LED光源DOAS方法的夜间NO_2气体浓度遥测研究
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摘要
针对现有利用自然光源的被动DOAS测量方法无法实现夜间对NO_2等痕量气体进行垂直分布探测的问题,提出构建一种基于窄带光源蓝光LED技术的DOAS测量NO_2的方法,搭建了仪器系统,成功地实现了夜间对NO_2气体浓度的测量。该系统主要分为灯源发射系统和望远镜接收系统两部分,采用主波长为450nm的LED作为光源,通过望远镜采集发光束的散射光,利用光纤耦合将望远镜接收到的散射光导入光谱仪中,结合DOAS原理运用计算机进行处理。DOAS的理论基础是朗伯-比尔(Lambert-Beer)定律,据此原理可将数据处理过程概述如下:首先采集相对干净光谱作为背景参考谱,用实际测量大气谱除以参考谱,利用数字高通滤波去除慢变化部分,然后取对数,即可获得光学厚度;其次将仪器函数与NO_2的高分辨率截面卷积,得到与所用仪器相匹配的低分辨率吸收截面;最后将差分吸收截面与处理后的差分光学厚度相结合,运用最小二乘法拟合并结合光程L即可获得NO_2浓度值。同时可通过调节灯源光速发射角度及望远镜接收角度,测出不同位置处NO_2浓度值,进而给出NO_2气体浓度的立体分布信息。在算法确定的情况下, LED灯谱质量对仪器系统的可靠性显得尤为重要。由于LED光谱受温度及驱动电流影响较大,为了保证LED处于最佳工作状态,开展了LED光谱温度及驱动电流敏感性实验。测试结果表明,要确保采集到的光谱稳定且具有较高质量, LED工作温度应低于20℃,驱动电流需控制在1.5 A以内,且两者波动范围较小。实验中, LED实际工作温度为10～15℃,驱动电流为1.4 A,控制精度±1 mA,能够满足实验要求。为了提高LED阵列密度、获得更加集中的发光束, LED底座基板采用正六边形结构,每块正六边形基板上7只LED串联,各个基板之间并联。经计算较采用矩形结构,其空间利用率提高了8%。各基板工作电流1.4 A,最大电压23.8 V,易于扩展,维护方便。为了验证方案可行性及系统的可靠性,进行了实验室测试及外场实验。实验室采用NO_2样气浓度为1 642.86 mg·m~(-3),不确定度5%。系统测量结果为1 607.54 mg·m~(-3),与标定值误差为2.15%,在标定的不确定度范围以内,经计算系统检测线为0.014 3 mg·m~(-3)(6.942 ppb),因此可认为测量结果准确。将外场实验测量结果与同时段国控站点给出的NO_2数据进行了对比,对应时间段结果偏差均在10%以内,两组数据线性拟合一致性较好,相关系数达0.967,表明该系统所测NO_2结果较准确。研究结果表明,在确保LED光源稳定的基础上,采用基于窄带光源蓝光LED的DOAS方法能够实现夜间对NO_2气体垂直分布情况测量。为大气痕量气体垂直分布测量、特别是在夜间条件下对痕量气体立体分布测量提供了一种新的思路。
Aiming at the problem that existing passive DOAS methods using natural light sourcecannot detect the vertical distribution of NO_2 and other trace gases at night, this paper proposes a method of DOAS measurement of NO_2 based on the blue LED technology of narrowband light source, and uses this method to build the instrument system. Successfully realized the concentration measurement of NO2 gas by using the instrument system. This system is mainly composed of two parts: a light source emitting system and a telescope receiving system. The LED with the dominant wavelength of 450 nm is used as a light source to collect the scattered light of the light beam through the telescope. The scattered light received by the telescope is imported into the spectrometer through fiber coupling, DOAS principle using a computer for processing. The theoretical basis for DOAS is Lambert-Beer's law, which describes the attenuation of the electromagnetic radiation energy as it penetrates the material. Based on this principle, the data processing can be summarized as follows: Firstly, a relatively clean spectrum is taken as the background reference spectrum, and the actual measured atmospheric spectrum is divided by the reference spectrum, and the digital high-pass filtering is used to remove the slow changes and then take the logarithm to obtain the optical thickness. Secondly, the instrument function is convolved with the high-resolution cross-section of NO_2 to get the low-resolution absorption cross-section matched with the instrument used. Finally, the differential absorption cross-section is combined with the processed differential optical thickness, and the least square method is used to fit out the NO_2 concentration value with the light path L. At the same time, by adjusting the angle of light emission and the receiving angle of the telescope, the NO_2 concentration at different positions can be measured, and then the three-dimensional distribution of NO_2 gas concentration can be obtained. Under the condition ofthe algorithm determined, the quality of the LED light spectrum is particularly important for the reliability of the instrument system. As the temperature and drive current have a greater impact on the LED spectrum, in order to ensure that the LEDs are in the best working condition, carried out the LED spectral temperature and drive current sensitivity experiments. The test results show that to make sure the acquired spectrum is stable and of high quality, the LEDs should operate at a temperature lower than 20 ℃, and the drive current needs to be controlled within 1. 5 A, and both of them should have a small fluctuation range. In the experiments, the LEDs are working with the temperature of 10～15 ℃, the driving current of 1.4 A and the accuracy of driving current of ±1 mA, and all the conditions can meet the experimental requirements. In order to improve the LED array density, to get a more focused beam of light, LED base block with a regular hexagonal structure is chose, and all the blocks have 7 LEDs connected in series, and the blocks are connected in parallel. Compared with using rectangular structure, the space utilization increased by 8% with using regular hexagon structure. At the same time, it is easier to expand and more convenient to maintenance with the working drive of 1.4 A and the maximum voltage of 23.8 V. In order to verify the feasibility of the program and the reliability of system, laboratory tests and outdoor experiments were conducted. The concentration of the sample gas of NO_2 used in the laboratory was 1 642.86 mg·m~(-3) and the uncertainty was 5%. The system measurement was 1 607.54 mg·m~(-3) with an error of 2.15% from the nominal value, within the uncertainty range of the calibration. The calculated system test line was 0.014 3 mg·m~(-3)(6.942 ppb), therefore, the measurement result can be considered as accurate. The results of outdoor experiments were compared with the data of NO_2 given by the national control station over the same period. The results showed that the deviations of the results were within 10% in the corresponding time periods. The linearity of the data fit well with the correlation coefficient of 0.967, indicating that the system NO_2 measured results were accurate. The results of this paper show that the DOAS method based on the blue LED with narrowband light source can measure the vertical distribution of NO_2 gas at night, while ensuring the stability of LED light source. It provides a new idea for measuring the vertical distribution of trace gases in atmosphere, especially for measuring the distribution of trace gases in nighttime conditions.
Aiming at the problem that existing passive DOAS methods using natural light sourcecannot detect the vertical distribution of NO_2 and other trace gases at night, this paper proposes a method of DOAS measurement of NO_2 based on the blue LED technology of narrowband light source, and uses this method to build the instrument system. Successfully realized the concentration measurement of NO2 gas by using the instrument system. This system is mainly composed of two parts: a light source emitting system and a telescope receiving system. The LED with the dominant wavelength of 450 nm is used as a light source to collect the scattered light of the light beam through the telescope. The scattered light received by the telescope is imported into the spectrometer through fiber coupling, DOAS principle using a computer for processing. The theoretical basis for DOAS is Lambert-Beer's law, which describes the attenuation of the electromagnetic radiation energy as it penetrates the material. Based on this principle, the data processing can be summarized as follows: Firstly, a relatively clean spectrum is taken as the background reference spectrum, and the actual measured atmospheric spectrum is divided by the reference spectrum, and the digital high-pass filtering is used to remove the slow changes and then take the logarithm to obtain the optical thickness. Secondly, the instrument function is convolved with the high-resolution cross-section of NO_2 to get the low-resolution absorption cross-section matched with the instrument used. Finally, the differential absorption cross-section is combined with the processed differential optical thickness, and the least square method is used to fit out the NO_2 concentration value with the light path L. At the same time, by adjusting the angle of light emission and the receiving angle of the telescope, the NO_2 concentration at different positions can be measured, and then the three-dimensional distribution of NO_2 gas concentration can be obtained. Under the condition ofthe algorithm determined, the quality of the LED light spectrum is particularly important for the reliability of the instrument system. As the temperature and drive current have a greater impact on the LED spectrum, in order to ensure that the LEDs are in the best working condition, carried out the LED spectral temperature and drive current sensitivity experiments. The test results show that to make sure the acquired spectrum is stable and of high quality, the LEDs should operate at a temperature lower than 20 ℃, and the drive current needs to be controlled within 1. 5 A, and both of them should have a small fluctuation range. In the experiments, the LEDs are working with the temperature of 10～15 ℃, the driving current of 1.4 A and the accuracy of driving current of ±1 mA, and all the conditions can meet the experimental requirements. In order to improve the LED array density, to get a more focused beam of light, LED base block with a regular hexagonal structure is chose, and all the blocks have 7 LEDs connected in series, and the blocks are connected in parallel. Compared with using rectangular structure, the space utilization increased by 8% with using regular hexagon structure. At the same time, it is easier to expand and more convenient to maintenance with the working drive of 1.4 A and the maximum voltage of 23.8 V. In order to verify the feasibility of the program and the reliability of system, laboratory tests and outdoor experiments were conducted. The concentration of the sample gas of NO_2 used in the laboratory was 1 642.86 mg·m~(-3) and the uncertainty was 5%. The system measurement was 1 607.54 mg·m~(-3) with an error of 2.15% from the nominal value, within the uncertainty range of the calibration. The calculated system test line was 0.014 3 mg·m~(-3)(6.942 ppb), therefore, the measurement result can be considered as accurate. The results of outdoor experiments were compared with the data of NO_2 given by the national control station over the same period. The results showed that the deviations of the results were within 10% in the corresponding time periods. The linearity of the data fit well with the correlation coefficient of 0.967, indicating that the system NO_2 measured results were accurate. The results of this paper show that the DOAS method based on the blue LED with narrowband light source can measure the vertical distribution of NO_2 gas at night, while ensuring the stability of LED light source. It provides a new idea for measuring the vertical distribution of trace gases in atmosphere, especially for measuring the distribution of trace gases in nighttime conditions.
引文
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[3] Platt U,Perner D,Patz H.Geophys.Res.,1979,84(10):6329.
[4] Platt U,Perner D,Patz H.Geophys.Res.,1979,84(10):6329.
[5] ZHENG Ni-na,XIE Pin-hua,LING Liu-yi,et al(郑尼娜,谢品华,凌六一,等).Acta Optica Sinica(光学学报),2013,33(3):0301007-1.
[6] Christoph Kern,Sebastian Trick,Bernhard Rippel,et al.Appl.Opt.,2006,45(9):2077.
[7] Chan K L,P?hler D,Kuhlmann G,et al.Atmos.Meas.Tech.,2012,5:901.
[8] ZHANG Bao,GAO Chao,GUO Yong-cai,et al(张豹,高潮,郭永彩,等).Acta Photonica Sinica(光子学报),2018,47(2):61.
[9] LING Liu-yi,XIE Pin-hua,QIN Min,et al(凌六一,谢品华,秦敏,等).Acta Optica Sinica(光学学报),2011,31(12):1230003.
[10] WANG Jian,HUANG Xian,LIU Li,et al(王健,黄先,刘丽,等).Chinese Journal of Luminescence(发光学报),2008,29(2):358.
[11] QIN Min,XIE Pin-hua,LIU Jian-guo,et al(秦敏,谢品华,刘建国,等).Spectroscopy and Spectral Analysis(光谱学与光谱分析),2005,25(9):1463.
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