Research On Mid-infrared Urban Atmospheric Methane Mobile Detection Technology

With the development of industry,atmospheric methane concentration has shown an increasing trend year by year.At present,as a kind of trace gas,the methane concentration level in the atmosphere is about 1.8 ppm(parts per million).However,the slight increase of methane concentration can exacerbate the greenhouse effect,which is closely related to global climate change and human activities.Due to the large area of city coverage,complex road traffic conditions,real-time climate or environment changes,at the same time,we consider the high cost of optical gas sensors,and it is impossible to build a sensor network to achieve large-scale detection.The mobile detection technology in vehicle or airborne mode has become the main technology for rapid and large-scale monitoring of urban air quality because of the above factors.When detecting gas in a vehicle,in addition to the gas concentration,it is necessary to detect the environmental variables such as the location of the measurement point,vehicle speed,driving direction,ambient temperature and humidity,atmospheric pressure,wind speed and wind direction in real time.Therefore,the study of mid-infrared urban atmospheric methane movement detection technology has important application value.We usually use infrared spectroscopy to detect trace gases.Tunable Diode Laser Absorption Spectroscopy(TDLAS)technology has the advantages of good selectivity,high sensitivity,and non-contact measurement,which has become one of the effective methods for real-time online detection of trace gases.In this thesis,based on TDLAS technology,we design and implement an urban atmospheric methane mobile detection system.The specific work is as follows:First of all,the principle of TDLAS technology is detailed.The absorption spectroscopy used for trace gas detection and the harmonic detection technology combining TDLAS and Wavelength Modulation Spectroscopy(WMS)are introduced.In order to extract the harmonic signal,the principle and design method of digital quadrature lock-in amplifier are presented.Mathematical modeling and simulation of the TDLAS-WMS gas sensor system were performed on the Matlab platform,and the feasibility of the technology was theoretically verified.Secondly,the overall structure and design scheme of the methane mobile detection system based on TDLAS-WMS technology are introduced.A methane sensor system based on TDLAS-WMS is designed,which includes the choice of absorption line,photoelectric characteristics of laser,gas chamber structure and detector performance.In order to control each sensor module and sample data,a main control circuit based on Digital Signal Processor(DSP)is designed.At the same time,the weather station module for mobile sample of atmospheric environmental parameters is introduced.Then,a mobile data acquisition and control platform based on LabVIEW is designed.A data communication module is designed for data interaction between the LabVIEW platform and the TDLAS system.A drive signal generation module is designed to drive and control the laser.A data processing module based on orthogonal phase-locked amplification is designed to extract the second harmonic from the absorption spectrum signal,at the same time,data fitting and wavelet denoising are used to further improve the detection accuracy of the system.A mobile monitoring interface is designed to display the collected sensor data and change trends in real time.In the end,the performance test of the designed urban atmospheric methane mobile detection system was carried out.The photographs of the integrated TDLAS sensor optical system and the DSP main control electrical system were presented.The modulation depth of the sensor system was optimized,and the gas absorption optical path was verified.The sensor was calibrated using several gas samples with certain concentration levels and the goodness of linear fitting between the amplitude of the second harmonic signal and the concentration of methane reached 0.99991.A long-term stability test was carried out,and the results showed that the lower detection limit of the system was 14.1 ppb(parts per billion)when the integrationtime was 1 s.The experiment proved the practicality of the urban atmospheric mid-infrared mobile detection system based on TDLAS technology.The main innovation of this thesis: In order to meet the needs of urban atmospheric methane mobile detection,the urban atmospheric mid-infrared mobile detection system based on TDLAS is designed to measure,collect and transmit methane concentration,location,vehicle speed,environmental temperature and humidity,wind speed and wind direction during the mobile detection in real time.