星载大气痕量气体差分吸收光谱仪狭缝函数研究
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摘要
星载大气痕量气体差分吸收光谱仪是一种新型光学遥感仪器,具有分辨率高(0.3～0.5 nm)、宽光谱范围(240～710 nm)、大视场角(114°视场对应地面2 600 km)的特点,载荷采用推扫方式,可实现1日全球覆盖监测。载荷通过探测地球大气或地表反射、散射的紫外/可见光,利用差分吸收光谱技术来获取全球大气痕量气体(NO_2, SO_2, O_3等)分布和变化。定标是遥感数据定量应用的前提,同时为获取载荷光谱特性,需要在地面完成载荷的光谱定标。根据大气痕量气体差分吸收光谱仪视场角度大、谱段范围宽、空间和光谱分辨率高等特点,搭建了一套基于二维转台的光谱定标系统,此系统能够完成全视场光谱定标。光谱定标采用标准谱线法,光谱定标光源使用汞灯。光谱响应函数是描述光谱仪光谱响应特性的重要参数,根据光谱响应函数可以获取载荷的光谱分辨率,同时也是基于DOAS反演的关键输入参数,光谱响应函数的精度直接影响大气痕量气体的反演结果。根据载荷实际测试的光谱响应数据,选取了Gauss, Lorentz和Voigt三种函数作为待选的光谱响应函数。为对三种函数模型进行筛选,进行了两种筛选对比测试,首先分别用Gauss函数、 Lorentz函数、 Voigt函数对载荷的单色光响应数据进行拟合,以三种函数的拟合残差平方和作为评判标准,拟合结果表明Gauss函数作为狭缝函数拟合的残差平方和最小为0.01, Lorentz和Voigt函数作为狭缝函数拟合的残差平方和分别为0.033和0.021。从载荷单色光响应数据函数拟合的结果分析, Gauss函数可以作为载荷的光谱响应函数模型。为了进一步验证这一结论,进行了DOAS反演NO_2样气的实验,考察三种函数模型对反演的影响。在实验室开展了NO_2样气测试,大气散射光通过30*40cm的石英窗口入射到载荷狭缝,将NO_2样品池放置在载荷狭缝和石英窗口中间,获取的数据为NO_2样气吸收谱,随后充入N_2气体获取反演的参考谱,实验在晴朗天气下进行,并能够在较短时间内完成,可以减少外界天气条件对反演结果的影响。实验中NO_2样气浓度为8.481 2×10~(16) molec·cm~(-2),在利用DOAS进行反演时,设置仪器狭缝函数分别为Gauss, Lorentz和Voigt函数,分析三组不同的函数模型对应的NO_2浓度结果,根据反演结果的相对偏差对函数模型进行评价。实验结果表明Gauss函数作为狭缝函数反演结果的相对偏差最小为5.6%,Lorentz和Voigt函数作为狭缝函数的反演相对偏差分别为28%和15.1%。由光谱响应数据的拟合结果及样气反演结果表明, Gauss函数可以作为载荷的光谱响应函数模型。
In this paper, spaceborne atmospheric trace gas differential absorption spectrometer is introduced. This instrument is a new optical remote sensing instrument whose spectral resolution is better than 0.5 nm. With high resolution(spectral resolution 0.3～0.5 nm), wide wavelength range(240～720 nm), large field(114°field of view corresponds to the ground 2 600 km) features, the load is pushed and swept to achieve 1 day global coverage monitoring. This instrument acquires high accuracy UV/Vis radiation scattered or reflected by air or earth surface, and can monitor distribution and variation of trace gases(NO_2, SO_2, O_3 and so on) based on differential optical absorption spectrum algorithm. Calibration is the premise when formally putting this instrument into operation. At the same time, in order to obtain the spectral characteristics of the load, on-ground spectral calibration is needed. According to the large field, wide wavelength range, high spatial resolution and high spectral resolution of this load, a set of spectral calibration system based on two dimensional turntables is set up. This system can finish the spectral calibration of full field of view. Spectral calibration was performed using standard spectral line method with mercury lamp as calibration source. The spectral response function is an important parameter to describe the spectral response characteristics of the spectrometer. The spectral resolution of the load can be obtained according to the spectral response function. It is also the key input parameter of inversion which is based on DOAS method. The accuracy of the spectral response function directly affects the inversion results of the atmospheric trace gas. According to the spectral response data of load tests, three function models of Gauss, Lorentz and Voigt are selected as the potential spectral response functions. In order to find the most suitable function model, two kinds of contrast tests are carried out. First, the Gauss function, Lorentz function and Voigt function are used to fit the monochromatic light response data of the load, and the sum of the squares of the three kinds of functions is used as the evaluation criterion, the fitting results show that the sum of the residual squares of the Gauss function as the slit function is 0.01, and the sum of the residual squares of the Lorentz and Voigt functions as the slit function is 0.033 and 0.021 respectively. From the analysis of the fitting results of monochromatic light response data, the Gauss function could be used as a spectral response function model of load. In order to further verify this conclusion, DOAS inversion of NO_2 experiment was carried out, and the influence of three kinds of function models on inversion was investigated. The NO_2 sample gas test was carried out in the laboratory. The atmospheric scattering light was incident through the 30 cm×40 cm quartz window to the load slit, and the NO_2 sample pool was placed in the middle of the load slit and the quartz window. The data obtained were NO_2 like gas absorption spectra, and then it was filled into the N_2 gas to obtain the reference spectrum of the inversion. The experiment was carried out in sunny weather and can be completed in a short time, which can reduce the influence of weather conditions on the inversion results. In the experiment, the concentration of NO_2 sample gas is 8.481 2×10~(16) molec·cm~(-2). During the inversion, Gauss function, Lorentz function, Voigt function were set as slit function respectively. The results of NO_2 concentration corresponding to the different functional models of three groups are analyzed, and the function model is evaluated according to the relative deviation of the inversion results. The experimental results show that the relative deviation of the Gauss function as a slit function is 5.6%, and the relative deviation of the Lorentz and Voigt functions as the slit functions is 28% and 15.1%, respectively. The fitting results of spectral response data and gas sample inversion results show that the Gauss function can be used as a spectral response function model of load.
In this paper, spaceborne atmospheric trace gas differential absorption spectrometer is introduced. This instrument is a new optical remote sensing instrument whose spectral resolution is better than 0.5 nm. With high resolution(spectral resolution 0.3～0.5 nm), wide wavelength range(240～720 nm), large field(114°field of view corresponds to the ground 2 600 km) features, the load is pushed and swept to achieve 1 day global coverage monitoring. This instrument acquires high accuracy UV/Vis radiation scattered or reflected by air or earth surface, and can monitor distribution and variation of trace gases(NO_2, SO_2, O_3 and so on) based on differential optical absorption spectrum algorithm. Calibration is the premise when formally putting this instrument into operation. At the same time, in order to obtain the spectral characteristics of the load, on-ground spectral calibration is needed. According to the large field, wide wavelength range, high spatial resolution and high spectral resolution of this load, a set of spectral calibration system based on two dimensional turntables is set up. This system can finish the spectral calibration of full field of view. Spectral calibration was performed using standard spectral line method with mercury lamp as calibration source. The spectral response function is an important parameter to describe the spectral response characteristics of the spectrometer. The spectral resolution of the load can be obtained according to the spectral response function. It is also the key input parameter of inversion which is based on DOAS method. The accuracy of the spectral response function directly affects the inversion results of the atmospheric trace gas. According to the spectral response data of load tests, three function models of Gauss, Lorentz and Voigt are selected as the potential spectral response functions. In order to find the most suitable function model, two kinds of contrast tests are carried out. First, the Gauss function, Lorentz function and Voigt function are used to fit the monochromatic light response data of the load, and the sum of the squares of the three kinds of functions is used as the evaluation criterion, the fitting results show that the sum of the residual squares of the Gauss function as the slit function is 0.01, and the sum of the residual squares of the Lorentz and Voigt functions as the slit function is 0.033 and 0.021 respectively. From the analysis of the fitting results of monochromatic light response data, the Gauss function could be used as a spectral response function model of load. In order to further verify this conclusion, DOAS inversion of NO_2 experiment was carried out, and the influence of three kinds of function models on inversion was investigated. The NO_2 sample gas test was carried out in the laboratory. The atmospheric scattering light was incident through the 30 cm×40 cm quartz window to the load slit, and the NO_2 sample pool was placed in the middle of the load slit and the quartz window. The data obtained were NO_2 like gas absorption spectra, and then it was filled into the N_2 gas to obtain the reference spectrum of the inversion. The experiment was carried out in sunny weather and can be completed in a short time, which can reduce the influence of weather conditions on the inversion results. In the experiment, the concentration of NO_2 sample gas is 8.481 2×10~(16) molec·cm~(-2). During the inversion, Gauss function, Lorentz function, Voigt function were set as slit function respectively. The results of NO_2 concentration corresponding to the different functional models of three groups are analyzed, and the function model is evaluated according to the relative deviation of the inversion results. The experimental results show that the relative deviation of the Gauss function as a slit function is 5.6%, and the relative deviation of the Lorentz and Voigt functions as the slit functions is 28% and 15.1%, respectively. The fitting results of spectral response data and gas sample inversion results show that the Gauss function can be used as a spectral response function model of load.
引文
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[5] ZHOU Hai-jin,LIU Wen-qing,SI Fu-qi,et al(周海金,刘文清,司福祺,等).Spectroscopy and Spectral Analysis(光谱学与光谱分析),2012,32(11):2881.
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[8] Barry P S,Shepanski J,Segal C.Proc.SPIE,2002,4480:231.
[9] DAI Cong-ming,WEI He-li,CHEN Xiu-hong(戴聪明,魏合理,陈秀红).Infrared and Laser Engineering(红外与激光工程),2013,42(1):174.
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