Strong low-pass filtering effects on water vapour flux measurements with closed-path eddy correlation systems
Strong low-pass filtering effects on water vapour flux measurements with closed-path eddy correlation systems
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DOI：
10.1016/j.agrformet.2007.07.007
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发表时间：
2007-12-10
影响因子：
6.2
通讯作者：
Pilegaard, Kim
中科院分区：
文献类型：
--
作者：
Ibrom, Andreas;Dellwik, Ebba;Pilegaard, Kim
使用闭路涡相关 (EC) 系统测量的湍流水蒸气通量会被系统无意中以随环境条件变化的方式进行低通滤波。这里描述了原因和方式。系统地纠正长期通量数据集的这种巨大测量误差的实用方法也是如此。与早期的研究相反，在分析中使用了大量的光谱和原始数据来定义EC系统的低通滤波特性。这表明闭路 EC 系统用于水蒸气浓度测量的截止频率随着相对湿度的增加呈指数下降。对这种意外过滤进行校正后，通量与使用开路传感器同时测量的 CO2 和 H2O 通量一致。丹麦西兰岛索罗山毛榉森林的水蒸气通量测量值的校正平均为测量通量的 42%，而使用同一 EC 系统测量的二氧化碳通量仅为 4%。我们建议使用所描述的方法来校正在任何闭路 EC 系统中测量的水蒸气通量，以消除意外的低通滤波效应。与 CO2 不同的是，水蒸气通量测量的校正幅度不能令人满意地高，即 EC 系统需要在技术上进行改进。我们的结果表明，通过将 EC 系统的整个气体传输系统中的相对湿度保持在 30% 以下，可以避免如此高的校正。通过加热进气过滤器和管道。 (c) 2007 Elsevier B.V. 保留所有权利。
Turbulent water vapour fluxes measured with closed-path eddy correlation (EC) systems are unintentionally low-pass filtered by the system in a manner that varies with environmental conditions. Why and how is described here. So is the practical method that systematically corrects long-term flux datasets for this substantial measurement error. In contrast to earlier studies, a large number of spectra and raw data have been used in the analysis to define the low-pass filtering characteristic of the EC system. This revealed that the cut-off frequency of the closed-path EC system for water vapour concentration measurements decreases exponentially with increasing relative humidity. After correction for this unintended filtering, the fluxes are consistent with CO2 and H2O fluxes that were measured with an open-path sensor at the same time. The correction of water vapour flux measurements over a Beech forest in Soro, Zealand, Denmark, amounted on average to 42% of the measured flux, while it was only 4% for the CO2 flux, which was measured with the same EC system. We recommend using the described method to correct water vapour fluxes measured in any closed-path EC system for unintended low-pass filtering effects. Other than for CO2 is the magnitude of the correction for water vapour flux measurements unsatisfactorily high, i.e. the EC system needs to be technically improved. Our results suggest that such high correction can be avoided by keeping relative humidity in the entire gas transport system of the EC system lower than 30%, e.g. by heating intake filters and tubes. (c) 2007 Elsevier B.V. All rights reserved.