Study On The System And Inversion Method Of Continuous Wave Nitrogen Dioxide Differential Absorption Lidar

Nitrogen dioxide（NO₂）is a common air pollution gas,which has great harm to the natural environment and biological activities.With the progress of industrialization,it is more necessary to detect and monitor it effectively.Compared with the point measurement scheme,the advantage of differential absorption lidar（DIAL）technology is that it has the ability to detect the spatial and temporal distribution of air pollutants.It has a very important application value in the field of atmospheric environment monitoring.It is developed based on the complex narrow band pulse laser technology.At present,it has achieved some phased results.However,most of them are still in the stage of scientific research due to the large amount of maintenance and high cost in practical application.SLidar is a new lidar technology based on Scheimpflug principle proposed by Lund University.It has the advantages of low cost,stable performance and high integration.The continuous wave NO₂-DIAL technology in this study is the exploration of SLidar technology in NO₂ concentration monitoring.The main content of this work includes three parts.Firstly,build a CW-NO₂-DIAL system.A high power continuous wave diode laser is used as the light source,and a CMOS image sensor with high sensitivity is used as the signal detector.The differential absorption wavelengthl_on（located at the NO₂ absorption peak）andl_off（deviated from the NO₂absorption peak）are obtained by quickly adjusting the input current of the laser.The laser beam is collimated by the cylindrical lens group and the refractive telescope,and then emitted into the atmosphere.The backscattering signal is received by the large aperture Newton telescope,and the echo signal is recorded by the CMOS image sensor.The whole system is triggered by the timing signal from CMOS image sensor to realize the synchronization of laser emission,spectrum acquisition and signal acquisition.In addition,completed the inversion algorithm of NO₂ concentration based on MATLAB.The absorption spectrum of NO₂ is obtained from Hitran database.The emission spectrum of 450nm diode laser is measured.The equivalent differential absorption cross section can be calculated by combining the absorption cross section of NO₂ with the emission spectrum of laser.In the case of weak absorption,the inversion of NO₂ concentration can approximately use the least square linear fitting method,and the average NO₂ concentration in a range can be calculated from the attenuation slope of the differential absorption curve and the equivalent differential absorption cross section.Finally,the DIAL system was used to measure NO₂ concentration in the near horizontal path of Dalian city for a long time.Two typical measurement results are given in this paper,which are in good agreement with the national monitoring station data.It can be considered that the system can effectively monitor the atmospheric background NO₂concentration.When the fitting distance is 500 m,the integration time is 5min,and the signal is filtered and smoothed,the detection sensitivity can be achieved±2.5μg/m³.The detection range is 3km.The continuous wave differential absorption lidar based on SLidar technology proposed in this paper has the advantages of low cost and stable performance.It creatively solves the problems encountered in the practical application of DIAL technology,and opens up a new possibility for outdoor NO₂ remote sensing application.