A research team led by Prof. GAO Xiaoming from the Hefei Institutes of Physical Science of the Chinese Academy of Sciences recently proposed a new dual-wavelength Faraday rotation spectroscopy (FRS) sensor in the two-component simultaneous detection of nitrogen oxides.

Results were published in Sensors and Actuators: B. Chemical.

Nitrogen oxides are at the center of atmospheric chemical reactions and influence the concentration of ozone, hydroxyl and peroxy radicals, which are important precursors for the formation of photochemical smog, acid rain and haze pollution. At the same time, nitrogen oxides emitted from agricultural fields, wetlands and other ecosystems play an important role in the global nitrogen cycle.

In this study, Prof. LIU Kun, CAO Yuan and their collaborators proposed the FRS sensor based on NdFeB ring magnet array in response to the slow measurement rate and lack of selectivity for nitric oxide and nitrogen dioxide in the traditional chemiluminescence detection of nitrogen oxides.

The interaction between two beams of linearly polarized light of different wavelengths and nitrogen oxides in a static magnetic field was facilitated by designing a single-cavity gas absorption cell with two optical paths and coupling it coaxially to a NdFeB ring magnet array.

In addition, they recommended that the absorption cell should be less than or equal to the length of the permanent magnet ring array. This is to address the problem that the left and right sides of NdFeB ring magnet array were opposite to the direction of the internal axial magnetic field, causing partial cancellation of the magneto-optical signal excited by the internal axial magnetic field.

By combining wavelength modulation spectroscopy with a static magnetic field, the FRS NO_x sensor achieved a detection sensitivity of 0.58 ppb NO₂ and 0.95 ppb NO over an optical length of 23.7 m and an integration time of 100 s.

This work lays the foundation for further development of eddy covariance method-based studies of the soil-plant-atmosphere NO_x interface fluxes in ecosystems.

Single-cavity, dual-optical path gas cell; Magnetic field intensity distribution of NdFeB ring magnet array. (Image by CAO Yuan)

Optical layout of the FRS NO_x sensor. (Image by CAO Yuan)