Seasonal source identification and formation processes of marine particulate water soluble organic nitrogen over an offshore island in the East China Sea.

Water soluble organic nitrogen (WSON) had great influences on the aerosol chemistry, hygroscopicity, marine primary productivity, as well as nitrogen biogeochemical cycles. Aerosol sampling was conducted over an offshore island in the East China Sea in four seasons of 2019, aiming to reveal the seasonal sources and secondary formation processes of marine WSON. The annual mean WSON concentration reached 1.05 ± 1.72 μg/m with a mean WSON/WSTN fraction of 27 %. In spring, WSON was associated with combustion emissions. The liquid-phase reaction of NH/NH with VOCs was a potential secondary formation process of WSON. In summer, WSON was mainly formed through the gaseous phase oxidation of marine biogenic precursors. In autumn, WSON showed miscellaneous sources from agricultural activities, biomass burning, and fossil fuel combustion. In addition to the contribution from primary urea, WSON could be also affected by the oxidation of biological proteinaceous matters. This explained the highest WSON concentrations and WSON/WSOC ratios in autumn. In winter, WSON was probably emitted from sea spray aerosols via the bubble-bursting processes. This study indicated that the sources of WSON over the coastal waters in the East China Sea were quite diverse, highlighting the need of more detailed characterization of marine WSON at the molecular level.