Research On Remote Sensing Technique Of CO/CO₂/NO Based On Tunable Laser Absorption Spectroscopy And Its Application To Roadside Vehicle Emission Detection

CO,CO₂ and NO are very important products in the vehicles emissions,as well as the key objects in many plants and various of combustion-related industries.The remote sensing study of CO,CO₂ and NO has been of great significance and practical value for the fields of vehicle emission detection and real-time control of industrial processes.In this context,based on the tunable laser absorption spectroscopy technology,this paper carries out the research on the online detection technology of CO,CO₂ and NO,builds the corresponding measurement system and applies it to the vehicle emission detection.The main research contents of the paper are as follows.In this paper,the theoretical basis of tunable absorption spectroscopy technology is firstly described.Detailed elaboration of the measurement principles of direct absorption spectroscopy and wavelength modulation spectroscopy technology based on tunable laser absorption spectroscopy technology and their application occasions are conducted.In order to apply the direct absorption spectroscopy techniques to gas remote sensing,the relevant spectral fitting algorithms are studied.In particular,for the DAS technique,the Levenberg-Marquardt algorithm is proposed for the online fitting of the spectra.The study of these detection technique theories and their implementation algorithms laid the theoretical foundation for the subsequent experimental measurements.Secondly,the simultaneous online detection of CO and CO₂ based on tunable laser absorption spectroscopy combined with time-division multiplexing technology was studied.Two DFB lasers operating at 2.0?2.3?m were used to detect the R(10)and R(16e)absorption line near 2.3?m and 2.0?m,respectively,to achieve the CO and CO₂concentration measurements.The structure design of the optical-mechanical integration ensures the portability and reliability of the multi-component gas remote sensing device.For the software design of the host computer,multi-threading technology is used to achieve parallel execution of multiple tasks to improve the program execution efficiency and achieve accurate and fast online measurement of multi-component gas concentrations.For this detection system,the detection limit of CO concentration can reach 2 ppm with accuracy approximately 25ppm.The detection limit of CO₂concentration can reach 6 ppm with accuracy approximately 20ppm under the condition of 1m optical length.Subsequently,the NO online detection technology based on mid-infrared laser was studied.The spectral band near 5.2?m was selected for NO detection,and two sets of mid-infrared gas detection systems based on ICL and QCL were designed and validated respectively.For each set of the detection and inspection system design,the selection of absorption lines,the composition of optical system,hardware design and algorithm design are descripted in detail.For the ICL-based measurement system:an ICL operates near 1926.29 cm^-1 was selected for NO detection.In the range of 0?2000ppm,the linear relationship between the second harmonic peak values and the gas concentration is good with the correlation coefficient greater than 0.99.However,in the actual measurement process,the laser power received by the detector was too weak(<1m W),which reduced the SNR of the detection system.To increase the laser power received by the detector,a portable NO detection device based on WMS was designed using a QCL operating at 1900.08 cm^-1 as the laser source.The modular device design and the flexible software design allows the device to achieve a temporal resolution of ms.A new method of NO measurement using software-based lock-in amplifier for normalized second harmonic detection combined with Gabor filtering is proposed.For the lock-in amplifier used in the wavelength modulation spectroscopy technique:a software-based lock-in amplifier is designed instead of the traditional analog lock-in amplifier,and a simulation study based on Lab VIEW was conducted.The simulation results show that the linear relationship between the normalized second harmonic peak and the gas concentration is good,and the fitting coefficient R² reaches 0.99987.For the noise reduction study of the system:it is found that when the original signal SNR is 0 d B,the SNR of the signal after noise reduction by Gabor filter reaches 18 d B,and the relative error of the filtered 2f peak value is less than 3%through simulation.The linear fit correlation coefficient R² between the normalized peak values of the 2f signals and the NO concentrations was found to be 0.9989.According to the Allan variance analysis,the detection limit could reach 42 ppb with the integration time of 3s,and the NEA coefficient reached 2.8×10^-10 cm^-1Hz^-1/2.Lastly,the remote sensing results of vehicle emissions for roadside motor vehicles are analyzed.Finally,the remote sensing measurements of different vehicle emissions were carried out.The quantitative analysis of CO/CO₂ and NO/CO₂ can be achieved based on the path integrated concentration ratios considering the influence of exhaust gas diffusion.The results showed that the CO/CO₂ content in exhaust emissions of diesel vehicles was slightly lower than that of gasoline vehicles,indicating that diesel vehicles burned more adequately compared to gasoline vehicles.However,for NO emissions,the NO/CO₂ in the exhaust emissions of diesel vehicles is significantly higher than that of gasoline vehicles,about 3 times,and this index can be directly used to distinguish the type of fuel used by vehicles.The use of remote sensing technique provides an efficient method to monitor vehicle emissions.Collection of more vehicle emission sample data can be used for further in-depth research about the emission characteristics of various types of vehicles more accurately.