Final Report Summary - PASSME (Air-sea gas exchange - PArameterization of the Sea-Surface Microlayer Effect)
The Earth’s ocean absorb about 11 billion tonnes of carbon dioxide (CO2) each year, about 25% of all anthropogenic CO2. The ocean is a huge reservoirs of CO2 potentially controlling the pace of climate change. A better understanding on how the ocean absorbs CO2 is critical for predicting climate change. Any absorption process between a liquid and gas phase is controlled by the boundary layer between the two phases. Therefore, the sea-surface microlayer (SML), the interfacial aqueous boundary layer between the ocean and atmosphere (typically 40-100 µm in thickness), plays an important role in the process of air-sea gas exchange. In this context, PassMe has focused on the role of the sea-surface microlayer (SML), the uppermost thin boundary layer between the ocean and atmosphere, in the air-sea CO2 exchange.
The CO2 transfer velocity, a key parameter for air-sea CO2 fluxes, has been estimated by parameterizations solely based on the wind speed forcing near surface turbulence. The PassMe team found that the zero intercept forced in those classical parameterizations is not real, and ignoring a nonzero intercept may bias the oceanic global CO2 uptake by 0.73 Gt C yr−1. The PassMe team found an abrupt reduction of air-sea CO2 fluxes by 23% in the presence of natural surfactants in the SML exceeding a concentration of 200 µg/L. The surfactants create a laminar layer with increasing thicknesses as verified for oxygen outgassing in laboratory studies using micro electrodes. Very low surfactant concentrations were found in the western Pacific and the PassMe team estimated an error of 20% for this regions if typical wind-based parameterizations are applied not developed within a low surfactants regime.
The air-sea CO2 transfer is also driven by biogeochemical processes in the SML, which is known to be an unique ecosystem. The PassMe team conducted microbiological studies confirming that the SML is mainly a heterotrophic system, but the microbial activities in the SML were insufficient to control oxygen micro gradients across the boundary layer. We found that plankton, but not neuston metabolic activity was the main driver of oxygen exchange across the SML, although neuston activity may exceed plankton activity by one order of magnitude. Furthermore, within PassMe a new analytical technique was developed to prove a hypothesis from 1969, that an enrichment of the enzyme carbonic anahydrase in the SML enhances the oceanic CO2 uptake. The enzyme catalyzed the slow interconversion between HCO3- and CO2 and, if enriched in the SML, may facilitate the exchange of CO2the interconversion able. We found nanomolar levels of eCA in the SML, and these levels are capable to enhance air-sea CO2 fluxes by 15%.
Overall, using the extensive PassMe database, we were able to improve the prediction of the CO2 transfer velocity k with a deviation of not more than ± 1 cm*h-1. It will allow prediction of the oceanic CO2 uptake with a higher certainty, and, therefore, how much of the anthropogenic CO2 emissions remain in the atmosphere forcing climate change.
The CO2 transfer velocity, a key parameter for air-sea CO2 fluxes, has been estimated by parameterizations solely based on the wind speed forcing near surface turbulence. The PassMe team found that the zero intercept forced in those classical parameterizations is not real, and ignoring a nonzero intercept may bias the oceanic global CO2 uptake by 0.73 Gt C yr−1. The PassMe team found an abrupt reduction of air-sea CO2 fluxes by 23% in the presence of natural surfactants in the SML exceeding a concentration of 200 µg/L. The surfactants create a laminar layer with increasing thicknesses as verified for oxygen outgassing in laboratory studies using micro electrodes. Very low surfactant concentrations were found in the western Pacific and the PassMe team estimated an error of 20% for this regions if typical wind-based parameterizations are applied not developed within a low surfactants regime.
The air-sea CO2 transfer is also driven by biogeochemical processes in the SML, which is known to be an unique ecosystem. The PassMe team conducted microbiological studies confirming that the SML is mainly a heterotrophic system, but the microbial activities in the SML were insufficient to control oxygen micro gradients across the boundary layer. We found that plankton, but not neuston metabolic activity was the main driver of oxygen exchange across the SML, although neuston activity may exceed plankton activity by one order of magnitude. Furthermore, within PassMe a new analytical technique was developed to prove a hypothesis from 1969, that an enrichment of the enzyme carbonic anahydrase in the SML enhances the oceanic CO2 uptake. The enzyme catalyzed the slow interconversion between HCO3- and CO2 and, if enriched in the SML, may facilitate the exchange of CO2the interconversion able. We found nanomolar levels of eCA in the SML, and these levels are capable to enhance air-sea CO2 fluxes by 15%.
Overall, using the extensive PassMe database, we were able to improve the prediction of the CO2 transfer velocity k with a deviation of not more than ± 1 cm*h-1. It will allow prediction of the oceanic CO2 uptake with a higher certainty, and, therefore, how much of the anthropogenic CO2 emissions remain in the atmosphere forcing climate change.