Carbon Monoxide Cycle in the Bohai Sea and the Yellow Sea: Spatial Variability, Sea-Air Exchange, and Biological Consumption in Autumn

As an important trace gas, carbon monoxide (CO) is considered to be mainly released by the ocean into the atmosphere. To further understand the role of the marginal seas for marine CO emission, we investigated sea-to-air flux and spatial distribution as well as biological consumption of CO using the Yellow and Bohai Seas as research objects. In the surface water, CO concentrations varied largely between 0.109 and 6.26 nmol/L. Strong vertical and diurnal variabilities were also observed, which could be attributed to the sunlight-induced photochemical production of CO at different sampling time and depth. The atmospheric CO mixing ratios generally decreased from the coast to the open ocean, possibly due to the influence of anthropogenic activity. Relative to atmospheric CO, the concentrations of CO were mostly supersaturated in the surface water, suggesting that both the Bohai and the Yellow Seas were significant CO source in the atmosphere. Average sea-to-air flux of CO was about 1.76 μmol · m^-2 · day^-1, which largely depended on the saturation factor and wind speed. As an important removal pathway, microbial consumption of CO exhibited the first order kinetics. The rapid turnover of CO (3.2−9.6 hr) indicated that microbial metabolism was the major CO consumer in the surface waters that could significantly regulate the emission of CO to the atmosphere. Overall, this study improved our understanding of the biogeochemical CO cycling in marginal seas as well as in coastal regions.