System Design And Detection Performance Simulation Of Mid-infrared Differential Absorption Lidar For Vocs Detection

VOCs is a key factor of air pollution and the key to whether the secondary air pollution can be effectively controlled in our country.Carrying out research on VOCs spatial stereo detection technology has important scientific significance and social value.It not only provides technical support for the analysis of the causes and temporal and spatial evolution of air pollution,but also provides technical guarantees for the treatment of air pollution.For this reason,this thesis has carried out the research of mid-infrared differential absorption lidar technology applied to the detection of VOCs concentration.Designed a detection system with formaldehyde gas as the research object.Focus on key technologies such as detection wavelength,emission and reception optical path,detection performance and error of the system etc.The main work of the thesis is as follows:(1)The mechanism of the interaction between the atmosphere and the laser and the broadening characteristics of the absorption spectrum of gas molecules are studied and analyzed.The gas concentration inversion formula is deduced and the factors affecting the inversion accuracy are analyzed based on the differential absorption lidar detection theory.Analyzed and discussed the possible error sources and elimination methods during system detection.(2)System design of mid-infrared differential absorption lidar.According to the principle of the detection wavelength selection of the differential absorption lidar system,the Aon andλoff detection wavelengths of the system are selected to be 2778.48 cm-1 and 2777.82 cm-1,respectively.The phase errors caused by the corresponding two interference gases are both less than 0.6%.Choose the even-order aspheric double-mirror collimation system and off-axis two-reverse beam expander system.Through the research and calculation of the optical path,the overall optical transmission efficiency is approximately 53.4%.The catadioptric telescope structure is chosen as the receiving telescope in this article.Through the research and calculation of the optical path,the receiving efficiency is 77.6%when only the central shading is considered.A narrowband filter with peak transmittance>90%is selected.It is planned to use VIGO-PVI-4TE-4 detector as the system photodetector and PicoScope 4824 as the data acquisition unit of the system.(3)The extinction coefficient and backscatter coefficient were corrected corresponding to the differential absorption lidar equation used for mid-infrared detection.The system parameters are optimized,and the echo signal power and signal-to-noise ratio are studied.The results show that The signal-to-noise ratio of the system within 1500m is greater than 10.The ideal detection distance is 904.5m based on considering the detection power of the detector.The influence of factors such as the output laser energy,the concentration of the gas to be measured,the optical receiving area,the concentration of interfering gas and the absorption cross section of the gas molecule on the detection performance of the system are analyzed.The results show that the system can detect H2CO gas with a concentration of 0.17-2.02 ppm in the range of 500-1500m.When the water vapor concentration is 10.17%,the minimum error is about 0.85%.And the simulated detection error is less than 1%when the methane concentration is less than 0.467 standard concentration based on the proportional relationship between the methane concentration and system error.Through the analysis of the influence of molecular absorption cross section on the simulation detection error,it can be concluded that as the temperature increases,the molecular differential absorption cross section continues to increase,and the simulation inversion error gradually decreases.It shows that the system is more suitable for summer measurement when the temperature is higher.In this thesis,the design and detection performance of the differential absorption lidar system for mid-infrared VOCs gas concentration detection were studied.Laid the technical foundation for the development of the mid-infrared VOCs gas concentration detection differential absorption lidar system and the monitoring of the unorganized emission of VOCs in the heavily polluted area of the chemical industry.And also has positive significance to the air pollution control in our country.