光腔衰荡光谱技术测定大气水汽稳定同位素校正方法研究

几乎所有小的气相分子（如H₂O，CO₂等）均具有独特的近红外吸收光谱，在负压条件下，每种微小的气相分子都拥有一对一的特征光谱线，基于这一原理人们开始使用激光光谱（IRIS）技术来准确分析气体样品中的同位素组成。该方法克服了传统同位素比质谱（isotope ratio mass spectrometry, IRMS）方法的局限性，已经成为公认的高精度、高灵敏度和高准确度的痕量气体检测方法。以大气水汽稳定同位素研究为例，大气水汽稳定同位素组成对水汽源区及其通道上的输送过程等水循环研究有着重要的指示意义。激光光谱技术使得大气水汽氢氧稳定同位素（δ18O和δD）野外原位连续高分辨率观测成为可能。但是，其观测精度和准确度受仪器运作特点、不同浓度大气水汽对特定光谱吸收性的敏感性差异等因素的影响，通常观测结果具有明显的非线性响应问题。因此，有必要对仪器观测过程中出现的各种偏差进行校正，但现阶段许多用户对新观测技术的国际校正方法尚不清楚。因此，基于波长扫描-光腔衰荡光谱（WS-CRDS）技术的大气水汽同位素观测系统（Picarro L2120-i），通过可调谐二极管激光器（Tunable Diode Laser, TDL）发射可被待测气体分子所吸收的不同波长的激光，测量不同波长下的衰荡时间（即有样品吸收的衰荡时间）；TDL发射不能被待测气体吸收的不同波长的激光，测量每个波长下的衰荡时间（相当于无样品吸收的衰荡时间）。通过分析有无样品吸收的衰荡时间差，高精度计算待测气体的分子浓度，进而计算水汽稳定同位素组成。从记忆效应、漂移效应、浓度效应等方面，系统建立了一套准确可靠的大气水汽稳定同位素观测流程与校正方法，为正在使用或将要使用此类设备的研究人员提供参考，以获得高精度和高可靠性的大气水汽稳定同位素观测数据。

Study on Calibration Method for Atmospheric Water Vapor Stable Isotopes Observed by Cavity Ring-Down Spectroscopy

Nearly every small gas-phase molecule (e. g., H₂O, CO₂) has a unique near-infrared absorption spectrum. At sub-atmospheric pressure, each tiny gas-phase molecule has a one-to-one characteristic spectral line. Based on this principle, it can use the Isotope Ratio Infrared Spectroscopy (IRIS) to accurately analyze the isotopic composition of gas samples, which overcomes the limitations of the conventional Isotope Ratio Mass Spectrometry (IRMS) and has become a recognized high-precision, high-sensitivity and high-accuracy method for trace-gas detection. In recent years, commercial laser spectroscopy gas composition analysis technology has gradually developed, and more and more scholars have made significant progresses in their respective fields using laser spectroscopy instruments. Among them, especially the study of atmospheric water vapor stable isotope has important significance to the study of hydrological cycle process. Laser spectroscopy makes it possible to conduct continuous and in-situ high-resolution measurement of atmospheric water vapor stable isotope (δ18O and δD). However, its observational precision and accuracy are affected by factors such as the operating characteristics, the sensitivity of different concentrations of atmospheric water vapor to specific spectral absorbance and so on, usually leading to the observations with obvious nonlinear response problems. Therefore, it is necessary to calibrate various deviations during the process of instrument observation. But at this stage, many users are not yet clear about the international calibration methods for the new observational technology. Therefore, based on Wavelength-Scanned Cavity Ring-Down Spectroscopy (WS-CRDS) technology, the atmospheric water vapor isotope observation system (Picarro L2120-i)measures the ring-down time at different wavelengths by Tunable Diode Laser (TDL) emitting laser of different wavelengths that can be absorbed by the target gas and that cannot be absorbed by the gas. And then by analyzing the ring-down time difference of the sample absorption and without any gas absorption, it can calculate molecular concentration of the target gas with high precision, and then determine the water vapor stable isotope composition. This paper establishes a set of accurate and reliable observation procedures and calibration methods in respect to memory effect, drift effect, concentration effect and so on, which provides a reference for researchers who are using or will use such equipment to obtain high-precision and high-reliability atmospheric water vapor stable isotope observation data.