Research On Spectra Of High Sensitivity Laser Absorption Of Nitrogen Gases Molecules

With the development of society,the social problems caused by pollutant and dangerous gases have been taken into great attentions.Nitrogen oxide are the main pollution gas in atmosphere,which will directly damage human health,and cause various kinds of physical diseases when exposing to high concentration of nitrogen oxide gas.Nitrogen oxide gas are important precursor of ozone and other photochemical secondary pollutants,and it is also the main source of acid rain,acid fog and photochemical smoke.Therefore,the detection of nitrogen oxide gases is of great significance to human health,environmental safety and industrial production.In this thesis,a Nitric oxide?NO?detection system for was established based on tunable diode laser absorption spectroscopy?TDLAS?.It was used for measuring the NO concentration of cigarette emission in different smoking behaviors.The method of Wavelength Modulation Spectroscopy?WMS?was employed to reduce the noise in the measurement process and improve the accuracy of the system.In the experiment,a single pass cell with a volume of 28cm³ and a length of 1 m was employed.Combination of drier and Cambridge filters filtered the particles from cigarette combustion.In order to avoid the interference of other species,R?8.5?of NO fundamental absorption band was selected due to its strong absorption strength.A Distributed Feedback-Quantum Cascade Laser?DFB-QCL?with output wavelength of 5.2?m was used and its current and temperature were monitored simultaneously.According to different manners of people smoking,the NO concentration and masses of three different kinds of cigarette combustion smoke were measured in both the main and side streams at different flow rates.By analysis of Allan variance,the minimum detection limit of NO is 28 ppb at the optimum integration time of 180 s.Beneficial to the low effective volume,the rise time and the fall time were as rapid as 3 s and 6 s,respectively.An adaptive Kalman filter was performed to reduce the noise on the concentration measurement.As a result,the measurement precision is improved by a factor of 4.5.This work provides a promising method for determining the emission of NO and other components in cigarette smoke.In the thesis,a measurement system of NO₂ was implemented based on photoacoustic spectroscopy?PAS?with a 450 nm blue laser of high output power.The pulse mode of laser was adopted in order to avoid the influence caused by mechanical chopper.TTL modulation signal was provided by a signal generator and transmitted to the lock-in amplifier as reference signal to demodulate the 1f signal.The best modulation frequency of modulation and conditions of pressure and flow velocity was obtained by varying the parameters of experiment.In addition,the linear response of the system was calibrated.Based on Allan variance analysis,it indicated that the optimum integration time is 274 s and the corresponding detection limit is46 ppt by analyzing an hour experimental data.The results of 48 hours measuring atmospheric NO₂ show that it is in good agreement with the data of the nearby environmental monitoring station.