基于O2-O2吸收的非相干宽带腔增强吸收光谱浓度反演方法研究

针对传统非相干宽带腔增强吸收光谱浓度反演方法的定量结果易受镜片反射率标定误差的影响问题, 提出了一种基于测量大气O₂-O₂吸收的浓度反演方法. 该方法是将非相干宽带腔增强吸收光谱技术的光学增强腔等效成吸收光程不随波长变化的多次反射池, 首先根据测得的宽带腔增强大气吸收谱和参考谱计算出光学厚度, 并应用差分光学吸收光谱算法拟合修正后的气体吸收截面到光学厚度, 反演得到大气中O₂-O₂以及被测气体的柱浓度, 然后根据O₂-O₂在大气中的含量已知且相对稳定这一特性, 确定出等效多次反射池的吸收光程, 最后从被测气体的柱浓度中扣除吸收光程信息得到被测气体的浓度值. 以监测大气中NO₂实验为例, 应用该方法在454-487 nm波段反演得到了大气NO₂的浓度(1-30 ppbv范围内), 并将反演结果与传统浓度反演方法的结果进行了对比, 发现两者的不一致性在7%以内. 实验结果表明, 非相干宽带腔增强吸收光谱技术可以利用大气O₂-O₂的吸收来定量其他被测气体的浓度, 而且定量结果对镜片反射率的标定误差不敏感.

A concentration retrieval method for incoherent broadband cavity-enhanced absorption spectroscopy based on O2-O2 absorption

Incoherent broadband cavity-enhanced absorption spectroscopy (IBBCEAS) has a very high detection sensitivity, nevertheless the quantitative results retrieved by the traditional concentration retrieval method are affected by the calibration error of mirror reflectivity. Therefore, in this paper we present another concentration retrieval method based on the measurements of atmospheric O₂-O₂ absorption. In this method the optical cavity of IBBCEAS is equivalent to a multi-reflection cell, in which the optical path length is independent of the wavelength, i.e. a constant. First, we get the slanting column concentrations of atmospheric O₂-O₂ and other trace gases measured by using the differential optical absorption spectroscopy (DOAS) to fit the corrected cross sections of the measured gases to the optical density from the IBBCEAS absorption spectra and reference spectra. Second, the optical path length of an equivalent cell is determined by the known concentrations of O₂-O₂ in the atmosphere. Third, the concentrations measured for trace gases are retrieved by the deduction of absorption optical path length from the obtained slanting column concentrations. The above method is demonstrated by measuring the atmospheric NO₂ with an IBBCEAS instrument in the range of 454-487 nm. Atmospheric NO₂ concentrations retrieved by this method are compared with those by the traditional method, and the difference between them is shown to be less than 7%. Experimental results show that the absorption of atmospheric O₂-O₂ can be used to quantify other trace gases when measured by IBBCEAS, and, above all, the quantitative results are almost insensitive to the calibration error from mirror reflectivity.