波长调制-直接吸收方法在线监测大气中CH4和CO2浓度

波长调制-直接吸收光谱(WM-DAS),同时具有直接吸收光谱(DAS)的免标定、可测量吸收率函数的优点和波长调制光谱(WMS)高信噪比、抗干扰能力强的优点.本文利用免标定、高信噪比的WM-DAS方法结合长光程Herriott池,在常压常温条件下,对大气中CH4(6046.952 cm^-1)和CO2(6330.821 cm^-1)分子两条近红外吸收谱线的吸收率函数进行了测量,其光谱拟合残差标准差可达到5.6×10^-5.随后,采取WM-DAS方法结合Herriott池,对大气中CO2和CH4浓度进行了连续监测,并将其与高灵敏度的连续波腔衰荡光谱(CW-CRDS)测量结果进行比较.实验结果表明:本文采用的长光程WM-DAS与CW-CRDS方法测量结果一致,两组数据线性拟合相关性达到0.99,其中基于WM-DAS方法的CO2和CH4的检测限分别达到170 ppb和1.5 ppb,略高于CW-CRDS检测限,但其测量速度远高于CW-CRDS,并且具有系统简单、对环境要求低、可长期稳定运行等优点.

Monitoring of ambient methane and carbon dioxide concentrations based on wavelength modulationdirect absorption spectroscopy

Wavelength modulation-direct absorption spectroscopy(WM-DAS)integrates the advantages of measuring absolute absorbance profile from calibration-free direct absorption spectrum(DAS)with the enhanced noise rejection and high sensitivity of wavelength modulation spectrum(WMS).This method can be used to precisely recover the crucial absorbance profile via the extraction of the characteristic frequency of the modulated transmitted light.In this paper,the WM-DAS method with non-calibration and high signal-to-noise ratio is integrated with a Herriott cell(about 128 m).Under the condition of atmospheric pressure and room temperature,the absorptance functions of two spectral lines of CO2(6330.821 cm^-1)and CH4(6046.964 cm^-1)in air are measured,and their standard deviations of spectral fitting residual are 5.6×10^-5 and 7×10^-5,respectively.Subsequently,the concentration of CO2 and CH4 in air are monitored on-line by the WM-DAS method integrated with the Herriott cell,and compared with those by the highly sensitive continuous wave cavity ring down spectroscopy(CW-CRDS).The experimental results show that the measured results of the long optical path WM-DAS method are consistent with those by the CW-CRDS method,and the linear correlation between the two methods is above 0.99.The detection limit of CO2 and CH4 by the WM-DAS method are 170 ppb and 1.5 ppb respectively,which are slightly higher than those by the CW-CRDS.However,the measurement speed by WM-DAS is much higher than that by CW-CRDS,and possesses the advantages of simpler operation,lower environmental requirements,long-term stability,etc.