Increase in Agricultural-Derived NHx and Decrease in Coal Combustion-Derived NOx Result in Atmospheric Particulate N–NH4+ Surpassing N–NO3– in the South China Sea

The atmospheric deposition of anthropogenic active nitrogen significantly influences marine primary productivity and contributes to eutrophication. The form of nitrogen deposition has been evolving annually, alongside changes in human activities. A disparity arises between observation results and simulation conclusions due to the limited field observation and research in the ocean. To address this gap, our study undertook three field cruises in the South China Sea in 2021, the largest marginal sea of China. The objective was to investigate the latest atmospheric particulate inorganic nitrogen deposition pattern and changes in nitrogen sources, employing nitrogen-stable isotopes of nitrate (δ15N–NO3–) and ammonia (δ15N–NH4+) linked to a mixing model. The findings reveal that the N–NH4+ deposition generally surpasses N–NO3– deposition, attributed to a decline in the level of NOx emission from coal combustion and an upswing in the level of NHx emission from agricultural sources. The disparity in deposition between N–NH4+ and N–NO3– intensifies from the coast to the offshore, establishing N–NH4+ as the primary contributor to oceanic nitrogen deposition, particularly in ocean background regions. Fertilizer (33 ± 21%) and livestock (20 ± 6%) emerge as the primary sources of N–NH4+. While coal combustion continues to be a significant contributor to marine atmospheric N–NO3–, its proportion has diminished to 22 (Northern Coast)–35% (background area) due to effective NOx emission controls by the countries surrounding the South China Sea, especially the Chinese Government. As coal combustion's contribution dwindles, the significance of vessel and marine biogenic emissions grows. The daytime higher atmospheric N–NO3– concentration and lower δ15N–NO3– compared with nighttime further underscore the substantial role of marine biogenic emissions.