Abstract. Carbon dioxide (CO₂) emissions from running waters represent a key component of the global carbon cycle. However, quantifying CO₂ fluxes across air-water boundaries remains challenging due to practical difficulties in the estimation of reach-scale standardized gas exchange velocities (k₆₀₀) and water equilibrium concentrations. Whereas craft-made floating chambers supplied by internal CO₂ sensors represent a promising technique to estimate CO₂ fluxes from rivers, the existing literature lacks of rigorous comparisons among differently designed chambers and deployment techniques. Moreover, as of now the uncertainty of k₆₀₀ estimates from chamber data has not been evaluated. Here, these issues were addressed analyzing the results of a flume experiment carried out in the Summer of 2019 in the Lunzer:::Rinnen – Experimental Facility (Austria). During the experiment, 100 runs were performed using two different chamber designs (namely, a Standard Chamber and a Flexible Foil chamber with an external floating system and a flexible sealing) and two different deployment modes (drifting and anchored). The runs were performed using various combinations of discharge and channel slope, leading to variable turbulent kinetic energy dissipation rates (2 x 10⁻³< ε < 8 x 10⁻² m²/s³). Estimates of gas exchange velocities were in line with the existing literature (4 k₆₀₀ k₆₀₀ for larger turbulent kinetic energy dissipation rates. The Flexible Foil chamber gave consistent k₆₀₀ patterns in response to changes in the slope and/or the flow rate. Moreover, Acoustic Doppler Velocimeter measurements indicated a limited increase of the turbulence induced by the Flexible Foil chamber on the flow field (26 % increase in ε, leading to a theoretical 6 % increase in k₆₀₀). The uncertainty in the estimate of gas exchange velocities was then estimated using a Generalized Likelihood Uncertainty Estimation (GLUE) procedure. Overall, uncertainty in k₆₀₀ was moderate to high, with enhanced uncertainty in high-energy setups. For the anchored mode, the standard deviations of k₆₀₀ were between 1.6 and 8.2 m/d, whereas significantly higher values were obtained in drifting mode. Interestingly, for the Standard Chamber the uncertainty was larger (+ 20 %) as compared to the Flexible Foil chamber. Our study suggests that a Flexible Foil design and the anchored deployment might be useful techniques to enhance the robustness and the accuracy of CO₂ measurements in low-order streams. Furthermore, the study demonstrates the value of analytical and numerical tools in the identification of accurate estimations for gas exchange velocities. These findings have important implications for improving estimates of greenhouse gas emissions and reaeration rates in running waters.

中文翻译：

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在受控的水槽实验中评估通过漂移和锚定通量室产生的流CO ₂脱气

摘要。自来水的二氧化碳（CO ₂）排放量是全球碳循环的重要组成部分。然而，由于在估算达到范围的标准化气体交换速度（k ₆₀₀）和水平衡浓度方面存在实际困难，量化跨空气-水边界的CO ₂通量仍然具有挑战性。尽管由内部CO ₂传感器提供的手工制作的浮舱代表着一种有前途的技术来估算河流中的CO ₂通量，但现有文献缺乏在不同设计的浮舱和部署技术之间进行严格比较的方法。而且，到目前为止，k ₆₀₀的不确定性来自腔室数据的估计值尚未评估。通过分析2019年夏季在Lunzer :::: Rinnen –实验设施（奥地利）进行的水槽实验的结果，解决了这些问题。在实验过程中，使用两种不同的腔室设计（即具有外部浮动系统和柔性密封的标准腔室和柔性箔腔室）以及两种不同的部署模式（漂移和锚定）进行了100次运行。使用排放和通道斜率的各种组合进行运行，从而导致可变的湍动能耗散率（2 x 10 ^-3 <ε<8 x 10 ^-2  m ² / s ³）。气体交换速度的估计值与现有文献一致（4 k₆₀₀  k ₆₀₀用于更大的湍动能耗散率。响应于斜率和/或流量的变化，柔性箔腔给出了一致的k ₆₀₀模式。此外，声学多普勒测速仪的测量结果表明，挠性箔腔在流场上引起的湍流增加有限（ε增加26％，理论上k ₆₀₀增加6％）。然后使用广义似然不确定度估算（GLUE）程序估算气体交换速度估算中的不确定性。总体而言，不确定度为k ₆₀₀从中等到高，高能量设置的不确定性增加。对于锚定模式，k ₆₀₀的标准偏差在1.6和8.2 m / d之间，而在漂移模式下获得的值明显更高。有趣的是，与柔性箔室相比，标准室的不确定度更大（+ 20％）。我们的研究表明，灵活箔设计和锚定部署可能是有用的技术，可增强CO ₂的鲁棒性和准确性低阶流中的测量。此外，该研究证明了分析和数值工具在识别气体交换速度的准确估算中的价值。这些发现对改善自来水的温室气体排放量和净化率具有重要意义。