Measurements of methane flux densities from paddy fields by eddy accumulation method

涡流积累法是理论上提出的一种直接测量源气体通量密度的微气象新方法，应用于稻田甲烷的生物圈/大气通量密度的测量。研制了一种满足该方法理论要求的新型自动气体取样器。垂直风速w由声速风速仪以5赫兹测量，并记录在数据记录仪上。流速通过在w为正时打开一个阀门到一个储层，在w为负时打开另一个阀门到另一个储层，来确定在恒定流量下的条件采样。每隔2小时采集30分钟气体样品，用气相色谱法测定平均浓度。利用这些数据，可以估算出甲烷通量密度f: f=bsigma_w（C_<up>-C_<down>）。为了检验涡流积累法测量结果的有效性，对比了涡流相关法、波文比法和涡流积累法估算的通量密度值。结果表明，两种方法吻合较好，表明涡流累积法的可靠性。采用涡流积累法和自动气体采样器，对泰国南部某稻田的甲烷通量密度进行了测量。通量密度的日变化过程在20 ~ 60mg /m^2/hr之间变化。此外，它们分别在下午和午夜之后出现最大值和最小值。

Eddy accumulation method, which had been proposed theoretically as a new micrometeorological method to directly measure the gas flux density from the sources, were applied to measure the biosphere/atmosphere flux density of methane from paddy fields.A new automatic gas sampler, which satisfies the theoretical requirements of this method, was developed. The vertical wind velocity w is measured with 5 Hz by the sonic anemometer and recorded on the data-logger. The velocity is used to determine the conditional sampling at a constant flow rate by opening one valve to a reservoir when w is positive and another valve to another reservoir when w is negative. The gas samples are collected for30 minutes in every 2 hours and average concentrations are measured by gas chromatography. Using these data, methane flux density f can be estimated as follows: f=bsigma_w(C_<up>-C_<down>)In order to check the validity of the measurement of the eddy accumulation method, the sensible he at flux densities estimated by the eddy correlation method, the Bowen ratio method, and the eddy accumulation method were compared. The result shows close agreement between two methods and suggests the reliability of the eddy accumulation method.Applying the eddy accumulation method with the automatic gas sampler, methane flux densities from a paddy field were measured in the south of Thailand. The daily courses of the flux densities changed between20 to 60 mg/m^2/hr. Also, they showed maxima and minima in the afternoon and after-midnight, respectively.