Distribution, emission, and cycling processes of carbon monoxide in the tropical open ocean

The carbon monoxide (CO) cycle in the marine mixed layer determines its emissions to the atmosphere and subsequently affects atmospheric chemistry and climate change. However, the contributions of oceanic CO transformation pathways and their impacting factors remain inadequately understood. Therefore, we investigated the distribution and cycle processes of CO in the Eastern Indian Ocean (EIO) and developed a CO budget model for the mixed layer. Surface seawater CO concentrations presented a diurnal variation due to periodic variations in solar radiation and rapid microbial consumption. The spatial distribution of CO in seawater was dominated by chromophoric dissolved organic matter (CDOM) and solar radiation. The EIO was a source of atmospheric CO and its daily CO emissions produced increases in the CO mixing ratio and hydroxyl radical consumption rate in the overlying atmosphere by 74.03 pptv and 6.48 pptv d⁻¹, respectively. Additionally, the budget model findings indicated that photoproduction (CDOM plus particulate organic matter), dark production, and phytoplankton emission accounted for about 67 %, 30 %, and 3 % of total CO production. The microbial consumption (94 %) and sea-air exchange (6 %) were the primary and secondary sink for CO within the mixed layer, respectively. Moreover, the photo-mineralization of dissolved organic carbon was estimated using CO as a proxy for CO₂ photoproduction. This study deepens our understanding of the oceanic CO cycle and the impact of photo-mineralization on the carbon cycle and is vital for refining global oceanic CO source-sink budgets and modelling studies.