Measurements of line strengths for NO2 near 6.2 μm using a quantum cascade laser spectrometer

https://doi.org/10.1016/j.jqsrt.2020.107047Get rights and content

Highlights

  • Line intensity of NO2 in the 6.2 μm region have been measured.

  • Calibration free absorption spectroscopy is used for determining spectroscopic parameters.

  • A self-established spectral analysis algorithm integrated with the continuous wavelet transform is developed to determine the integrated absorbance areas of individual peaks.

  • Mid-infrared QCLs are promising spectroscopic sources for developing analytical instrumentation.

Abstract

A high-resolution quantum cascade spectrometer was developed to study nitrogen dioxide (NO2) line intensities near 6.2 μm. The spectral region ranging from 1629.7 cm−1 to 1632.5 cm−1, which is suitable for the in situ laser sensing of trace NO2 in the atmosphere, was investigated using a thermoelectrically cooled continuous-wave distributed feed-back quantum cascade laser. Forty-three lines of nitrogen dioxide were experimentally studied within the pressure region ranging between 0 and 90 mbar. The measured intensities were thoroughly compared to the latest HITRAN16 database, a good agreement within ±3% was obtained for most of the established lines.

Introduction

Nitrogen dioxide (NO2) is a common pollutant that comes primarily from the emissions of burning fossil fuels, natural lightning, and microbial processes in soil [1]. Atmospheric NO2 contributes to the formation of ground-level ozone [2]. It can cause photochemical smog and lead to increased acidity of rain [3]. Continuous exposure to high NO2 concentration may result in a wide variety of short- and long-term adverse health effects on the respiratory system of humans and animals. Therefore, developing a cost-effective and robust sensor system for NO2 monitoring is crucial.

Many technical solutions have been developed for NO2 detection [4], [5], [6], [7]. The chemiluminescence and wet chemical analysis are commonly used for NO2 detection [8]. However, these methods have a slow response time and suffer from low selectivity in discriminating between NO and NO2, which limit their application. Optical methods based on absorption spectroscopy provide powerful access for trace gas analysis with extremely high sensitivity, selectivity, and fast response. NO2 features several absorption bands in the UV–vis and infrared (IR) region [9,10]. The IR rotational and vibrational bands are more suitable for NO2 detection than the electronic transitions in the UV–vis because they have substantially simpler and unblended spectra. Laser-based absorption spectroscopy techniques in mid-IR molecular fingerprint region are ideal for trace gas analysis because most atmospheric species have strong fundamental vibrational transitions in this spectral region, which allows highly sensitive and selective detection of trace gases [11]. The commercial available continuous-wave (CW) quantum cascade lasers (QCLs) in the mid-IR spectral region have been widely used for developing spectroscopic techniques for quantitative analysis of NO2 [12,13]. These lasers have the advantages of high output power, room-temperature (RT) operation, extremely narrow linewidth, and wide tunability except compactness. A spectrometer based on two time-division multiplexed, CW-QCLs emitting at ~1600 and 1900 cm−1, respectively, able to perform direct absorption spectroscopic measurements of tropospheric NO and NO2 with a multi-pass gas cell (MPGC) of 204 m optical path-length has been reported [14]. Recently, a spectrometer using a wavelength modulation-division multiplexing (WMDM) scheme and multi-pass absorption spectroscopy is reported. A CW distributed-feedback (DFB) QCL and a CW external-cavity (EC) QCL are used to perform measurements of NO and NO2 with a multi-pass gas cell of 76 m optical path-length. The minimum detection limits of NO and NO2 can reach sub-ppbv concentration levels with averaging times of 100 and 200 s, respectively [15]. Using cavity-enhanced methods, Courtillot et al demonstrate NO2 detection on the ppbv level with extended cavity diode lasers (ECDL) at around 410 nm [16]. The nitrogen dioxide (NO2) sensor system based on Faraday Rotation Spectroscopy (FRS) has been developed, and a NO2 detection sensitivity of ~95 ppt is obtained by averaging the associated data for 300 s [17]. The tunable laser-based photoacoustic spectroscopy is used to achieve the sub-parts-per-billion-level detection of NO2 [18]. A quantum cascade laser photoacoustic sensor using pulsed QCLs with 6.2 μm and 8 μm wavelengths are used to measure concentrations of NO2 and N2O in the sub-ppmv range at ambient pressure [19].

The accurate measurement of line strength of NO2 is important for trace NO2 detection in the atmosphere. The spectral lines of NO2 in mid infrared region show the strong absorption features; for instance, 6.2 μm corresponds to the ν3 band of NO2 [20], whereas 3.4 μm corresponds to the ν1 + ν3 band of NO2 [21]. NO2 spectroscopic line parameters have been studied well using various spectroscopy techniques, such as cavity ring-down spectroscopy [22,23], photoacoustic spectroscopy [24], incoherent broadband cavity-enhanced absorption spectroscopy [25], Faraday rotation spectroscopy [26], tunable diode laser absorption spectroscopy [27], wavelength modulation spectroscopy [28], and Fourier transform absorption spectroscopy [29].

In this work, a mid-IR CW-QCL-based laser absorption spectrometer is constructed in our laboratory to revise the spectral region from 1629 cm−1 to 1632 cm−1. Thanks to the customized laser collimation and heat dissipate package, the laser beam passes through the least transmissive optics, so there is minimal power loss and interference fringes. Several NO2 absorption lines that belong to the ν3 bands are studied. The spectral parameter of line strength is measured for improving the performance of our newly developed spectroscopic sensor system for NO2 gas detection. The results are compared with NO2 line strength values in HITRAN database and will be valuable to the spectroscopic databases of NO2.

Section snippets

Experimental details

The schematic of the CW-QCL-based spectroscopic setup used to investigate the NO2 absorption spectroscopy line parameters is shown in Fig. 1. A CW RT QCL chip (Thorlabs Inc.) is packaged in a thermoelectrically (TE) cooled beam collimation package (Q-qubeTM, HealthyPhoton Co., Ltd.), which is driven by an integrated temperature and low noise current controller (QC750-TouchTM, HealthyPhoton Co., Ltd.). The laser source is operating in the wavelength region from 1629 cm−1 to 1632 cm−1 without

Theory

The relationship between the input and output intensities at frequency ν through a uniform medium can be given by the Beer–Lambert law as:I(ν)=I0(ν)exp(α(ν)L),where I0(ν)is the incident beam intensity at frequency of ν, I(ν)is the transmitted beam intensity after through a length of L (cm), and α(ν)is the absorption coefficient of the sample given by:a(ν)=σ(ν)N.

N is the number density (molecules per unit volume) of the absorbing species, which is a function of temperature T (K) and pressure P

Results and discussion

Fig. 2 shows the QCL wavelength tuning range at different injection currents, from 290 to 470 mA, with a step of 10 mA. The wavelength changes from 1629 to 1633.5 cm–1, corresponding to the current tuning coefficient Δν/ΔA of –0.0195 cm−1/mA. In order to investigate the thermal behavior, we measured the wavenumber at the different heat sink temperatures, with a step of 2°C and the results is also shown in Fig. 2. The experimentally determined temperature tuning coefficient Δν/ΔT is around

Conclusion

In this study, a compact spectroscopic sensor based on a TE cooled RT CW-QCL was developed for trace NO2 detection. The spectra of NO2 and N2 mixtures with high resolution were detailedly investigated at RT (~296 K) and in the pressure range of 0–90 mbar. Absorption spectra were fitted with a standard Voigt profile. Accurate measurements of line intensities and N2 pressure-induced broadening coefficients for 43 lines of NO2 around 6.2 μm were performed. This spectral region is highly suitable

CRediT authorship contribution statement

Sheng Zhou: Conceptualization, Writing - original draft. Lei Zhang: Investigation. Yin Wang: Formal analysis. Ningwu Liu: Software. Benli Yu: Supervision. Jingsong Li: Methodology, Writing - review & editing.

Declaration of Competing Interest

The authors declare no conflict of interest.

Acknowledgments

This work is partly supported by the financial support from the National Natural Science Foundation of China (61905001, 41875158, 61705002 and 61675005), the National Program on Key Research and Development Project (2016YFC0302202), the Natural Science Foundation of Anhui Province (1908085QF276, 1808085QF198 and 1508085MF118), the Natural Science Research Project in Universities of Anhui Province (KJ2018A0034) and the Doctoral Start-up Foundation of Anhui University (Y040418131) .

References (30)

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