波长调制-直接吸收光谱(WM-DAS)在线监测大气CO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)可测量吸收率函数和波长调制光谱(WMS)高信噪比的优点,本文首先采用WM-DAS光谱,在50 cm光程和室温低压下,CO分子近红外4300.7 cm^-1谱线吸收率检测限低至4×10^-7(200 s);然后结合120 m长光程Herriott池,在室温大气压下,吸收率函数拟合残差标准差达到5.1×10^-5(1 s).最后利用长光程WM-DAS测量系统,对不同浓度(体积分数为0.44×10^-6—9.6×10^-6)CO进行了动态测量,并将其与腔衰荡光谱(CRDS)进行比较;实验结果表明:本文采用的长光程WM-DAS与CRDS方法测量结果相同,其中长光程WM-DAS系统CO浓度检测限低至0.9×10^-9(200 s),系统简单且测量速度远快于CRDS.与此同时,利用建立的长光程WM-DAS测量系统连续监测1个月时间内大气痕量CO浓度及其变化趋势,测量结果与中国环境监测总站测量结题高度一致.

Monitoring of ambient carbon monoxide concentrations based on wavelength modulation direct absorption spectroscopy

Wavelength modulation-direct absorption spectroscopy(WM-DAS)has the advantages of both direct absorption spectroscopy(DAS)measurable absorptivity function and wavelength modulation spectrum(WMS)with high signal-to-noise ratio(SNR).In this paper,the WM-DAS spectrum is used to measure the absorptivity of 4300.7 cm^-1 line of CO molecule and the detection limit is as low as 4×10^-7(200 s)at 0.5 m optical path,room temperature and low pressure.Then,through combining the WM-DAS spectrum with a 120 m long optical path Herriott cell,at room temperature and atmospheric pressure,the standard deviation of the fitting residual error of the absorptivity function is reduced down to~5.1×10^-5(1 s).Finally,different concentrations of CO are continuously monitored by long-path WM-DAS measurement system,and compared with the results obtained from the cavity ring-down spectroscopy(CRDS).The experimental results show that the measurement results from the long-path WM-DAS and CRDS method are the same.The detection limit of CO concentration in long-path WM-DAS system is as low as 0.9 ppb(200 s),and the WM-DAS system is simple and the measurement speed is much faster than CRDS.At the same time,the long-path WM-DAS system is used to continuously monitor the atmospheric trace CO concentration and trend for one month,and the measured results are highly consistent with those from the China Environmental Monitoring Station.