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.