Telemetry Research of NO2 Concentration in the Night Based on LED and DOAS Method
YANG Lei1, 2, LI Ang1*, XIE Pin-hua1, 2, HU Zhao-kun1, 2, LIANG Shuai-xi1, 2, ZHANG Ying-hua1, 2, HUANG Ye-yuan1, 2
1. Key Laboratory of Environmental Optics and Technology, Anhui Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, Hefei 230031, China
2. University of Science and Technology of China, Hefei 230026, China
Abstract：Aiming at the problem that existing passive DOAS methods using natural light sourcecannot detect the vertical distribution of NO2 and other trace gases at night, this paper proposes a method of DOAS measurement of NO2 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 NO2 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 NO2 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 NO2 concentration at different positions can be measured, and then the three-dimensional distribution of NO2 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. 5A, 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 NO2 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 NO2 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 NO2 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 NO2 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.
杨 雷，李 昂，谢品华，胡肇焜，梁帅西，张英华，黄业园. LED光源DOAS方法的夜间NO2气体浓度遥测研究[J]. 光谱学与光谱分析, 2019, 39(05): 1398-1405.
YANG Lei, LI Ang, XIE Pin-hua, HU Zhao-kun, LIANG Shuai-xi, ZHANG Ying-hua, HUANG Ye-yuan. Telemetry Research of NO2 Concentration in the Night Based on LED and DOAS Method. SPECTROSCOPY AND SPECTRAL ANALYSIS, 2019, 39(05): 1398-1405.
