Technical note: Resolving Vertical CO₂ Concentration Gradients at the Air-Water Interface Using a Novel Membrane Equilibration Technique

Abstract. Quantifying diffusive greenhouse gas emissions from inland waters often relies on the concentration gradient between dissolved gases in the water and their equilibrium concentration in the atmosphere. However, while gas exchange occurs directly at the surface, dissolved CO₂ is mostly measured further below the surface, potentially introducing bias. Recent studies have shown the presence of vertical CO₂ gradients during calm conditions, which can lead to systematic errors in flux estimations when measurements are not taken at the correct depth. To address this issue, we developed a novel Floating Membrane Equilibrator (FME)—a thin (0.6 cm), flexible device equipped with gas-permeable silicone tubing arranged in a flat plane with a theoretical vertical resolution of 1 cm. By directing airflow through the tubes, for instance using an infrared gas analyzer, CO₂ in the water equilibrates with the gas phase inside the tubing. The resulting CO₂ concentration reflects the local dissolved CO₂ in the adjacent water layer. We tested the FME in a controlled pool experiment using CO₂-supersaturated water. Two FMEs were deployed at 1 cm and 25 cm depth. Results show that the FME provides reliable equilibration with the surrounding water and delivers accurate CO₂ measurements. Compared to conventional CO₂ probes, the FME shows faster response times and higher temporal resolution, enabling detection of short-term fluctuations that are typically missed by standard sensors. Moreover, our measurements revealed a distinct vertical CO₂ gradient in the pool, with higher concentrations at 25 cm depth compared to 1 cm, which is consistent with surface outgassing. This highlights the risk of misestimating fluxes when relying on deeper CO₂ measurements. The FME is a valuable tool for resolving near-surface CO₂ profiles with fine vertical resolution, thereby improving our understanding of the dynamics and drivers of lake-atmosphere gas exchange. Ultimately, the FME helps reduce uncertainty in CO₂ flux estimates and supports the development of more accurate models for greenhouse gas emissions from aquatic systems.