| The monitoring of greenhouse gases in complex ecological environment,on different space and time scale,is the basis of understanding the their source and sink distributions.At present,the technical means that adapt to the long-term continuous monitoring of greenhouse gases in ecological environment still remains to be studied.Tuned Diode Laser Absorption Spectroscopy(TDLAS)is a non-invasive spectroscopy technique with high selectivity,high sensitivity and high resolution.Combing TDLAS with the new type of infrared quantum cascade laser(QCL)can achieve the "base frequency" absorption spectral measurements,to further improve the detection sensitivity,and meet the monitoring needs of greenhouse gas in area environments.In this paper,the open-path mid-infrared wavelength modulated TDLAS technique was detailly studied.A open-path simultaneous and continuous monitoring system for CH4 and N20 was designed.The spectral retrieved algorithm for multi-component measurements,with gas cross interference,was further studied.And the rapid and continuous monitoring of muli greenhouse gases is realized.In this paper,the two major greenhouse gases,CH4 and N2O,were selected as the target gases.The absorption spectra of the actual atmospheric environment were simulated and analyzed based on HITRAN database.Two neighboring absorption lines of CH4 and N2O near 1275cm-1 were selected,to realize two-component measurement with a single laser.The modulation characteristics of CW-QCL laser around 1275cm-1 were studied,and combining with the actual measurement environment,the modulation parameters were optimized.In the system design process,the dichroic mirror was used to solve the problem of infrared multi-beam coaxial coupling with long optical path(?km).The reflective off-axis laser collimation structure and the send-receive one light structure based on off-axis parabolic mirror were designed,with which can achieve the monitoring of greenhouse gases with long optical path on km seal.In this paper,we deeply studied the multi-component spectral-concentration retrieving algorithm based on calibration and calibration-free method,respectively.For the calibration method,a multi-component retrieving method of WMS with multiple linear fitting was firstly proposed.The accurate measurements of CH4 and N2O were carried out using this method and the measurement errors were all less than 5%,which proved the feasibility of the cross interference eliminating method.In the WMS calibration-free fitting algorithm,the influence of frequency and intensity nonlinear response on the harmonic signal was studied based on the simulation and the actual measured laser frequency and intensity response.The WMS calinration-free method was optimized by combining measured absorption-free intensity signal and more accurate laser frequency response model.Using non-absorbing light intensity signals,not only avoided the problem of the accurate analytical model establishment wtih the unconventional intensity response,especially the obvious nonlinear response,but also contained all the light intensity information including nonlinear response characteristics,Parasitic interference noise that can not be eliminated and background absorption characteristics.An accurate laser response model,including the first and second order frequency response terms and their time-dependent coefficient,was used to realize accurate laser frequency response,solved the problem of application limitation of the conventional frequency response of conventional laser.The optimized WMS calibration-free method is more universal and suitable for any modulation depth,any absorbance and obvious non-linear non-conventional intensity response cases.The WMS calibration-free method was well experimental varified with CH4 samples,the WMS calibration-free residuals were all within 20%in the concentration range of 60?1200ppm*m,or integrated absorbance range of 0.029?0.57cm-1,and the measurement linearity reached 0.99996.Based on the research of the system and concentration retrieved algorithm,the performance of the measurement system,including stability,linearity and detection limit were tested and analyzed.The test results indicated that the measurement system fully meet the simultaneous measurement requirements of atmospheric CH4 and N2O.The field experiment was carried out on Hefei Science Island,the continuous and high sensitive monitoring of environmental atmospheric CH4 and N2O,with optical path of 690m,was realized,laying a foundation for the high sensitive measurement of greenhouse gases in ecological environment on different area scale and timeresolution. |
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