Increase\nin Agricultural-Derived\nNH<sub><i>x</i></sub> and Decrease in Coal Combustion-Derived\nNO<sub><i>x</i></sub> Result in Atmospheric Particulate\nN–NH<sub>4</sub><sup>+</sup> Surpassing N–NO<sub>3</sub><sup>–</sup> in\nthe South China Sea

The atmospheric deposition of anthropogenic\nactive nitrogen\nsignificantly\ninfluences marine primary productivity and contributes to eutrophication.\nThe form of nitrogen deposition has been evolving annually, alongside\nchanges in human activities. A disparity arises between observation\nresults and simulation conclusions due to the limited field observation\nand research in the ocean. To address this gap, our study undertook\nthree field cruises in the South China Sea in 2021, the largest marginal\nsea of China. The objective was to investigate the latest atmospheric\nparticulate inorganic nitrogen deposition pattern and changes in nitrogen\nsources, employing nitrogen-stable isotopes of nitrate (δ¹⁵N–NO₃^–) and ammonia (δ¹⁵N–NH₄⁺) linked to a mixing model.\nThe findings reveal that the N–NH₄⁺ deposition\ngenerally surpasses N–NO₃^– deposition,\nattributed to a decline in the level of NO_<i>x</i> emission from coal combustion and an upswing in the level of NH_<i>x</i> emission from agricultural sources. The\ndisparity in deposition between N–NH₄⁺ and N–NO₃^– intensifies from\nthe coast to the offshore, establishing N–NH₄⁺ as the primary contributor to oceanic nitrogen deposition,\nparticularly in ocean background regions. Fertilizer (33 ± 21%)\nand livestock (20 ± 6%) emerge as the primary sources of N–NH₄⁺. While coal combustion continues to be a significant\ncontributor to marine atmospheric N–NO₃^–, its proportion has diminished to 22 (Northern Coast)–35%\n(background area) due to effective NO_<i>x</i> emission controls by the countries surrounding the South China Sea,\nespecially the Chinese Government. As coal combustion’s contribution\ndwindles, the significance of vessel and marine biogenic emissions\ngrows. The daytime higher atmospheric N–NO₃^– concentration and lower δ¹⁵N–NO₃^– compared with nighttime further underscore\nthe substantial role of marine biogenic emissions.