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

Nitrogen oxides (NO_x), comprised of nitric oxide (NO) and nitrogen dioxide (NO₂), 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 NO₂ from the coastal seas to the open ocean in the northwestern Pacific Ocean (NWPO). The concentrations of NO and NO₂ 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^-1) and the East China Sea (11.8 and 23.3 pmol L^-1), while the oligotrophic sea and open ocean showed decreased levels, like the South China Sea (2.4 pmol L^-1 and below the detection limit, LOD) and the NWPO (3.5 pmol L^-1 and below the LOD). Apart from nitrogen nutrients, sea surface temperature might be another important influencing factor on the NO and NO₂ 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 NO₂.