High sensitive and selective detection of OH radicals with Faraday rotation spectroscopy

The hydroxyl (OH) radical plays a critical role in atmospheric chemistry due to its high reactivity with volatile organic compounds (VOCs) and other trace gaseous species. Because of its very short life time and very low concentration in the atmosphere, interference-free high sensitivity in-situ OH monitoring by laser spectroscopy represents a real challenge. We report on the development of a Faraday rotation spectrometer operating at 2.8 µm for sensitive and selective detection of OH radical. Faraday rotation spectroscopy (FRS) relies on the particular magneto-optic effect observed for paramagnetic species, which makes it capable of enhancing the detection sensitivity and mitigation of spectral interferences from diamagnetic species in the atmosphere [1, 2]. When an AC magnetic field is used, the Zeeman splitting of the molecular absorption line (and thus the magnetic circular birefringence) is modulated. This provides an “internal modulation” of the sample, which permits to suppress the external noise like interference fringes [3, 4]. An alternative FRS detection scheme is to use a static magnetic field (DC-field) associated with laser wavelength modulation to effectively modulate the Zeeman splitting of the absorption lines [5]. In the DC field case, the lock-in amplifier detects the wavelength modulation of the laser frequency, which can provide excellent performance compared to most of the sensing systems based on direct absorption and wavelength modulation spectroscopy. Both of these techniques will be presented.