Sniffle: a step forward to measure in situ CO2 fluxes with the floating chamber technique

Abstract

Understanding how the ocean absorbs anthropogenic CO2 is critical for predicting climate change. We designed 'Sniffle', a new autonomous drifting buoy with a floating chamber, to measure gas transfer velocities and air–sea CO2 fluxes with high spatiotemporal resolution. Currently, insufficient 'in situ' data exist to verify gas transfer parameterizations at low wind speeds (<4 m s−1), which leads to underestimation of gas transfer velocities and, therefore, of air–sea CO2 fluxes. The 'Sniffle' is equipped with a sensor to consecutively measure aqueous and atmospheric 'p'CO2 and to monitor increases or decreases of CO2 inside the chamber. During autonomous operation, a complete cycle lasts 40 minutes, with a new cycle initiated after flushing the chamber. The 'Sniffle' can be deployed for up to 15 hours at wind speeds up to 10 m s−1. Floating chambers often overestimate fluxes because they create additional turbulence at the water surface. We correct fluxes by measuring turbulence with two acoustic Doppler velocimeters, one positioned directly under the floating chamber and the other positioned sideways, to compare artificial disturbance caused by the chamber and natural turbulence. The first results of deployment in the North Sea during the summer of 2016 demonstrate that the new drifting buoy is a useful tool that can improve our understanding of gas transfer velocity with 'in situ' measurements. At low and moderate wind speeds and different conditions, the results obtained indicate that the observed tidal basin was acting as a source of atmospheric CO2. Wind speed and turbulence alone could not fully explain the variance in gas transfer velocity. We suggest that other factors like surfactants, rain or tidal current will have an impact on gas transfer parameterizations.

Keywords

• Air-water CO2 flux
• coastal area
• ocean technology
• gas transfer velocity
• carbon cycle
• Wadden Sea