Abstract
For decades, the effect of surfactants in the sea-surface microlayer (SML) on gas transfer velocity (k) has been recognized; however, it has not been quantified under natural conditions due to missing coherent data on in situ k of carbon dioxide (CO2) and characterization of the SML. Moreover, a sea-surface phenomenon of wave-dampening, known as slicks, has been observed frequently in the ocean and potentially reduces the transfer of climate-relevant gases between the ocean and atmosphere. Therefore, this study aims to quantify the effect of natural surfactant and slicks on the in situ k of CO2. A catamaran, Sea Surface Scanner (S3), was deployed to sample the SML and corresponding underlying water, and a drifting buoy with a floating chamber was deployed to measure the in situ k of CO2. We found a significant 23% reduction of k above surfactant concentrations of 200 µg Teq l−1, which were common in the SML except for the Western Pacific. We conclude that an error of approximately 20% in CO2 fluxes for the Western Pacific is induced by applying wind-based parametrization not developed in low surfactant regimes. Furthermore, we observed an additional 62% reduction in natural slicks, reducing global CO2 fluxes by 19% considering known frequency of slick coverage. From our observation, we identified surfactant concentrations with two different end-members which lead to an error in global CO2 flux estimation if ignored.
Highlights
Around half the carbon dioxide (CO2) produced by humans since the Industrial Revolution has been dissolved into the ocean [1]
We provide the first in situ assessment of k reduction by natural surfactant to investigate the effect of natural surfactant in the surface microlayer (SML) and slicks at various geographical locations and wind regimes on the k of CO2
We suggest that the high k660 in the our study, exclusively observed in the Western Pacific, is due to the low resistance of air–sea CO2 transfer by the lowest concentration of surfactants observed on a global scale [2,41]
Summary
Around half the carbon dioxide (CO2) produced by humans since the Industrial Revolution has been dissolved into the ocean [1]. Despite the importance of k parametrization in the estimation of the global uptake of climaterelevant gases by the ocean, no data exist, to the best of our knowledge, on in situ measurements of air–sea gas exchanges and natural surfactants in the SML. The lack of in situ data leads to uncertainties in k parametrizations, and on the estimate of the oceanic CO2 uptake [22] For this reason, we provide the first in situ assessment of k reduction by natural surfactant to investigate the effect of natural surfactant in the SML and slicks at various geographical locations and wind regimes on the k of CO2. Our study leads to an understanding of how surfactants at the sea surface affect gas exchange processes under natural conditions
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