Research On The UV-enhanced DOAS Technology For Air Quality Monitoring Based On Optical Fiber Coupling

With the development of economy and improvement of living standards, air quality has drawn more and more attention of the society. But with the development of industry and transportation, air pollution becomes worse every day. That’s why we need an effective method monitoring the air quality. Differential Optical Absorption Spectroscopy (DOAS) has a great potential in monitoring the atmosphere. In view of the problem of insufficient UV intensity and SNR of existing Differential Optical Absorption Spectroscopy (DOAS) atmospheric monitoring system, the paper developed a system of UV-enhanced DOAS system with optical fiber coupling for air quality monitoring, and do a comprehensive performance test for the system. The main contents are as follows.Firstly, the paper studied the principle of Differential Optical Absorption Spectroscopy. On the base of Lambert-Beer Law, Rayleigh scattering, Mie scattering and the effects of the extinction of other factors should be understood. And the amended Lambert-Beer Law is demonstrated.Secondly, the paper analyzes the principle and characteristics of the existing DOAS atmospheric monitoring system. On this basis, the paper proposed a new atmospheric monitoring system. The system is optimized based on the structure of combined transmitting-receiving fibers. The diameter of main mirror of the optical fiber coupling telescope is 20.1cm, while the diameter of its emitting collimated light at the spot of 20m is only 24cm.The paper use UV-enhanced xenon lamp as light source of the system and the light source system uses fiber-couple output design. To test the UV intensity of the optimized DOAS system in the open air, and the results shows that once the integration time of spectrometer is only 15ms the received light intensity reach more than 80% of the maximum.In addition, compared with the traditional DOAS system, the new DOAS system has many advantages such as high flexibility, high portability, and it is easier to adjust the optical path.Thirdly, the paper research and improved the algorithm of DOAS technology. Savitzky-Golay filter fitting is used to separate fast and slow changing parts of the absorption spectrum instead of the traditional polynomial and it improved the accuracy of measurement.Fourthly, SO2, NO2 standard gas is used to conduct calibration experiments of the system, The experimental results show that the maximum measurement error of SO2 is 4.19% and the maximum measurement error of NO2 is 5.22%.Finally, The system is used to continuously monitor the atmospheric pollutants SO2, NO2 and O3 in the industrial field and a comparative analysis is done. The results show that the performance of the system is advanced, and it can fully meet the requirement of on-line monitoring of atmospheric quality on low visibility weather such as fog.