Research On Spectra Of Atmospheric Carbide Gases Molecules Based On Absor-ption Spectroscopy With High Sensitivity

With the progress of the world's industrialization, greenhouse gases emission increases tremendously, especially for the last a hundred years, years which has caused the greenhouse effect and has influenced the global climate. Ozone, nitrous oxide and Carbide gases?CO₂ and CH₄? are the most important parts of greenhouse gases, so the concentration of CO₂ is an important index in environmental monitoring. Moreover, CO and CO₂ are main products in the combustion of hydrocarbons. Therefore, it is of great importance to measure the carbide gases?such as CO and CO₂? concentration in the combustion diagnosis and industrial process control systems.In this dissertation, CO and CO₂ absorption spectra are studied based on tunable diode laser absorption spectroscopy?TDLAS?. In order to improve the detection sensitivity, we used a Herriott-type cell with base length of 0.95 m, in which the laser radiation could be reflected by 58 times and the effective absorption sample length could reach 55.1 m. In addition, in order to further reduce 1/f noise, we use the wavelength modulation spectroscopy?WMS? associated with Boxcar averaging technology and detect the P?4? line in the?3,0? band of CO by a distributed feed-back?DFB? diode laser at 1.578 ?m. There was an enhanced factor of ³8 for second harmonics signal WMS?WMS-2f? in comparison with direct absorption spectroscopy, which can achieve a detection limit?1?? of 4.32 ppm. Under the different pressure and different concentrations, they emerge no-linear correlations between the WMS-2f amplitudes and gas concentrations, which cause errors in the actual measurement. Therefore, we propose a forward pressure free calibration of WMS-2f based on first order Taylor expansions of the Lambert-Beer of validity to different pressure?less than 80 Torr?.In the paper, we reported on a simultaneous measurement of the absorption P?3? line in 3? band of CO and the absorption P?12e? CO₂ line of the 3?1 band with the detection sensitivity?1?? of 47 ppm and 29 ppm, respectively. With 2.0 % CO/N₂ and 2.0 % CO₂ /N₂ under different pressures, we observed the nonlinear relationships between the WMS-2f amplitudes and the concentrations with gas pressure. We find there is an error when the experimental results are fitted to the WMS-2f approximate solutions of the transmission expanded by a first order Taylor series. Therefore,we derive the WMS-2f approximate solutions of the transmission expanded by a second order Taylor series, which is in good agreement with the experimental results. Finally, the Allan variances show that the limitation sensitivities of system are 13.98 ppm for CO around 60 seconds and 7.53 ppm for CO₂ around 55 seconds, respectively.