Portable laser-flash photolysis Faraday rotation spectrometer for real-time in-situ measurement of total OH reactivity

The total OH reactivity (k_OH′), which is equal to the reciprocal of the lifetime (τ_OH) of the hydroxyl (OH) radical in the atmosphere, is an important parameter for quantitatively assessing the atmospheric oxidation capacity. Although k_OH′ was first measured in the laboratory more than 20 years ago, the required instrumentation is costly and complex, and only a few research groups can perform such measurements. Long-term observation of k_OH′ remains challenging and difficult to achieve. In this presentation, we report the development of a portable laser-flash photolysis Faraday rotation spectroscopy (LP-FRS) instrument for real-time and in-situ measurement of k_OH′. OH decay is directly measured using a time-resolved FRS spectrometer at 2.8 μm. Since FRS relies on the detection of the rotation of the polarization state of the probe light induced by paramagnetic molecules in a longitudinal magnetic field, the laser noise and molecule interferences are significantly reduced, which enables the FRS system to directly and highly sensitive monitor OH concentration without chemical interferences. The LP-FRS instrument has a k_OH′ detection precision of 1.0 s^-1 with an averaging time of 300 s. The instrument’s optical box measures 130 cm × 40 cm × 35 cm, making its convenient for field applications.