Spatio-temporal distribution and production-consumption dynamics of nitric oxide in coastal waters off Qingdao, China.

Nitric oxide (NO) is an atmospheric pollutant and climate forcer, as well as an important intermediate in the marine nitrogen cycle. However, the distributions of NO in the ocean and its production-consumption mechanisms remain unclear. This study investigated the spatial distributions of surface NO in the Jiaozhou Bay and adjacent coastal waters off Qingdao based on two cruises conducted in autumn 2023 and spring 2024. Dissolved NO concentrations in the surface seawater ranged from 315.0 to 446.7 pmol L-1 (average: 355.5 ± 44.6 pmol L-1) and 117.4-299.8 pmol L-1 (average: 219.8 ± 63.5 pmol L-1) during spring and autumn, respectively. The NO2- enrichment caused by terrestrial inputs could amplify photochemical production potential. The photodegradation experiments using three size fractions <1 kDa, 1 kDa-0.2 μm, and <0.2 μm of surface seawater were conducted. The mean photodegradation rates inside the bay were 1.48 × 10-10, 0.67 × 10-10, and 0.61 × 10-10 mol L-1 h-1 for these size fractions, respectively, while the rates outside the bay were 0.31 × 10-10, 0.68 × 10-10, and 0.59 × 10-10 mol L-1 h-1. The degradation rate in the <1 kDa size fraction inside the bay was significantly higher than outside the bay. This difference was attributed to elevated dissolved inorganic nitrogen concentrations within the bay, which served as the primary precursor for NO photoproduction. In natural seawater, hydrogen peroxide (H2O2) and fulvic acid (FA) strongly suppressed NO photoproduction in a concentration-dependent manner. This inhibitory effect was attributed to the consumption of reactive oxygen species (ROS) generated by seawater photolysis, where H2O2-derived hydroxyl radicals and FA-derived peroxide radicals exhibited NO scavenging abilities. The competitive quenching by ROS played a critical role for the consumption of NO. The photolysis production rate of NO increased sharply with increasing temperature within 10-15 °C, while the rising slowed down significantly within 15-25 °C, suggesting that the photolysis process had significant temperature dependence and a shift in rate-limiting steps across thermal regimes. The sea-air exchange fluxes of NO were 3.4 × 10-16 and 6.4 × 10-16 mol m-2 s-1 inside and outside Jiaozhou Bay, respectively. The flux outside bay was higher than that inside bay, which was mainly due to the difference of the surface water temperature. This study clarifies the synergistic regulation of NO dynamics by temperature, ROS, and terrestrial inputs, providing a theoretical foundation for modeling nitrogen cycles and assessing reactive gas emission in eutrophic bays.