| With the improvement of living standard,it is becoming the trends for green development and green life.Nitrogen dioxide（NO2）is one of the important pollutant gases,although its concentration is more than a dozen ppb level in the atmosphere.It is a crucial reactant in controlling the ozone in the troposphere and promoting the formation of acid rain.NO2 is deadly for human respiratory and lung function at high concentration exposure.Methane（CH4）is an explosive trace gas.In the atmosphere,the main sources are natural gas and biogas.In addition,exceeding the specified concentration（5%）in some mines and processing will seriously endanger production safety.CH4 is low in content（several ppm）as one of the greenhouse gases.However,CH4 has produced about of 20%amount for global warming.Therefore,the detection of NO2 and CH4 is of significance to human health,production security and environment protection.（1）A NO2 sensor with sub-ppb sensitivity was developed based on resonant photoacoustic spectroscopy in this thesis.The output of laser light source is 3.5W and emits at 450 nm,because the absorption of NO2 is extremely strong around450 nm.Furthermore,a TTL signal was produced by a wavefunction generator and used to modulate the amplitude of laser replacing a mechanical chopper for avoiding the noise due to mechanical instability.A mirror was placed to reflect the beam back for increasing the effective optical path-length and enhance the detection sensitivity.In addition,the resonance frequency,the parameters of cell pressure and flow were optimized for obtaining maximum signals.The limitation of detection（LOD）could reach 46 ppt according to Allan deviation analysis.Finally,it was validated via monitoring the atmospheric NO2 for 48 hours.A Kalman filtering algorithm was applied to reduce the noise.It illustrated that our results are in good agreement with the ones of CNEMC.（2）Herein,a Herriott multi-pass enhanced–resonant photoacoustic cell was developed for sensing CH4 gas.The signal intensity can be enhanced through increasing the optical pathlength between the light and matter.Eventually,the sensitivity is significantly improved by a long pathlength absorption.Two concave mirrors were installed at both ends to construct the Herriott cell.The reflections can reach 34 times,so its pathlength be increased to 6.8 m from 0.2 m.It illustrated the amplitude of signal was improved 13 times.A near infrared DFB laser was chosen as light source for measuring CH4 gas.The parameters of resonant frequency,optimal pressure and flow were derived by observing the amplitude of signals.The wavelength modulation spectroscopy was employed to suppress the background and noise from environment.The LOD of CH4 sensor could achieve 117 ppb at the integrated time of 54.6 s.Furthermore,it was deployed for 2 day’s monitoring for verifying its feasibility. |
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