Recently, researchers at Anhui Institute of Optics and Fine Mechanics (AIOFM), Hefei Institutes of Physical Science successfully developed a superconducting-magnet-based Faraday rotation spectrometer (FRS) for real time in-situ measurement of hydroxyl radical (OH), which will help to study the cause of air pollution in China.

This work entitled "Superconducting-Magnet-Based Faraday Rotation Spectrometer for Real Time in Situ Measurement of OH Radicals at 10⁶ Molecule/cm³ Level in an Atmospheric Simulation Chamber" was published in Analytical Chemistry.

OH radical is the most important oxidant in the atmosphere, which plays a crucial role in the degradation of trace gases and pollutants in the troposphere and in controlling the atmospheric oxidation capacity.

The source and sink of OH radical and its reaction processes are closely related to a lot of environmental problems, such as regional and global climate change, atmospheric photochemical pollution, acid rain, and so on.

Therefore, the concentration of OH radical in the atmosphere can be used as an index of atmospheric oxidation capacity.

However, due to the high activity, short life and low concentration of OH radical in the atmosphere, it is very difficult to measure it directly and accurately, which is a very important and challenging research in the field of atmospheric chemistry.

Faraday rotation spectroscopy (FRS) relies on the particular magneto-optic effect observed for paramagnetic species (including most radicals and some compounds with unpaired electrons), which can significantly reduce excess laser noise and make it capable of enhancing the detection sensitivity and mitigation of spectral interferences from diamagnetic species in the atmosphere.

In this newly published work, Prof. ZHANG Weijun's group at AIOFM reported the first development of a superconducting magnet based Faraday rotation spectrometer operating at 2.8 μm for real time in-situ measurement of OH in chamber.

The combination of a static magnetic field (DC-field) with laser wavelength modulation spectroscopy (WMS) was used, which provided an alternative method to effectively modulate the Zeeman splitting of the absorption lines with excellent performance.

Now, a 1σ detection limit of 1.6×10⁶ molecule/cm³ can be achieved with an absorption pathlength of 108 m and 4 s data acquisition time.

"The developed system provides a suitable self-calibrated analytical instrument for in-situ direct measurement of absolute concentration of OH for study of the OH-based atmospheric oxidation process, and this will be a 'new weapon' for air pollution research", said Prof. ZHAO Weixiong, a research fellow of ZHANG’s group.

This instrument was jointly developed with Prof. CHEN Wenge from High Magnetic Field Laboratory of the Chinese Academy of Sciences, and ZHANG Haifeng from Vacree Technologies Co., Ltd.

Schematic diagram and photograph of the superconducting magnet based FRS system. (Image by ZHAO Weixiong) 

Evolution of OH FRS signals in the O₃ photolytic process. (a) FRS signals at different photolysis time. (b) FRS signals close to the end of the photolytic process when O₃ was almost exhausted. (Image by ZHAO Weixiong)