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CH4 flux measurement by Fast Methane Analyzer with Eddy Covariance System
|關鍵字:||fast methane analyzer;快速甲烷分析儀;eddy covariance system;紊流協變性系統||出版社:||環境工程學系所||摘要:||
目前國際間以渦流協變性系統(ECs 系統)相關方法進行通量觀測，過去對於甲烷通量研究大部分是利用Chamber法進行甲烷濃度的量測，並進而求得甲烷通量。本研究目的在於利用Cavity-enhanced laser absorbtion技術所設計的快速甲烷分析儀搭派渦流協變性系統直接量測水稻田內甲烷之通量變化方法之建立，由於此項儀器可以連續性高頻率分析採樣氣體，對於以往方法無法進一步了解短時間甲烷通量的變化的情況，有直接的改變，並可搭配土壤及氣象資料，即時的了解到甲烷通量與大氣跟植物間收支動態變化行為。
在氣象觀測結果顯示，平均垂直風速、與摩擦風速其範圍約在-0.3 ~ 0.1 m s-1及0.03~0.4 m s-1，而在大氣溫度則為23~31 ℃。另外在土壤溫度則有明顯的日夜變化，其範圍約在20~30 ℃左右；而在土壤含水率部分，由於水稻田持續的在灌溉，所觀測到的土壤含水率也都在40%以上。對於甲烷濃度，從觀測資料可看出在實驗期間甲烷濃度有明顯的日夜變化，其甲烷濃度值在中午會達最低，約為1.9 ppm；而在夜晚接近清晨則會產生最大值2.08 ppm，另外，也將觀測到的甲烷濃度與附近環保署大里測站作一比較，發現日夜變化之趨勢相近。在甲烷通量部分，甲烷通量訊號並不明顯，唯一在小時平均值上，可看出甲烷通量稍微有日夜變化，其範圍約在-0.13~0.11 μg m-2 s-1間。在此次實驗中， 造成甲烷通量訊號不明顯的原因推測可能為：初始延遲時間設定出現誤差，以及儀器經清理過後，無法維持良好的精確度使得產生過大的雜訊讓甲烷通量不明顯。
At present, the flux measurement in the world is often measured using the Eddy Covariance System. In the past, Chamber method was often used to measure the methane concentration and flux, however it had several limits, such as of small spatial distribution. This study uses the fast methane analyzer based on Cavity-enhanced laser absorption with the Eddy covariance System to directly measure the methane flux at the rice paddy. The fast methane analyzer can continually measure with a high frequency. We also use soil data (such as soil moisture, soil heat flux, and soil temperature) and meteorological data to realize the methane flux between the atmosphere and the surface on a specific time. The study site is located at Wung-Fong Agricultural Research Institute where the interference caused by buildings which affect the wind direction to Eddy Covariance System can be reduced to the minimum. The measurement was made from 28th April to 5th June in 2009 through automated measurement and data logging system. The measurement in the rice paddy started in the growing period and ended in harvest period. The soil of the rice paddy was covered by water during measured period, and this condition provides a good anaerobic environment for methane production. The vertical wind velocity and the friction velocity are -0.3 - 0.1 m s-1 and 0.03-0.4 m s-1, respectively. The air temperature is 23 -31 ℃, and the diurnal variation on soil temperature is distributed in 20-30℃. The soil water content is more than 40% because the rice paddy was flooded all the time when the measurement was in proceeding. For the methane concentration, there was an obvious diurnal variation during the measurement period. From the composite diurnal methane concentration, the minimum concentration occurred around the noon and the mean concentration was about 1.9 ppm during daytime. The maximum concentration is about 2.08 ppm at dawn. The observed methane concentrations at Wung-fong station and Dali station were compared and the result showed similar diurnal variation. Although there was no obvious signal on the day-after-day methane flux measurement, the diurnal variation of methane flux was -0.13~0.11 μg m-2 s-1. A reason which leads to unobvious methane flux signal may be caused by the inadequate setting of the delay-time. Another reason may be caused by the suspended particle which dirtied the instrument and produced stronger noise to influence the precision.
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