差分吸收光谱法(DOAS)是基于朗伯比尔定律的光谱法测量气体的重要方法, 按此原理建立的测量系统是测量痕量气体的主要方法。 用于测量痕量气体的DOAS系统的关键是其检出限的校准, 传统的方法是使用标准气体进行校准。 但是由于标准气体自身的量值确定问题, 在ppb甚至ppt级的不确定度大于10%, 而一般的ppt级的DOAS测量系统本身的不确定度也会高于标准气体, 导致传统方法失效。 提出一种基于光谱密度的DOAS系统校准方法, 利用朗伯比尔定律将DOAS系统的检出限和光谱密度建立关系。 由于光谱密度作为光学量值可以测量到10-6甚至更高, 所以通过该方法可以实现DOAS系统在ppb乃至ppt级的校准。 本方法需要根据待校准的测量系统光学结构的基本参数计算其总的标准光学密度值, 然后把标准光学密度片放入测量系统光程中, 测得其光学密度值, 根据前后两次光学密度计算测量系统的测量偏差, 进而分析计算测量系统的标准不确定度和标定的扩展不确定度, 所得到的标定的扩展不确定度即为测量系统的检出限。 该方法完全基于光学测量, 不需引入标准气体评估, 基于光学密度的精密测量和测量系统光学结构的装调误差, 实现测量系统在较小不确定度水平上的标定, 提高检出限标定的精度。 本方法在开放光程式的DOAS系统上进行了实验验证。

DOAS (differential optical absorption spectroscopy) is an important gas measurement system based on the Lambert-Beer law especially for the trace gas measurement. The measurement key of DOAS is the calibration of the measurement limit. The traditional calibration method is using standard gas. But the concentration value of standard gas has a large measurement uncertainty which can be large than 10% at ppt. This uncertainty is large than the corresponding specification of instruments which are calibrated. So this traditional method is invalid. A new calibration method is given which is based on spectral optical density. The relationship between measurement limit and optical density can be established using Lambert-Beer law. The optical density as a physical quantity can be measured in 10-6 or more precisely, so the corresponding measurement limit can also be calibrated in ppb or ppt. Firstly, the kinds of optical densities and the total optical density of the measurement system which be calibrated should be analyzed and calculated according to the optical structure (or optical design diagram) of the system. This step is the key of this method because the types of the standard optical densities which are necessary for the measurement of the standard optical density film on the optical density standard equipment. Meanwhile, the optical structure should be understood in order to achieve this calculation. The optical structure include the optical path, every optic cell specification, sphere aberration, astimigation distribution, etc. Secondly, the standard optical density film should be filled in the optical path of the measurement system and the corresponding optical density value is measured. Especially, the optical density film must be filled in the proper station which is confirmed at the step 1 when calculate the different type of the optical density. The error station would make large measurement error which would make large effect into the end data. Then the two above optical density can be compared and the difference of the two value can be considered as the measurement limit. This method is based on the optical measurement without the standard gas. The evaluation error sources only are the measurement error of optical density and the alignment error of the optical structure of the calibrated system. So the calibration of the measurement system can be achieved on small uncertainty, and the measurement limit can be more precise. This method is applied on an open path DOAS and the corresponding experiment data are given.