Highly-sensitive NO_x/NH₃ Measurements Based On Mid-infrared Wavelength Modulation Spectroscopy

In recent years,global atmospheric pollution is increasingly serious.As the main precursors of secondary fine particulate matters,NOx and NH3 in the air play an important role in the formation of haze and are fundamental indicators of air quality.The accurate data of their distribution,sources and sinks can be instructive to meteorological observation and human's living.Nowadays,more studies on accurate and rapid detections of NOx and NH3 in the atmosphere are needed.This paper performed detailed studies on mid-infrared wavelength modulation spectroscopy.WMS can detect gas samples with high selectivity,high sensitivity and high resolution and can increase the detection frequency to reduce low frequency noise.Combined with maturing quantum cascade lasers,WMS will have the ability to measure strong absorption lines of target gases with a wide spectral range.This technique will greatly improve the detection limits and meet the requirements of atmospheric monitoring.This paper performed detailed research on the mid-infrared absorption lines and spectral features in the range of 1900.0 cm-1,1600.0 cm-1 and 1103.4 cm-1 was selected for NO,NO2 and NH3.To analysis the influence of H2O and CO2 quantitatively,the simulation of absorption spectra of NO,NO2,NH3,together with typical ambient mixing ratios of atmospheric absorbers were carried out.Small short-term concentration fluctuations of H2O and CO2 have slight contribution to the absorption spectroscopy and background 1f/-normalized 2f signals were then subtracted from the demodulated measurement signals to remove their contribution.Meanwhile,the correction method of the spectrum eliminated the impacts of temperature and pressure in different measurement environments.Mid-infrared QCLs always need large input current,high precision temperature control and good heat dissipation.As a result,compact drives were designed to satisfy the demand in the design of electronic system.The precision of the output current,temperature control and double-stage refrigeration module were 0.18 mA,0.001 ? and 0.15 ?respectively.Output mid-infrared beams were combined by wedged dielectric beam splitters in the mechanical structure.Compared with extractive cell,the 60-m open-air multi-pass cell was able to make measurements directly without any sample handling.The measuring software was based on Labview,including modules of time division multiplexing signals generation,digital lock-in,signal normalization and background signal subtraction.This software can display operating status and gas concentrations in real time.Based on the design above,we developed a mobile instrument for monitoring NO,NO2 and NH3 in the open air with fast response and high dection limits.The minimum detection limits and long-term stability of the system were optimized and the performance was tested.Based on actual measurement conditions,the tuning characteristics of three QCLs were measured and the modulation parameters of WMS were optimized.The calibration and Allan variance analysis were performed with an equivalent optical path.Good linear relationships with the correlation factor larger than 0.996 between gas concentration and peak values was observed.The 1-s detection limits for target gases were less than 1 ppb.With an optimum integration time around 100 s,the detection limits for each of all three gases were less than 0.2 ppb.To suppress long-term wavelength drifts of QCLs,we maintain a high temperature stability of laser shells and environment and the daily wavelength drift was minimized to less than 0.01 cm-1.According to the monitoring results,the zero drift and span drift were less than ±2.0%during 25 hours.The indication error,repeatability error and stability error were less than ±5.0%,±5.0%and ±5.0%respectively.The above tests indicated that our system can be used to monitor NOx and NH3 in the open air.Comparision experiment was performed in Jiangsu Environmental Monitoring Center with a commercial instrument(Teledyne,Model T201).And a correlation factor(R2)larger than 0.85 was observed with the regression analysis of the detected data,which proved the reliability.The WMS system was installed in a car to monitor NOx and NH3 on the ring road of Hefei city.And the concentration distribution of NOx and NH3 was measured on the ring road,which was fundamental to NOx and NH3 measurements with different time resolution in different conditions.