Quantitative measurement of OH radical using Faraday rotation spectroscopy at 2.8 µm

Hydroxyl (OH) radicals play a vital role in the degradation of trace gases and pollutants in the troposphere and in controlling the atmospheric oxidation capacity. Due to its short lifetime and low concentration, interference-free high sensitivity in situ OH monitoring by laser spectroscopy represents a challenge. In this presentation, we will report the development of Faraday rotation spectroscopy (FRS) instruments operating at 2.8 µm for quantitative measurement of OH concentrations in an atmospheric simulation chamber and the total atmospheric OH reactivity (k’_OH). The Q (1.5) double lines (²Π_3/2 (ν=1<-0)) at 3568 cm^-1 were selected for the detection. Different detection methods have been studied. The FRS technology 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 makes it capable of enhancing the detection sensitivity and mitigation of spectral interferences from diamagnetic species in the atmosphere. With the use of a multipass enhanced FRS, a detection limit of 3.2 × 10⁶ OH/cm³ (2σ, 4s) was achieved with an absorption path length of 108 m. We demonstrated that FRS method provides a unique method for atmospheric chemistry research.