Key Techniques Of No₂ Measurement By Mid-infrared Differential Absorption Lidar

As many molecules have abundant gas absorption peaks in the mid-infrared band,it is essential to study gas absorption spectrum in the infrared band for trace gas detection,and the propagation in the atmosphere during the day is less affected by visible light.It is of great significance to study the absorption spectrum of gas in the mid-infrared band for detecting trace gases.At the same time,the study of the differential absorption lidar in the mid-infrared band is of great significance for the subsequent detection of other gas molecules in this band.In order to measure the absorption cross-section of NO₂ gas with strong absorption in mid infrared band,the absorption cell experiments were carried out using two wavelengths?_?and?_ttemitted from OPA lasers.The absorption cross section of NO₂ gas in the range of 3410-3433nm is measured by a laser whose spectral line-width is less than 0.05nm.The absorption cross sections at 291K,308k and 363k were acquired respectively.The experimental results show that the absorption cross-section is negatively correlated with temperature,consisted with HITRAN database and the correlation coefficient between the measured data and the HITRAN data is above 0.92.The absorption spectra from 3400 nm to3435 nm were also acquired by a laser whose spectral line-width is 10nm.The absorption spectra were corrected through convolution between?_ttand absorption sections.The measured results were in good agreement with the corrected ones.Considering the measurement results of absorption cross section,the concentration error of differential absorption lidar?DIAL?is simulated,which verified the feasibility of the scheme of DIAL.A small prototype of mid-infrared differential absorption lidar is used to detect the external field.The echo signal of the building at 1800 m is detected as far away as possible,and the aerosol and other particulate matter are also detected.The aerosol can be detected by a small system.Particulate signals such as water vapor,smoke,etc.The receiving and transmitting coaxial of mid-infrared differential absorption lidar is debugged.The debugging results show that the receiving and transmitting optical paths of the system are at the same point at 300m and 1200m,and the coaxial property of the system is good.The external field verification proves that the mid-infrared band laser has good propagation characteristics and detection ability in the atmosphere.The lidar system is used to detect the hard target,cloud and gas,and the system can detect the gas signal.The concentration detection was carried out,and the variation curves of NO₂ column content at different concentrations were calculated by adding different concentrations of NO₂,into the gas absorption cell,and the calculated results were in good agreement with the gas concentration filled into the absorption cell.NO₂was released in the atmosphere and the concentration was measured.The measured results are in good agreement with the experimental process.