| 摘要: | 本研究將高頻之三維超音波風速計、水氣與二氧化碳濃度資料，以渦流相關法(EC)與權重渦流累積法(REA)兩種通量觀測技術進行計算評估。目前公認最能準確估計通量的技術為渦流相關法，但由於所需儀器成本相對上較為昂貴，而權重渦流累積法雖然不需要高頻之氣體分析儀，但需要給定若干控制參數方可進行通量估算，且目前尚無商品化之完整觀測套件可供使用，但近年來在量溫室氣體通量的監測上已有越來越多之應用。本研究將EC所計算出的通量作為比較基準，探討REA中的滯區(deadband)與經驗係數(b)之變化特徵。
以台灣中部蓮華池試驗集水區2012年觀測的風速、溫度、水氣密度與二氧化碳濃度資料建立REA估算通量所需參數，再利用2013年的資料驗證這些參數在REA方法計算通量之適用性。在滯區的設定部分，採用兩個時間周期前之垂直風速標準差(σ_w)以及平均日變化趨勢σ_w與定值三種方法，根據2012年的分析發現當滯區為0.6~0.8倍垂直風速標準差時，b值最為穩定。接著依照實際所得滯區與垂直風速標準差之倍率，探討在不同紊流發展程度下b值大小或使用可感熱b值作為水氣與二氧化碳通量估算的b值之差異。並針對權重渦流累積法中的濃度差項進行資料挑選(水氣濃度差範圍介於-20至100(m-mole/m3)，二氧化碳濃度差範圍介於-0.5至0.3(m-mole/m3)後可以得到良好的結果，最後將REA與EC所求得通量進行比較，在熱通量、水氣通量與二氧化碳通量的相關係數分別可達為0.98、0.88與0.90，本研究對於滯區與b值得分析可提供未來實際REA元件施測時之參考。
;This study uses high resolution time series (10Hz) of wind components (u, v, w), sonic temperature (Ts), carbon dioxide (CO2), and vapor(H2O) density data to compute surface fluxes with application of Eddy Covariance method (EC) and Relaxed Eddy Accumulation method (REA). Eddy Covariance method is the most accurate method for measuring surface fluxes at the atmospheric boundary layer. However, the cost of fast response equipment required for EC measurement is very expensive for some trace gases. Recently, REA measurement, without the need of fast response equipment, is widely applied to measure greenhouse gas flux. Nevertheless, this method requires setting parameters before estimating, and has not yet been commercialized. This study takes flux es estimated by the EC method to discuss characteristics of empirical constant (b) and deadband (wd) required in REA techniques.
Observation data in 2012 was used to help determine the adequate deadband and empirical constant setting of REA with referenced fluxes estimated by EC. Observation data in 2013 was used to verify obtained REA parameter setting. Standard deviation of b is smaller with the deadband set as 0.6 to 0.8 times standard deviation of vertical wind speed. For vapor and carbon dioxide fluxes, using b value of sensible heat and eliminating data with vapor flux differences ranging between -20 and 100 m-mole/m3 or with carbon dioxide flux differences ranging between 0.05 and -0.2 m-mole/m3 can obtain good result. Comparing the surface flux calculated by Eddy Covariance method and Relaxed Eddy Accumulation method, the R-squared values are 0.98, 0.88, and 0.90 in sensible heat, vapor, and carbon dioxide fluxes, respectively. This study provides analysis of deadband and b value of great benefit to actual assembly of REA system in the future. |
