Distribution and sea-to-air exchange of carbon monoxide in surface microlayer and subsurface seawater in the eastern marginal seas of China

Abstract. Sea-surface microlayer (SML) is the boundary interface between the atmosphere and ocean, exhibiting an enrichment of dissolved organic matter (DOM) and participating in sea-to-air gas exchange. However, how DOM enrichment in the SML controls the flux of several gases in sea-to-air exchange remains poorly understood. In our study, incubation experiments and in-situ investigation in the eastern marginal seas of China were conducted to determine the distribution of carbon monoxide (CO) and its production and consumption rates in the SML during winter. Chromophoric dissolved organic matter (CDOM) (92 %) and fluorescent dissolved organic matter (FDOM) (92 %) were frequently enriched in the SML during winter, and the enrichment of CO was lower than the enrichments of CDOM and FDOM. CO ranged from 0.48 to 2.81 nmol L−1 (1.22±0.85 nmol L−1) and 0.50 to 3.61 nmol L−1 (1.54±1.61 nmol L−1) in the subsurface layer (SSW) and the SML, respectively, with higher concentration observed in the SML due to CDOM enrichment. Enrichment in the SML was expressed as enrichment factors (EFs) defined as the ratio of values in the SML to those in the SSW. Although CO, CDOM and FDOM concentrations decreased from in-shore regions to open oceans, higher enrichment factors (EFs > 2) of CO and CDOM and FDOM in the SML were generally observed in the off-shore areas. Considering the photoproduction rate (mean value: 12.41 nmol L−1 d−1) of CO in the SML was significantly higher than that in the SSW (10.32 nmol L−1 d−1), the enrichment and concentration of CO in the SML showed diurnal variation, with higher values observed in the early afternoon. The flux of CO exhibited a significantly negative correlation with the CDOM absorption coefficient at 254 nm and marine-humic-like FDOM in the SML, suggesting that elevated DOM could stimulate the photoproduction of CO, but may also decrease sea-to-air CO exchange in the SML. Given the importance of the organic-rich SML as a diffusion layer in the air–sea exchange of climate-relevant gases and heat, understanding the layer's enrichment processes is crucial.