森林と大気のガス交換を「渦」で捉える～半世紀前に考案された理論を実用化～

2025-08-26 京都大学

図１ 滋賀県南部の森林観測タワー（高さ 29m、桐生水文試験地）に設置された真の渦集積法（TEA 法）に基づく大気サンプリングシステム。超音波風速計が捉える空気の上下の動き（鉛直風速）に応じて、上昇流と下降流を分離・採取する。

＜関連情報＞

• https://www.kyoto-u.ac.jp/ja/research-news/2025-08-26-3
• https://www.kyoto-u.ac.jp/sites/default/files/2025-08/web_2508_Sakabe-fb856086a434a20da4b600258965ea3c.pdf
• https://agupubs.onlinelibrary.wiley.com/doi/10.1029/2025JD044412

微量ガスフラックス測定のための真の渦積算システム：森林上空における実用化と評価 A True Eddy Accumulation System for Measuring Trace Gas Flux: Practical Implementation and Evaluation Above a Forest

Takuya Saito, Ayaka Sakabe, Satoru Takanashi, Yoshiko Kosugi
Journal of Geophysical Research: Atmospheres  Published: 31 July 2025
DOI:https://doi.org/10.1029/2025JD044412

Abstract

The true eddy accumulation (TEA) technique for turbulent flux measurements is an alternative to the conventional eddy covariance (EC) technique for measuring trace gases that are difficult to measure by the EC technique. However, the TEA technique has rarely been applied to studies of air–surface gas exchange because of the limited availability of reliable instrumentation. In this study, we developed an air sampling system based on the TEA technique using fast-response mass flow controllers. The TEA system collects updrafts and downdrafts at sampling flow rates proportional to the vertical wind speed, unless the wind speed exceeds a certain threshold. We evaluated the TEA system above a temperate forest in Japan from May to December 2022 by comparing CO₂ fluxes measured by the TEA system with those measured using EC. The comparison showed a moderate correlation (R = 0.86) with a small but significant negative bias in the TEA system (2%), likely because of the wind threshold. Our results suggest that the TEA system is a promising alternative for measuring turbulent fluxes of trace gases.

Key Points

• An air sampling system for turbulent trace gas flux measurements was developed based on the true eddy accumulation (TEA) technique
• Field evaluation of TEA was conducted above a temperate forest by comparing CO₂ fluxes by the conventional eddy covariance technique
• The comparisons showed good agreement, indicating that the TEA system is a promising alternative for measuring turbulent trace gas fluxes

Plain Language Summary

Trace gases released from Earth’s surface into the atmosphere have significant impacts on air quality and climate. Exchanges of trace gases between the Earth’s surface and atmosphere can be most directly measured using the eddy covariance (EC) technique. The true eddy accumulation (TEA) technique offers an alternative approach for measuring fluxes of trace gases, particularly when EC equipment is unavailable because of the slow response of the gas analyzer. We developed a new instrument that uses the TEA technique to measure trace gas fluxes. We tested our instrument by measuring CO₂ fluxes in a temperate forest in Japan. We compared the measured CO₂ fluxes between the TEA technique and the EC technique and found good agreement although TEA fluxes tended to slightly underestimate EC fluxes.