol Nitrogen Analyzer and investigated its chemical nature on the molecular level. The AON accounted for 14%−64% of the total nitrogen in the total suspended particles. The contribution of AON was equal to inorganic N (NO₃⁻-N+NH₄⁺-N) over the oligotrophic subtropical gyre and tropical areas. A total of 57% of the AON was external input from transported combustion emissions and aged pollutants. External AON compounds were characterized by a lower C number and higher oxidation state than the recycled ones. Carboxylic-rich alicyclic and aliphatic/peptides-like compounds were the dominant classes among the external AON, which could be potentially utilized by phytoplankton as a nitrogen source. External AON was also important to the aerosol light absorption or brown carbon in the marine boundary layer, especially over the East Asia Marginal Seas. Externally transported AON was contributed by aged anthropogenic sources, combustion, and dust (in spring). Our work highlights the importance of AON for aerosol light absorption over marginal seas, and for nitrogen supply over open oligotrophic oceanic regions.

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