The Ocean Serves as a Net Source of Atmospheric Nitric Oxide but a Net Sink of Nitrogen Dioxide.

Nitrogen oxides (NO), comprised of nitric oxide (NO) and nitrogen dioxide (NO), play a crucial role in the global nitrogen cycle, but the oceanic occurrence remained unclear. Here, we show an integrated underway observation of oceanic and atmospheric NO and NO from the coastal seas to the open ocean in the northwestern Pacific Ocean (NWPO). The concentrations of NO and NO showed similar distribution patterns that the coastal seas with rich nitrogen nutrients showed higher levels, like the Yellow Sea (7.6 and 19.9 pmol L) and the East China Sea (11.8 and 23.3 pmol L), while the oligotrophic sea and open ocean showed decreased levels, like the South China Sea (2.4 pmol L and below the detection limit, LOD) and the NWPO (3.5 pmol L and below the LOD). Apart from nitrogen nutrients, sea surface temperature might be another important influencing factor on the NO and NO distribution. In the surface water (<10 m), photoproduction was a major NO source compared to the microbial process, while in the mixed layer, the microbial process played a more critical role. The saturation values showed that the sea was a net source of atmospheric NO but was a net sink of NO.