DEVELOPMENT OF OPEN PATH METHANE SENSOR

甲烷是大气中一种重要的微量气体，它对向外发射的长波辐射具有很强的吸收能力。它的吸收潜力大约是二氧化碳的20倍。众所周知，大气中的甲烷浓度正在增加。甲烷浓度增加的主要来源被认为是与水稻种植和畜牧业有关的人类活动。涡旋相关技术是测量大气中甲烷通量最有前途的方法。然而，该技术需要快速响应和高分辨率的甲烷测量仪器。本文研制了一种具有开放传感路径的快速响应CH4仪器样机。实验室和现场测试的结果总结如下：(1)仪器的灵敏度约为15.6 ppm /伏特。测量频率为30hz。(2)对CH4浓度的噪声限制灵敏度约为0.6 ppm。(3)随着光学系统的调整可能发生变化，灵敏度实际上是不变的。(4)初步试验表明，该仪器可用于野外条件下涡流相关技术测量CH4通量。然而，目前的文书有若干限制。最严重的问题是氦氖激光器振荡不足。为了降低仪器的噪声水平，需要进一步控制氦氖激光器的振荡。

Methane is an important trace gas in the atmosphere because its high potential to absorb the outgoing longwave radiation. The absorbing potential is about 20 times that of carbon dioxide. It is well known that the methane concentration in the atmosphere is increasing. The major source of this increasing methane concentration has been assumed to be human activities associated with rice cultivation and animal husbandry. The eddy correlation technique is the most promising method to measure methane flux in the atmosphere. However, this technique requires a fast response and high resolution instrument for methane measurement.A prototype fast response CH4 instrument with open sensing path is developed in the present study. Results of the laboratory and fild test are summarized as follows : (1) The sensitivity of the instrument is about 15.6 ppm per volt. The measuring frequency is 30 Hz. (2) The noise-limited sensitivity is about 0.6 ppm peak-to-peak in CH4 concentration. (3) The sensitivity is practically invariant with a possible change in the adjustment of the optical system. (4) Preliminary test shows that the instrument is feasible for measurements of CH4 flux by the eddy correlation technique under field conditions. However, the present instrument suffers from several limitations. Most serious problem is the inadequate oscillation of He-Ne laser. In order to reduce the noise level of the instrument, further effort is required to control the oscillation of He-Ne laser.