Recently, the joint research team from Anhui Institute of Optics and Fine Mechanics, HFIPS, Chinese Academy of Sciences, Advanced Laser Technology Laboratory of Anhui Province, University of Science and Technology of China, Laboratoire de Physicochimie de l′ Atmosph`ere, Universit´e du Littoral Cˆote d′ Opale, published an academic papers Dual mid-infrared wavelength Faraday rotation spectroscopy NO_x sensor based on NdFeB ring magnet array.

Nitrogen oxides (NO_x, the sum of nitrogen dioxide (NO2) and nitric oxide (NO)) are important precursors of tropospheric ozone, and they also influence the concentration of hydroxyl and peroxyl radicals. Most of the compounds that are oxidized and removed from the air or converted to other chemical species are in direct or indirect contact with NO_x. At typical hydroxyl radical levels, the life time of NO_x depends on the season and the photochemical reaction rate, which is typically a few hours. According to the IPCC sixth assessment report, the emissions of NO_x result in net-positive warming from the formation of short-term ozone (warming) and the destruction of ambient methane (cooling). Additionally, NO_x contributes to acid deposition and the formation of chemical smog and aerosols. Since NO and NO₂ play a central role in atmospheric photochemical reactions, their simultaneous detection helps to understand the sources and sinks of these two gases, in addition to studying the NO_x exchange fluxes between terrestrial ecosystems and the atmosphere.

Chemiluminescence detection (NO+O₃→NO₂+O₂+hν) is the conventional method for measuring NO_x. NO₂ first needs to be converted to NO at high temperature (~325 ^◦C) before it can be measured by chemiluminescence reaction (Mo+3NO₂→MoO₃+3NO). Although this method is more widely used, other oxidized nitrogen compounds, such as peroxyacetyl nitrate (PAN) and nitric acid (HNO₃), can cause cross-interference in the measurement of NO_x concentrations. Simultaneously, this method is non-selective in discriminating between NO and NO₂. The infrared absorption method can also be used for NO and NO₂ measurements. In this method, NO₂ usually needs to be reduced to NO by the converter. As NO and NO₂ are paramagnetic molecules, Faraday rotation spectroscopy (FRS) can be used as a potential method to achieve their highly sensitive and selective detection. FRS enables highly sensitive detection of species concentrations by detecting changes in the polarization state of light induced by a gaseous medium immersed in a longitudinal magnetic field. This method realizes the detection of gas concentration by measuring optical dispersion, so it has a higher dynamic measurement range than absorption spectroscopy (dynamic range upper limit ≤10%) based on Beer-Lambert law. Another significant advantage of FRS is that it is reasonably immune to diamagnetic species (e.g., water and carbon dio_xide), which allows it to exhibit high sample specificity.

Most of these reported FRS sensors use solenoid coils to provide an external longitudinal magnetic field, which makes them suffer from high power consumption and excessive Joule heat generation. The high-current alternating current circuit required to generate the target magnetic field produces uncontrolled electromagnetic interference (EMI), which usually deteriorates the long-term stability of the FRS sensors. In addition, the currently reported FRS sensors are only capable of single-component measurements in the absorption cell and cannot meet the demand for simultaneous multi-component measurements in complex environments.

In the present work, a novel low-power FRS sensor based on a neodymium-iron-boron (NdFeB) ring magnet array was proposed to achieve simultaneous detection of NO and NO₂ in a single absorption cell. The magnetic field distribution characteristics of a ring magnet array coaxial to a dual-wavelength Herriott cell (DWHC) were analyzed. Two room-temperature continuous wave mid-infrared quantum cascade lasers (QCL) with wavelengths of 5.33 µm (1875.81 cm⁻¹) and 6.2 µm (1613.25 cm⁻¹), respectively, were used simultaneously to probe magneto-optical effects within the DWHC. The 1/f noise was effectively suppressed by high-frequency modulation of the laser wavelength. The performance of the dual-wavelength FRS NO_x sensor was optimized with respect to modulation amplitude, modulation frequency, sample gas pressure, and analyzer offset angle. The FRS sensor proposed in this work provides a preferable solution for field deployable trace gas detection equipment. The laser detected by two TEC-cooled mid-infrared thermoelectrically cooled mercury-cadmium- telluride (MCT) photodetectors (Healthy Photon, model HPPD-B- 10–150 K).

(a) Schematic diagram of the dual mid-infrared wavelength FRS NO_x sensor based on a NdFeB ring magnet array;

(b) Optical layout of the FRS NO_x sensor.

thermoelectrically cooled mercury-cadmium- telluride (MCT) photodetectors (Healthy Photon, model HPPD-B- 10–150 K),

Conclusion

In this work, a dual mid-infrared wavelength FRS sensor based on a NdFeB ring magnet array was developed for the simultaneous detection of NO₂ and NO. The detection limits for NO₂ and NO were 0.58 ppb and 0.95 ppb, respectively, at an optical path length of 23.7 m and an integration time of 100 s. High frequency laser wavelength modulation was combined with an external static magnetic field to minimize the effect of low frequency noise on the FRS signal. The magnetic field distribution characteristics of the used permanent magnet array were analyzed based on the finite element method, which helped to determine the length of the absorption cell coupled to it. A dual-wavelength Herriott cell was used to amplify the interaction between two different wavelengths of linearly polarized light and nitrogen oxide molecules, thus achieving highly sensitive detection of two paramagnetic molecules within a single absorption cell. The FRS NO_x sensor presented in this work shows great potential for further development into a portable, field-deployable instrument with applications in atmospheric environmental monitoring or ecosystem NO_x flu_x observation.

Reference:

Yuan Cao, Kun Liu, Ruifeng Wang, Guishi Wang, _xiaoming Gao, Weidong Chen，Dual mid-infrared wavelength Faraday rotation spectroscopy NO_x sensor based on NdFeB ring magnet array, Sensors & Actuators: B. Chemical 388 (2023) 133805

https://doi.org/10.1016/j.snb.2023.133805