基于可调谐二极管激光吸收光谱的气池光程可溯源测量

在碳中和的国际大背景下，精确可靠地定量测量大气温室气体浓度对实现碳中和目标具有重要意义，开发测量结果可直接溯源至国际单位制SI的气体分析仪是精确可靠监测温室气体浓度的重要方法。可调谐二极管激光吸收光谱（TDLAS）技术是常用的气体浓度测量方法，根据比尔-朗伯定律，实现仪器的测量浓度直接溯源至SI的必要条件之一是可直接溯源的气池光程，气池光程的不确定度直接影响气体浓度的测量不确定度，对气池光程的可溯源精确测量有利于发展测量结果可直接溯源的气体分析仪。针对光程标称为81 cm的三次反射型气池光程可溯源测量需求，使用校准的米尺测量该气池光程得到的直接测量结果为(81.21±0.80) cm，较大的测量不确定度（0.80 cm）是综合考虑定位误差和三段光路与测量路径可能不重合导致的测量误差估算得到的。为了减小测量不确定度，本文搭建了TDLAS气池光程测量系统，测量系统以1 576 nm分布式反馈激光器为光源，通过在激光控制器上加载斜坡扫描电压来测量待测气池内标准高纯二氧化碳（CO₂，99.999%）在6 344.68 cm-1附近的吸收光谱，使用测量结果可直接溯源的压力传感器和温度传感器分别测量气池内的压强和气体温度，采用美国国家标准技术局最新测量得到的30012-00001跃迁带P 4e支线强（相对标准不确定度为0.15%）反演气池光程，使用二次速度依赖Voigt线型精确拟合不同气压（36~75 Torr）下的光谱吸光度信号获得对应气压的积分吸光度，全面分析各参量的测量不确定度及其传递过程，对不同气压下的积分吸光度进行线性回归分析，计算得到可直接溯源的气池光程为(81.61±0.42) cm，相对标准不确定度为0.51%，测量不确定度范围落在直接测量结果范围内，测量不确定度小于直接测量结果。本文气池的光路结构是多次反射长光程气池的简化，该系统同样适用于多次反射长光程气池光程的可溯源测量。

Traceable Measurement of Optical Path Length of Gas Cell Based on Tunable Diode Laser Absorption Spectroscopy

In the context of carbon neutrality, reliable and accurate quantification of atmospheric greenhouse gas is of great importance. Thus, it is necessary to develop gas analyzers which can provide analytes amount-of-substance fraction results traceable to the international system of units (SI). Tunable diode laser absorption spectroscopy (TDLAS) is a common way to measure gas concentration. According to Beer-Lambert Law, one of the necessary conditions to realize the direct traceability of the measured concentration of the instrument to the SI is the directly traceable gas cell optical path length (OPL). The gas cell OPL directly affects the measurement uncertainty of the gas concentration. Accurate measurement is conducive to developing gas analyzers whose measurement results can be directly traced.In this paper, the directly traceable measurement of a gas cell OPL with three reflection points is carried out. The direct measurement result obtained by measuring three parts of the optical path of the gas cell using a calibrated meter is 81.21±0.80 cm. The large measurement uncertainty (0.80 cm) is a careful consideration, estimated considering positioning error and the measurement error caused by the possible misalignment of the three optical paths and the measurement path. In order to reduce the OPL measurement uncertainty, a TDLAS gas cell OPL measurement system is built in this paper. A 1 576 nm distributed feedback laser with a ramp sweep voltage loading on the laser controller is used to measure the absorption spectrum of standard high-purity carbon dioxide (CO₂, 99.999%) in the gas cell to be measured near 6 344.68 cm-1. A traceable pressure and temperature sensors are used to measure the pressure and gas temperature in the gas cell. The key parameter of line strength of the P 4e branch of CO₂ at 30012-00001 transition band, which has relative uncertainty of 0.15%, is from theNational Institute of Standards and Technology. A quadratic speed-dependent Voigt line profile is used to fitting the acquired absorbance at various pressure from 36 to 75 Torr. The uncertainty and the transfer process of various input parameters are comprehensively analyzed. A generalized linear regression (GLR) is applied to absorbance data with uncertainty at various pressure. The slope from the GLR yield the optical path length of the gas cell L= 81.61±0.42 cm with a standard uncertainty of 0.51%.This uncertainty range falls within one of the direct measurement results. The optical path structure of the gas cell used in this paper is a simplified multi-reflection gas cell with long OPL. The system can be equally applied to the traceable measurement for OPL of the multi-reflection gas cell.