Urban Traffic Pollution Gas Monitoring Based On Differential Optical Absorption Spectroscopy

The introduction of Differential Optical Absorption Spectroscopy（DOAS）has enabled us to acquire new technologies for monitoring polluted gases in the atmosphere.The low construction cost and high monitoring efficiency of the DOAS system have become an important method for monitoring the air quality in the atmospheric environment.Based on the principle of differential optical absorption spectroscopy,this thesis develops a differential absorption spectroscopy system for urban traffic pollution gas monitoring.The air over the urban road is monitored for a long time,and the collected data is analyzed and processed.Based on the theoretical basis of absorption spectroscopy,considering the effects of rice scattering,Rayleigh scattering and other extinction terms in atmospheric light absorption process,the Lambert-Beer law formula is modified and a gas concentration measurement formula suitable for actual monitoring is proposed.In the process of pre-processing the spectrum,considering the influence of the deviation of the instrument,the molecular absorption spectrum suitable for the actual monitoring system is obtained by convolving the spectrum measured under laboratory conditions with the instrument function of the monitoring equipment.Based on the modified Beer’s law,this paper proposes a method for filtering the broadband absorption in the spectrum using high-pass numerical filtering.In addition,in order to measure the high frequency noise of the system existing in the spectrum,this topic designs a FIR low-pass filter to filter out the system high-frequency noise.This project designed and developed a new Long Path Differential Optical Absorption Spectroscopy（LP-DOAS）prototype.Considering to improve the signal-to-noise ratio of the system,a system light source with high stability is adopted.The telescopic structure of the transceiver is designed,and the position of the secondary mirror and the primary mirror is determined by repeated calculations,so that the reflected light can be accurately focused on the incident end of the optical fiber.In this paper,the parameters of the spectrometer are calculated and selected,and the appropriate slit width and grating are selected to improve the utilization of light.The system structure is simplified as a whole,and the accuracy of system detection is improved.Regarding the gas concentration inversion part,this project designed a data acquisition software that can automatically collect spectra and simultaneously invert gas concentration.The software realizes the function of automatically adjusting the integration time,and can perform on-line pre-processing of the spectral signals collected by the spectrometer.For the inversion algorithm,the differential absorption spectrum algorithm based on least squares method is used to calculate the gas concentration.Finally,the real-time concentration curve is displayed on the software interface,and the measurement data is automatically stored in the computer.This project uses the developed DOAS system equipment to conduct long-term monitoring of SO₂,NO₂ and O₃ pollutant gases near Yinchuan Road,Haier Road,Laoshan District.In this paper,the measured data is compared with the data of the neighboring Laoshan District Environmental Monitoring Station,and the correlation is good,which verifies the accuracy of the data.The measured data verified that the correlation between vehicle exhaust emissions and NO₂ concentration above the road is high,which proves that motor vehicle emissions are the main source of NO₂ pollution in the region.The experiment verifies that the DOAS system can be used to monitor the pollutant content of urban traffic emissions,and provides a reference for the monitoring and control of urban pollutants.