Nitric oxide (NO) in the Bohai and Yellow Seas

Abstract. Nitric oxide (NO) is a short-lived compound of the marine nitrogen cycle; however, our knowledge about its oceanic distribution and pathways is rudimentary. Here we present the measurements of dissolved NO in the surface and bottom layers at 75 stations in the Bohai Sea (BS) and Yellow Sea (YS) in June 2011. Moreover, NO photoproduction rates were determined at 27 stations in both seas. The NO concentrations in the surface and bottom layers were highly variable and ranged from below the detection limit (i.e. 32 pmol L −1 ) to 616 pmol L −1 in the surface layer and to 482 pmol L −1 in the bottom layer. There was no significant difference between the mean NO concentrations in the surface (186 ± 108 pmol L −1 ) and bottom (174 ± 123 pmol L −1 ) layers. A decreasing trend of NO bottom layer concentrations salinity indicates a NO input by submarine groundwater discharge. NO in the surface layer was supersaturated at all stations during both day and night and therefore the BS and YS were a persistent source of NO to the atmosphere at the time of our measurements. The accumulation of NO during daytime was resulting from photochemical production and photoprodcution rates were correlated to illuminance. The persistent nighttime NO supersaturation pointed to a, so far unknown, NO dark production. NO sea-to-air flux densities were much lower than the NO photoproduction rates. Therefore, we conclude that the bulk of the NO produced in the mixed layer was rapidly consumed before its release to the atmosphere. Overall, the oceanic NO emissions to the atmosphere were negligible compared to anthropogenic NOx sources such as emissions from ships.