Detection Technology And Application Of Trace Ethane In Urban Atmosphere

Ethane（C₂H₆）is the predecessor of tropospheric ozone.The chemical reaction between C₂H₆ and hydroxyl radicals will affect the oxidation capacity of the atmosphere,and which reaction products are also one of the sources of major environmental pollution.Although the environmental problems had improved in recent years and the concentration of C₂H₆ in the atmosphere had decreased significantly,C₂H₆as the second largest hydrocarbon in the atmosphere and an important gas component in fossil energy,its detection value and research value are still very high.C₂H₆ is the second largest component of natural gas.With the increasing popularity of natural gas as a clean energy,related safety accidents are also increasing year by year.Therefore,it is very necessary to build a sensor detection network and realize large-scale and rapid inspection.Based on the above problems,according to the principle of infrared gas detection,a mid-infrared tunable diode laser absorption spectroscopy（TDLAS）urban atmospheric trace C₂H₆ sensor was developed,which realized the high-precision detection of atmospheric C₂H₆.Firstly,the gas absorption line was selected and the detection scheme was formulated,and then the overall structure of the sensor was designed.The sensor was divided into three parts:optical subsystem,electrical subsystem and gas sampling subsystem.The optical part adopted a commercial interband cascade laser（ICL）,a multi-pass gas cell（MPGC）and a mercury cadmium telluride（MCT）thermoelectric refrigeration mid-infrared detector,which formed a compact linear optical path structure.According to the selected absorption line,the working current and working temperature of the ICL were determined.The electrical part adopted a self-developed low-power,small and low-noise digital lock-in amplifier（DLIA）module,a main control module and a pressure control module to realize the functions of driving signal generation,concentration extraction,sensor automatic control,MPGC pressure control and so on.The gas sampling subsystem,as the first checkpoint of the sensor gas flow,realized the cleaning and drying of the gas.On the basis of considering compactness and maintainability,the whole sensor achieves small size and high space utilization.Then,the noise performance of the key electrical module DLIA was analyzed.The causes and suppression methods of noise were analyzed from three aspects:zero input noise,digital signal processing noise and driving signal phase noise,and these noise values were quantified.Aiming at the problem of insufficient sampling rate of DLIA,an equivalent sampling method based on DSP was proposed.The combination of equivalent sampling method and high-order harmonic extraction technology not only solved the problem of sampling rate,but also expanded the detection function of the sensor system.These two technologies have achieved a win-win situation.Then,aiming at the problem of large concentration fluctuation in 2f detection mode,a 2f spectral linear inversion method based on Lorentz linear harmonic analysis was proposed.The 2f spectral line-type was accurately inverted by MATLAB,and then the line-type inversion formula was transplanted to the embedded system.Through the repeated iteration and fitting of the inverted line-type and the measured 2f spectral waveform of the sensor,the real concentration value was found out,and the further filtering processing of the concentration detection results was realized.Finally,the overall performance of the designed sensor system was evaluated.The excellent linearity of the system was verified by calibration fitting（goodness of fit:0.9996）.The stability experiment for a long time showed that the lower detection limit of the sensor was 3.45 parts per billion（ppb）in the average time of 0.5 s.In the system response time test,the fastest response time of the sensor was 6 s.The above experiments showed that the sensor could realize fast and high-precision C₂H₆concentration detection.In order to verify the application capability of the sensor system on site,application experiments such as fixed-point,mobile,small-scale detection and large-scale inspection were carried out.The results showed that the sensor system could realize the detection of trace C₂H₆ in urban atmospheric and stable field application.the portable mobile urban atmospheric trace C₂H₆ sensor system,the equivalent sampling method and high-order harmonic extraction technology based on DSP,and the 2f spectral linear inversion filtering method based on Lorentz linear harmonic analysis were the innovations of this paper.