Study On Theory And Technology Of Trace Gas Detection Based On Mid-infrared Laser Absorption Spectroscopy

Mid-infrared laser-based absorption spectroscopy techniques have been identified as powerful tools in trace gases detection for applications in environmental monitoring,atmospheric chemistry,industrial process analysis and control,medical diagnostics,etc.A number of trace gas sensors have been developed and demonstrated by taking advantage of recent,significant advances in the commercial availability of high performace mid-infrared lasers,such as diode lasers,interband cascade lasers(ICL)and quantum cascade lasers(QCL),in the spectral range of 3-12 ?m known as the "fingerprint" region of trace gases.Trace gases usually exhibit weak absorption fearures due to their relative scarcity in the ambient air,so that the ability to perform sensitive and precise measurements of trace gases is required for the sensors.The utilization of a multi-pass gas cells(MPGCs)in the trace gas sensor system is a common way to increase the effective gas absorbing path-length for the improvement of the detection sensitivity.Another method called wavelength modulation spectroscopy(WMS)is widely used in trace gas detection.In WMS,the wavelength of the laser is modulated at a high frequency,which leads to a modulation of the transmitted intensity in the vicinity of the absorption line.The transmitted intensity detected is then demodulated resulting in harmonic signals.The second harmonic(2f)signal of WMS is always extracted to represent the absorption feature.Therefore,the combination of a MPGS and the WMS can significantly increase the detection sensitivity.Water vapor,nitrous oxide(N2O),and methane(CH4),known as the non-carbon dioxide(CO2)greenhouse gases,have contributed to climate forcing,whicah influences human life and natural system.In chapter 4,we will discuss the recent development of a continuous wave,external-cavity-quantum cascade laser(CW EC-QCL)-based sensor for simultaneous monitoring of H2O,HDO,N2O and CH4 with 1 s sampling interval in a narrow spectral range of 1281.05-1281.66 cm-1.An effective optical path-length of 57.6 m was achieved by using a dense pattern MPGC with a compact physical size of 17×6.5×5.5cm3.A strategy of using 1f-normalized peak-to-peak amplitudes of WMS-2f was implemented to process the acquired spectral data.The noise level of the sensor system was evaluated by the Allan-Werle variance method.The minimum detection limits of 1.77 ppmv for H2O,3.92 ppbv for HDO,1.43 ppbv for N2O,and 2.2 ppbv for CH4 can be achieved with averaging times at 50 s,50 s,100 s,and 129 s,respectively.The long-term stability of the four-gas sensor was verified by performing a 10-hour and a one-week long ambient monitoring campaigns.Nitric oxide(NO)and nitrogen dioxide(NO2)are prominent atmospheric pollutants,playing important roles in controlling the photochemical production of tropospheric ozone(O3),determining the concentration of the hydroxyl radical(OH)and contributing to the formation of secondary organic aerosols as well as acid precipitation.Simultaneous detection of NO and NO2 is critical for environmental monitoring and climate research.In chapter 5,we will report a dual QCL-based sensor system for simultaneous detection of the two most common atmospheric polluting nitrogen oxides(NOx)with a sampling rate of 1 Hz by using an astigmatic Herriott MPGC with an effective optical path-length of 76 m.A wavelength modulation-division multiplexing(WMDM)strategy was implemented to realize the simultaneous monitoring of NO at 1900.075 cm-1 and NO2 at 1630.33 cm-1 with a large spectral interval of?270 cm-1.Four implemented Labview-programmed lock-in amplifiers were cost-effective and simplified for the sensor system.According to the Allan-Werle variance analysis,the minimum detection limits can be sub-ppbv concentration levels,which are sensitive enough to quantify atmospheric NOx.