The\nOcean Serves\nas a Net Source of Atmospheric Nitric\nOxide but a Net Sink of Nitrogen Dioxide

Nitrogen oxides (NO<sub>x</sub>),\ncomprised of nitric\noxide (NO)\nand nitrogen dioxide (NO<sub>2</sub>), play a crucial role in the\nglobal nitrogen cycle, but the oceanic occurrence remained unclear.\nHere, we show an integrated underway observation of oceanic and atmospheric\nNO and NO<sub>2</sub> from the coastal seas to the open ocean in the\nnorthwestern Pacific Ocean (NWPO). The concentrations of NO and NO<sub>2</sub> showed similar distribution patterns that the coastal seas\nwith rich nitrogen nutrients showed higher levels, like the Yellow\nSea (7.6 and 19.9 pmol L<sup>–1</sup>) and the East China Sea\n(11.8 and 23.3 pmol L<sup>–1</sup>), while the oligotrophic\nsea and open ocean showed decreased levels, like the South China Sea\n(2.4 pmol L<sup>–1</sup> and below the detection limit, LOD)\nand the NWPO (3.5 pmol L<sup>–1</sup> and below the LOD). Apart\nfrom nitrogen nutrients, sea surface temperature might be another\nimportant influencing factor on the NO and NO<sub>2</sub> distribution.\nIn the surface water (<10 m), photoproduction was a major NO source\ncompared to the microbial process, while in the mixed layer, the microbial\nprocess played a more critical role. The saturation values showed\nthat the sea was a net source of atmospheric NO but was a net sink\nof NO<sub>2</sub>.