Superconducting-Magnet-Based Faraday Rotation Spectrometer for Real Time in Situ Measurement of OH Radicals at 10⁶Molecule/cm³Level in an Atmospheric Simulation Chamber

Abstract

Atmospheric simulation chambers play vital roles in the validation of chemical mechanisms and act as a bridge between field measurements and modeling. Chambers operating at atmospheric levels of OH radicals (10-10 molecule/cm) can significantly enhance the possibility for investigating the discrepancies between the observation and model predications. However, few chambers can directly detect chamber OH radicals at ambient levels. In this paper, we report on the first combination of a superconducting magnet with midinfrared Faraday rotation spectroscopy (FRS) for real time in situ measurement of the OH concentration in an atmospheric simulation chamber. With the use of a multipass enhanced FRS, a detection limit of 3.2 × 10 OH/cm (2σ, 4 s) was achieved with an absorption path length of 108 m. The developed FRS system provided a unique, self-calibrated analytical instrument for in situ direct measurement of chamber OH concentration.

Time-Resolved Laser-Flash Photolysis Faraday Rotation Spectrometer: A New Tool for Total OH Reactivity Measurement and Free Radical Kinetics Research

“…Details can be found in our previous work. 19,20 The total uncertainty in OH concentration determination was estimated to be < 5%. 20 The relationship between the peak intensities of FRS 2f signals and OH concentrations is shown in Figure 4.…”

“…19,20 The total uncertainty in OH concentration determination was estimated to be < 5%. 20 The relationship between the peak intensities of FRS 2f signals and OH concentrations is shown in Figure 4. The FRS signal was linearly proportional to the OH concentrations with a fit uncertainty of less than 1%: [OH] (molecule/cm 3 ) = 2.23(±5.08)×10 6 (molecule/cm 3 ) + 5.39(±0.05) ×10 9 (molecule/cm 3 /mV) × FRS signal (mV).…”

“…19 Wavelength modulation spectroscopy (WMS) combined with a static magnetic field (DCfield) was used to efficiently modulate the Zeeman splitting of the absorption line. 20 The laser frequency was tuned in two ways: 21 (1) a point-by-point tuning method, using a GPIB card to control the laser injection current at each step; (2) a wavelength sweeping method, scanning the laser injection current with a continuous ramp provided by a function generator. For WMS detection, a sinusoidal modulation with a frequency (f m ) of 50 kHz from a lock-in amplifier (SR865, Stanford Research) was added to the injection current.…”

“…The signal detected by the detector is demodulated (second-harmonic, 2f detection) by the lock-in amplifier with a time constant of 300 μs to obtain the FRS signal. For weak absorptions, the FRS signal is pro-portional to the radical concentration (N) and absorption pathlength (L): [19][20][21]…”

“…The OH decay was directly measured using a time-resolved FRS spectrometer at 2.8 μm. [19][20][21] FRS is based on the magneto-optic effect of paramagnetic species, which provides a powerful tool for gas phase chemical kinetic studies with high precision, high selectivity, and free of interferences from precursor's absorption. [22][23][24][25] Further improvements can make the system more compact, portable, and affordable, which provides a reliable approach for long-term and network k' OH observations.…”