Fig. 1. Schematic of Herriott-type pump‒probe optical multi-pass cell. (a) Structure of multi-pass cell; (b) beam spot pattern on mirror surface

Fig. 2. Light spots on mirror surface and multi-pass cell center plane, and cross-section of ultraviolet photolysis beam under different angles θ. (a) 50.4°; (b) 79.2°; (c) 122.4°; (d) 158.4°

Fig. 3. Distributed light spots on the mirror surface when laser beam waist is at different positions of Herriott-type pump‒probe optical multi-pass cell. (a) -10 mm; (b) 350 mm; (c) 386 mm; (d) 900 mm

Fig. 4. 2.8 μm DFB laser output beam waist fitting

Fig. 5. Schematic of laser-flash photolysis Faraday rotation spectrometer for time resolved •OH measurements

Fig. 6. Time-resolved FRS signal of •OH decay and corresponding exponential fitting results

Fig. 7. Detection accuracy assessment of the LP-FRS system. (a) Measured series of zero air; (b) Allan deviation analysis; (c) statistical histograms of the measurements

Fig. 8. Measurement of chemical reaction kinetic rate constants. (a) Decay curves of •OH radicals at different molecular concentrations of CH₄ under the experimental condition of 100 mbar and 298 K; (b) measured reaction rate constant of CH₄ with •OH radicals

Fig. 9. Measurement kOH' in smog chamber. (a) Measured kOH' time-series; (b) linear correlation analysis between calculated values and measured values

Fig. 10. Measured diurnal variation of atmospheric kOH' in Shouxian in May 2024