基于傅里叶变换红外光谱技术观测海边大气水汽及其稳定同位素
Water vapor, the most prevalent greenhouse gas in Earth’s atmosphere, plays a pivotal role in atmospheric chemistry and climate dynamics. Its sources, sinks, and transportation mechanisms are integral to the hydrological cycle. Analyzing the isotopic composition of water vapor sheds light on diverse hydrological processes. Furthermore, concurrent observations of atmospheric water vapor and its isotopes offer insights into the origins of atmospheric humidity across various regions. Ground-based Fourier Transform Infrared Spectroscopy (FTIR) technology is a powerful tool for remotely sensing atmospheric gases through the collection of solar spectra, characterized by high accuracy and precision. A portable FTIR spectrometer is employed to conduct continuous observations at the Shenzhen Observatory over a period of approximately two weeks. It successfully collects the solar near-infrared (NIR) spectra of the coastal atmosphere, leading to the derivation of measurement results for water vapor and its stable isotopes in the ambient atmosphere through sophisticated inversion techniques.
The experimental setup for this study primarily included a portable FTIR spectrometer, an automated sun tracker, a comprehensive meteorological station, and a dedicated computer system. This spectrometer utilized natural sunlight as its primary incident light source, while the sun tracker continuously and accurately followed the sun’s position in real time. The solar rays were precisely channeled into the interferometer, where the resulting interference pattern was captured by a sensitive detector. This pattern was then transformed into a detailed NIR spectrum through a Fourier transform process. The spectrometer was designed to capture a NIR spectral range spanning 5000‒11000 cm-1, offering a spectral resolution of 0.5 cm-1. The core of the analysis lay in the utilization of a sophisticated non-linear least squares iterative algorithm, which enabled precise inversion of the vertical column concentration of the target gas. This involved a two-step process: initial forward modelling followed by meticulous spectral iterative fitting calculations. Upon determining the vertical column concentration of the sample gas, the dry air mole fraction (DMF) was then derived by correlating it with the total column concentration of dry air.
In our study, several specific spectral window bands are meticulously selected for the inversion of characteristic absorption features of atmospheric H2O and its isotope, HDO, in NIR spectroscopy. The average root mean square error of the residual fits for H2O and HDO spectra stands at 0.026% and 0.032%, respectively (Fig.2, Fig.3). This small residual indicates a high-quality fit to the solar spectra. During the observation period, the mean molar mixing ratio of water vapor (
This study utilizes near-infrared solar absorption spectra captured by a portable Fourier Transform Infrared (FTIR) spectrometer. We employ the advanced PROFFAST inversion algorithm to accurately derive the column concentrations of atmospheric H2O and its isotope, HDO. A key aspect of our experiment involves vigilant monitoring of the Instrument Line Shape (ILS) and Xair, revealing that the spectrometer maintains excellent long-term measurement stability. We simultaneously correct portable FTIR instrument readings with high-resolution FTIR instrument readings. Our research, leveraging the portable FTIR spectrometer, focuses on the coastal atmosphere of Shenzhen. We successfully measure water vapor and its isotopes, yielding valuable data on the column concentration of water vapor, the water vapor isotope ratio, and the characteristics of water vapor evapotranspiration during the measurement period. These results demonstrate the spectrometer’s capability to precisely monitor variations in water vapor and its stable isotopes in a coastal atmospheric setting. The data provided by these measurements offer a robust scientific foundation for understanding and tracking the diffusion and transport dynamics of water vapor in the ambient atmosphere. In future, our research endeavors will concentrate on the accurate retrieval of the water vapor isotope
1 引言
水汽是大气中最丰富的温室气体,在大气化学和气候变化中发挥着重要作用[1]。在水循环过程中,水汽的源(海洋或陆地蒸发)、汇(降雨)和输送(扩散、湍流或云内过程等)是至关重要的环节[2]。不同的物理过程、化学过程和生物学过程以不同的方式分馏同位素,稳定同位素是这些过程中的天然示踪剂[3]。并且,不同的排放源有不同的同位素成分,同位素的测量能提供源的相对贡献信息。鉴于水汽稳定同位素变化通常伴随着水汽的相变,所以水汽稳定同位素可以为大气中的水循环提供重要信息[4]。水蒸气的同位素成分可用于分析各种水文过程,因为它对大气中水汽的来源高度敏感,大气水汽及其同位素联合观测可以用于分析不同区域大气湿度的来源[5]。观测海边大气中的水汽及其稳定同位素对于陆-海水循环研究具有重要意义。
目前,人们已经通过多种技术和方法实现了环境大气中水汽及其稳定同位素的测量。机载或星载遥感成像可以观测大区域水汽浓度的时空分布,具有大规模测量的优势[6-8],但遥感测量数据的不确定性较大,在数据分析和应用之前必须对该遥感数据进行验证。可调谐红外激光吸收光谱(TDLAS)、腔衰荡光谱(CRDS)等原位测量光谱技术是经常用来测量大气中水汽及其同位素的方法[9-12],但其在测量地面附近气体浓度时很容易受到地形和气团垂直传输的影响。地基遥测技术可以减小这些干扰因素的影响,并且可以提供高精度的水汽及其同位素的柱浓度信息。
地基傅里叶变换红外光谱(FTIR)技术通过采集太阳光谱对大气气体进行遥感测量,具有较高的精度和准确度。此外,地基FTIR测量结果具有长期稳定性,可以用于验证卫星观测和模型模拟结果的准确性。Wunch等[13]利用总碳柱观测网(TCCON)的FTIR光谱技术测量了近红外太阳吸收光谱,进而实现了水汽的测量。Makarova等[14]利用MetOP-A卫星搭载红外大气探测干涉仪(IASI)、“Optik”Tu-134机载Picarro G2301-m气体分析仪和地基FTIR光谱技术观测了大气中的水汽柱浓度,并对观测结果进行了比较。Schneider等[15-16]通过多年的实验证明了高分辨率FTIR技术可以高精度地获取对流层水汽及其同位素比值δD的长时间序列。然而,基于高分辨率FTIR技术反演水汽及其同位素也存在一些问题,例如仪器造价昂贵且无法移动,无法实现不同区域包括偏远地区和基础设施条件较差区域大气水汽及其稳定同位素的探测。
本课题组利用便携式FTIR光谱仪,在深圳市天文台进行了约半个月的连续测量,采集沿海大气的太阳近红外光谱,进而反演获得了环境大气中水汽及其稳定同位素的测量结果。笔者首先介绍了地基遥感观测的测量仪器、光谱反演算法以及利用光谱反演算法拟合光谱的结果,并分析了仪器线型。最后,获得了海边大气水汽及其同位素比值δD的时间序列,分析了δD与水汽柱浓度之间的相关性,并利用Keeling比值分析方法研究了沿海地区水汽蒸散特征
2 测量仪器和方法
2.1 实验地点和装置
将便携式FTIR光谱仪(EM27/SUN)放置在深圳市南山区南山顶的天文台内(东经114.56°,北纬22.48°),此处海拔高度为430 m,临近海边。实验装置主要由便携式FTIR光谱仪、气象站以及计算机组成,其中便携式FTIR光谱仪主要由太阳追踪仪、干涉仪、探测器组成。观测原理如
图 1. FTIR光谱仪及其观测原理。(a)便携式FTIR光谱仪实物图;(b)观测原理示意图
Fig. 1. FTIR spectrometer and its observation principle. (a) Photo of the portable FTIR spectrometer; (b) schematic of observation principle
2.2 光谱反演方法
利用PROFFAST算法对测量光谱进行反演。该算法由德国卡尔斯鲁厄理工学院研发,基于非线性最小二乘迭代算法准确反演温室气体和痕量污染气体的柱浓度[18]。反演算法主要有前向建模和光谱迭代拟合计算两个步骤。在光谱前向建模中,基于大气辐射传输模型模拟光谱仪接收到的光谱辐射结果
式中:
在计算大气状态向量时
在反演过程中,考虑到未知量远多于实测量,通常采用Tikhonov-Phillips正则化约束未知状态向量。利用高斯-牛顿法求解非线性方程,将
式中:
气体的垂直柱浓度为
式中:
获得待测气体的垂直柱浓度后,将其与干空气的总柱浓度相比就能得到该气体的干空气柱平均摩尔混合比(DMF)。O2的柱浓度结果通常被作为干空气的柱浓度结果,它是通过反演7765~8005 cm-1光谱区域得到的。最终得到待测气体的DMF为
2.3 测量光谱的拟合结果
在晴朗无云的天气条件下采集太阳近红外吸收光谱并将其记录下来,采集光谱的时间设置与世界标准时间(UTC时间)相同[17]。针对近红外光谱中大气水汽及其同位素HDO的特征吸收,分别选取3个光谱窗口波段进行反演,反演的光谱窗口和干扰气体如
表 1. 水汽和HDO拟合的光谱窗口
Table 1. Spectral windows for fitting H2O and HDO
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图 2. 三个光谱窗口内 光谱拟合结果及残差。(a)6076.9 附近的光谱窗口;(b)6125.9 附近的光谱窗口;(c)6177.6 附近的光谱窗口
Fig. 2. Typical spectral fitting and residuals of in three spectral windows. (a) Spectral window near 6076.9 ; (b) spectral window near 6125.9 ; (c) spectral window near 6177.6
图 3. 三个光谱窗口内 光谱拟合结果及残差。(a)6330.0 附近的光谱窗口;(b)6377.4 附近的光谱窗口;(c)6458.6 附近的光谱窗口
Fig. 3. Typical spectral fitting and residuals of HDO in three spectral windows. (a) Spectral window near 6330.0 cm-1; (b) spectral window near 6377.4 cm-1; (c) spectral window near 6458.6 cm-1
3 仪器线型和测量稳定性监测
3.1 仪器线型监测
EM27/SUN光谱仪的仪器准直性能可以通过测试仪器的线型进行评估[20]。仪器线型(ILS)会影响反演信息所基于的吸收线形状,所以了解ILS函数对于正确表征仪器性能至关重要。ILS函数由振幅调制效率(ME)和相位误差(PE)表征[19],ME反映了光谱仪内两光束相干的程度,PE表示ILS函数的对称程度。通过ME和PE的监测结果来表征光谱仪的仪器线型是否是理想的sinc函数。在实验室中,利用卤素灯作为光源,基于LINEFIT软件分析7000~7400 cm-1光谱波段的水汽吸收谱线,从而获得ILS函数的ME和PE值[20]。将光源和光谱仪的距离分别设置为255、354、460、550、648 cm,对测量得到的光谱进行分析,得到了ILS函数中的两个参数,如
表 2. ME和PE随距离的变化
Table 2. Variation of ME and PE with distance
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由
3.2 仪器测量稳定性监测
式中:
3.3 与地基高分辨FTIR仪器观测结果的比对
为验证系统测量的准确性和可靠性,将便携式FTIR光谱仪与高分辨率FTIR光谱仪同时观测的水汽及其稳定同位素柱浓度进行比较[19]。高分辨率FTIR仪器是由德国Bruker Optics公司生产的,型号为IFS125HR,同样坐落于深圳天文台。该光谱仪的分辨率为0.02 cm-1,采集的太阳光谱覆盖范围为4000~11000 cm-1。为了保证测量时间的一致性,用两台仪器同时进行测量,测量日期为2023年2月27日。两台仪器当天同时测量的气体数据的相关性如
图 5. 便携式和高分辨率FTIR光谱仪测得的 和 的日均值比较。(a) ;(b)
Fig. 5. Daily average values of and measured with portable and high resolution FTIR spectrometers. (a) ; (b)
4 结果与分析
4.1 水汽柱浓度观测结果
为了分析观测期间大气水汽的变化规律,筛除了受云和气溶胶影响的光谱以及部分饱和光谱,选取2月27日至3月11日的有效数据进行分析,得到了海边大气水汽柱浓度的时间变化序列,如
4.2 水汽同位素比值
在生态系统中,大多数元素的轻同位素的丰度要远高于重同位素的丰度,所以同位素的比值经常被用来表示同位素的组成[22]。水蒸气同位素比值δD被定义为
式中:
为了更详细地研究水汽同位素比值δD,这里使用Rayleigh蒸馏模型来阐述δD与水汽柱浓度之间的关系[12]。
式中:
4.3 水汽蒸散同位素特征
为了更好地了解海边城市的水汽蒸散特征
图 11. 实验期间测得的水汽蒸散特征
Fig. 11. of water vapor evapotranspiration characteristics measured during the experiment
5 结论
笔者基于便携式傅里叶变换红外光谱仪采集的近红外太阳吸收光谱,采用PROFFAST反演算法获得了大气水汽及其同位素HDO的柱浓度。外场实验前对仪器的仪器线型函数进行监测,发现仪器的光学准直性较好,为后续的准确测量提供了保障。同时,对高分辨率FTIR光谱仪与便携式FTIR光谱仪进行观测,将二者测量的气体柱浓度的比值作为修正系数,对便携式FTIR仪器的测量数据进行校正。
采用便携式FTIR光谱仪对深圳市海边大气水汽及其同位素的柱浓度进行测量,测量期间
通过以上分析可以看出,便携式FTIR光谱仪能够准确观测海边大气水汽及其稳定同位素的变化,测量结果可以为追踪环境大气中水汽的扩散和输送提供科学依据。接下来的研究工作将集中于从太阳吸收光谱中准确检索水汽同位素H
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Article Outline
吴鹏, 单昌功, 王薇, 谢宇, 祝钱钱, 梁彬, 曾祥昱, 彭璇, 刘诚. 基于傅里叶变换红外光谱技术观测海边大气水汽及其稳定同位素[J]. 中国激光, 2024, 51(5): 0511005. Peng Wu, Changgong Shan, Wei Wang, Yu Xie, Qianqian Zhu, Bin Liang, Xiangyu Zeng, Xuan Peng, Cheng Liu. Observation of Atmospheric Water Vapor and Its Stable Isotopes at the Seaside Based on Fourier Transform Infrared Spectroscopy[J]. Chinese Journal of Lasers, 2024, 51(5): 0511005.