Retrieval of Atmospheric H2O Column Concentration Based on Mid-Infrared Inter-Band Cascade Laser Heterodyne Radiometer
ZHANG Shang-lu1, 2, HUANG Yin-bo1, LU Xing-ji1, 2, CAO Zhen-song1, DAI Cong-ming1*, LIU Qiang1, GAO Xiao-ming1, RAO Rui-zhong1, WANG Ying-jian1
1. Key Laboratory of Atmospheric Optics, Anhui Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, Hefei 230031, China
2. Science Island Branch of Graduate School, University of Science and Technology of China, Hefei 230026, China
摘要： 水汽是地球大气的重要组成部分，也是平衡地气系统辐射收支的一个重要因素，对天气和气候变化有着重要的影响。常用的水汽柱浓度测量设备，如无线电探空仪、激光雷达、微波辐射计、太阳光度计、DOAS仪器以及傅里叶变换红外光谱仪等，难以兼顾高分辨率以及便携机动等应用需求。为此，基于一种高灵敏度、高分辨率光谱探测技术，围绕水汽柱浓度的探测开展了相关研究，取得的主要成果有：(1) 基于激光外差光谱技术，利用窄线宽带间级联激光器作为本振光源，与太阳跟踪仪结合，建立了一套高分辨率激光外差太阳光谱测量装置，光谱分辨率达到了0.002 cm-1。(2) 采用Langley-plot方法对高分辨率激光外差太阳光谱测量装置进行了现场定标，并于云南紫金山天文台观测站开展了外场测量，获得了2 831~2 833 cm-1波段太阳光谱的直接测量数据。对实测的太阳光谱进行归一化处理后，获得了高分辨率的整层大气透过率谱。(3) 利用逐线积分辐射传输模式（line by line radiative transfer model，LBLRTM）计算了整层大气透过率谱，并与实测的透过率谱进行了非线性最小二乘拟合，实现了水汽柱浓度的反演。同时利用微波辐射计进行了水汽柱浓度的观测，将反演结果与实测结果进行了对比分析，两者的一致性相对较好，最小相对偏差为16.59%，最大相对偏差为21.69%。(4) 反演结果与实测结果的偏差主要由反演算法误差和装置测量误差所导致。反演算法误差包括辐射传输模式的计算误差、实际大气温度的测量误差、甲烷浓度不确定性引入的误差、HDO丰度与自然丰度的偏差，装置测量误差包括装置定标误差、波长标定误差、系统噪声影响、背景信号以及直流信号的微弱起伏引起的误差。(5) 文中选取的2 831~2 833 cm-1波段同时包含了水汽和甲烷的吸收，在反演水汽柱浓度的同时，同步进行了甲烷柱浓度的反演。以甲烷初始柱浓度作为参考值，发现反演后的甲烷柱浓度相对初始柱浓度的数值平均增加了14.41%。高分辨率激光外差太阳光谱测量装置结合反演算法是一种有效的整层大气透过率以及水汽、甲烷柱浓度探测的综合设备，在多组分气体浓度探测方面具有广泛的应用前景。
关键词：激光外差；整层大气透过率谱；非线性最小二乘；水汽柱浓度
Abstract：Water vapor is an important component of the atmosphere. It is also an important factor to balance the radiation budget of the atmosphere system, which has an important influence on weather and climate change. The commonly used equipment for measuring the concentration of water vapor column, such as Radiosonde, Lidar, Microwave Radiometer, Solar Photometer, DOAS instrument and Fourier Transform Infrared Spectrometer are difficult to meet the requirements of high-resolution and portable mobility. Based on a high-sensitivity and high-resolution spectral detection technology, related researches have been carried out around the detection of water vapor column concentration. The main achievements are as follows: (1) Based on the laser heterodyne spectroscopy technology, a set of high-resolution laser heterodyne solar spectrum measuring devices with a narrow-line broadband inter-cascade laser as the local oscillator and the sun tracker is estabished, with a spectral resolution of 0.002 cm-1. (2) The Langley-plot method is used to calibrate the high-resolution heterodyne solar spectrum measuring device. The field measurement is carried out at the Purple Mountain Observatory in Yunnan, and the direct measurement data of the 2 831~2 833 cm-1 band solar spectrum are obtained. The high-resolution total atmospheric spectral transmittance is also obtained. (3) The Line by Line Radiative Transfer Model (LBLRTM) is used to calculate the total atmospheric spectral transmittance, and the nonlinear least square fitting is carried out with the measured spectral transmittance. The inversion of water vapor column concentration is realized. The concentration of water vapor column is also observed by the Microwave Radiometer. The consistency between the inversion results and the measured resultsis is relatively good, where the minimum relative deviation is 16.59%, and the maximum relative deviation is 21.69%. (4) The error of the inversion results and the measured results is mainly caused by the error of the inversion algorithm and the measurement error of the device. Inversion algorithm errors include the calculation error of the radiative transfer model, the actual temperature measurement error, the methane concentration uncertainty into the error, the deviation of HDO abundance and the natural abundance. The device measurement error includes the calibration error of device, the wavelength calibration error, the noise influence, the error caused by the weak fluctuation of the background signal and the DC signal. (5) The 2 831~2 833 cm-1 band selected contains the absorption of water vapor and methane, and the concentration of methane column is also retrieved. With the initial column concentration of methane as the reference value, it is found that the numerical average of the concentration of the methane column after the inversion is 14.41% higher than the initial column concentration. The high-resolution laser heterodyne solar spectrum measurement device combined with its inversion algorithm is an effective integrated equipment for detecting the whole atmospheric transmittance and the concentration of water vapor and methane column. It has a wide application prospect in the detection of multi-component gas concentration.
Key words：Laser heterodyne; Total atmospheric spectral transmittance; Nonlinear least squares; Water vaporcolumn concentration
