Detection of Atmosphere NO2 Based on Quartz Enhanced Photoacoustic Spectroscopy Technique
The Quartz-enhanced Photoacoustic Spectroscopy(QEPAS)technology boasts high sensitivity,rapid response,and compact size.It has emerged as one of the prominent methods for detecting atmospheric pollutants in recent years.The on-beam Quartz Tuning Fork(QTF),with its superior anti-interference capability and stronger photoacoustic coupling effect compared to off-beam quartz tuning forks,serves as the foundation for designing a on-beam quartz-enhanced photoacoustic spectroscopy system for atmospheric NO2 detection.The quartz tuning fork,adopting a T-shaped structure,reduces resonance frequency while maintaining a high quality factor,thus increasing the amplitude of the photoacoustic signal.Finite element analysis using COMSOL Multiphysics software simulates the tuning fork,revealing its first six modal shapes and resonance frequencies.The fourth vibration mode is identified as symmetrical and effective,with a simulated characteristic frequency of 12 418 Hz.Experimental calibration of the tuning fork's resonance frequency under standard atmospheric pressure yields 12 464.5 Hz with a quality factor of 12 850,indicating a relative error of 0.37%compared to the simulation model,affirming the model's validity.The acoustic detection module of the NO2 sensor features a chamber volume of only 7 cm3,offering the advantages of small size and light weight.The laser source for the NO2 quartz-enhanced photoacoustic spectroscopy sensor employs a 150 mW Xilong Optoelectronics DL_405 semiconductor laser emitting at a wavelength of 403.56 nm with a full width at half maximum of 0.84 nm.The NO2 absorption cross-section in the central wavelength spectral region is approximately 5.948 5×10-19 cm2/molecule.The coupled power after optical coupling,measured using a Hamamatsu S1223-01 photodiode,is 45.6 mW.Experimentation optimizes the impact of gas flow rate on noise,with noise levels rapidly increasing above 80 sccm,thus selecting 60 sccm as the optimal flow rate.Evaluation of the NO2 quartz-enhanced photoacoustic spectroscopy sensor's performance across different NO2/N2 concentration mixtures,ranging from 100×10-9 to 10×10-6 NO2 levels,utilizes linear regression equations to assess sensor response,fitting NO2 concentration to photoacoustic signal values,resulting in an R-square value of 0.999.Allan variance analysis examines the long-term stability of the NO2 sensor,introducing pure N2 into the chamber,yielding a detection sensitivity of 3.09×10-9 and a corresponding normalized noise equivalent absorption coefficient of 1.32×10-8 cm-1·W·Hz-1/2 at 1 s averaging time and standard atmospheric pressure.When extending the averaging time to 434 s,the lowest detection limit is 0.32×10-9,indicating diminishing maximum NO2 concentration values and Allan variance with increasing integration time.To validate the real-time monitoring performance of the T-shaped NO2 quartz-enhanced photoacoustic spectroscopy sensor,continuous real-time monitoring is conducted for two weeks from April 4th to April 17th,2024,outside the laboratory of the School of Artificial Intelligence,Anhui University of Science and Technology,Huainan,Anhui Province,China.During the experiment,NO2 concentrations ranging from 3×10-9 to 18×10-9 are measured,with an average concentration of 8.6×10-9.The consistency between the two systems is satisfactory:the nonlinear fitting slope is 0.83±0.012,the intercept is 1.35±0.11,and the correlation coefficient is 0.87.The experimental results align closely with those of the environmental monitoring station,affirming the system's capability for ppb-level online NO2 detection and real-time monitoring of NO2 concentration fields,validating the sensor's stability and reliability.
Photoacoustic spectroscopyQuartz tuning forkFinite element simulationNO2Gas Sensing
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