Carbon Monoxide in the Marine Atmosphere and Seawater: Spatiotemporal Distribution and Photobiogeochemical Cycling.

Carbon monoxide (CO) plays a vital role in the biochemical processes of marine microorganisms, climate regulation, and global carbon cycle. In this study, seasonal and spatial variations of CO and oceanographic factors influencing these variations were investigated in the East China Sea (ECS) during two cruises from 16 to 30 May 2020 and from 8 to 30 October 2021. ECS was as a net source of atmospheric CO in both spring and autumn. The average values of seawater CO concentrations ([CO]surf), atmospheric CO mixing ratio ([CO]atm) and sea‐to‐air flux of CO were 2.22 ± 0.47 nmol L−1 and 0.97 ± 0.86 nmol L−1, 182.8 ± 71.8 ppbv and 102.7 ± 38.8 ppbv, 96.14 ± 25.82 nmol m−2 hr−1 and 31.96 ± 35.61 nmol m−2 hr−1 in spring and autumn, respectively. A steady‐state model was used to estimate the biogeochemical cycling rates of [CO]atm over the ECS. The effect of CO sea‐to‐air fluxes on atmospheric ·OH concentration was likely decreased from spring to autumn. Photoirradiation experiments further showed that the actual contributions of ultraviolet radiation and photosynthetically active radiation to CO photoproduction in the surface waters were 72.2% ± 13.8% and 27.8% ± 13.8%, respectively. The sea‐to‐air fluxes of CO in autumn were 5.15 μmol m−2 day−1 lower than in spring, which were due to larger bacterial consumption and lower [CO]surf in autumn in the ECS. Further, this study is expected to improve our understanding of the biogeochemical processes of CO in the eastern marginal seas of China. Plain Language Summary: Over the past decade, there has been a significant increase in global carbon emissions. CO is a potent greenhouse gas, and chromophoric dissolved organic matter (CDOM) photochemical processes control CO emission from the ocean to the atmosphere. Coastal waters dominate global oceanic carbon emissions, but the dynamics, controls and roles of CO remain largely unconstrained in the marginal seas around China. Here, we conducted a variety of biogeochemical analyses to investigate the controls of CO dynamics cycling in the East China Sea during spring and autumn. [CO]surf, [CO]atm, and sea‐to‐air flux of CO showed significant seasonal variability between spring and autumn, with higher values observed in spring. A variety of environmental factors such as CDOM concentration, wind speed and direction, physical mixing, air‐sea exchange, bacterial consumption, and dark production could affect CO distribution. Compared to air‐sea exchange, bacterial consumption represented the main CO‐sink and contributed over 90% of the total CO loss. These results provide a supplement to the global database of CO cycling processes and improve our current understanding of CO seasonal distribution in coastal ecosystems. Key Points: [CO]surf, [CO]atm, and sea‐to‐air flux of CO in spring were significantly higher than in autumn in the East China SeaThe effect of CO sea‐to‐air fluxes on ·OH was decreased from spring to autumnDark production accounted for about 23% of the total CO production in autumn [ABSTRACT FROM AUTHOR]