The use of (en)closed-path Infrared Gas Analysers (IRGA) in the measurement of Eddy Covariance (EC) fluxes results in inadvertent high-frequency tube attenuation due to diffusion and mixing of sampled gas inside the tube. The application of tube heating and installation of particulate filters along the tube length also contributes to high-frequency attenuation. The goal of this research is first, to quantify the attenuation effects of different tube heating and filter configurations on CO2 and H2O fluxes. And second, to present a modified power spectral approach (PSA) based on theoretical power spectra to calculate the effective cut-off frequency fc. Measurements for each experimental configuration were performed at an Integrated Carbon Observation System (ICOS) station equipped with the standard LI-7200 enclosed-path IRGA and Gill HS-50 3D sonic anemometer. Correction factors for each dataset were determined and implemented in post-processing. We found only very small attenuation effects of CO2 fluxes between the examined configurations. In agreement with previous studies, we found attenuation worsens with increasing relative humidity rH, in the fluxes of H2O. As expected, the highest (best) fc for H2O was found in the lowest examined rH class of 45-50 % with the configuration of heating on, no filter. The lowest (worst) fc for H2O was in the highest rH class of 90-95 % with the configuration of heating off with the 7 μm filter. Our results confirm that tube attenuation effects for the standard ICOS setup are negligible for CO2 and small for H2O, depending on tube heating settings and use of particulate filters. We also show that the post-processing of attenuation effects, especially for H2O, could improve the accuracy of long-term EC measurements. We recommend that this novel approach be considered by users of datasets collected with the LI-7200 enclosed-path IRGA.

中文翻译：

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LI-7200 IRGA 使用各种加热和滤波器配置进行涡流相关测量中的高频衰减 - 一种光谱校正方法


在测量涡流相关差 （EC） 通量时使用闭路红外气体分析仪 （IRGA） 会导致由于采样气体在管内的扩散和混合而意外地产生高频管衰减。管道加热的应用和沿管道长度安装颗粒过滤器也有助于高频衰减。本研究的目标首先是量化不同管加热和过滤器配置对 CO2 和 H2O 通量的衰减效应。其次，提出一种基于理论功率谱的改进功率谱方法 （PSA） 来计算有效截止频率 fc。每种实验配置的测量都在配备标准 LI-7200 封闭路径 IRGA 和 Gill HS-50 3D 声波风速计的集成碳观测系统 （ICOS） 站进行。确定每个数据集的校正因子并在后处理中实施。我们发现 CO2 通量在检查的配置之间只有非常小的衰减效应。与以前的研究一致，我们发现 H2O 通量中的衰减随着相对湿度 rH 的增加而恶化。正如预期的那样，H2O 的最高（最佳）fc 是在 45-50% 的最低 rH 等级中发现的，配置为加热开启，无过滤器。H2O 的最低（最差）fc 处于 90-95% 的最高 rH 等级，使用 7 μm 过滤器加热。我们的结果证实，标准 ICOS 设置的管道衰减效应对 CO2 可以忽略不计，而对 H2O 的衰减影响很小，具体取决于管道加热设置和微粒过滤器的使用。我们还表明，衰减效应的后处理，尤其是 H2O 的衰减效应，可以提高长期 EC 测量的准确性。 我们建议使用 LI-7200 封闭路径 IRGA 收集的数据集的用户考虑这种新方法。