Hydroxyl radical (?OH), the most significant oxidant in the atmosphere, initiates oxidation reactions of most natural and anthropogenic trace gas species, determines the atmospheric lifetimes of these pollutants, and regulates the atmosphere’s self-cleaning capacity. Time-resolved measurements of ?OH provide an essential tool for researching chemical reaction kinetics and field measurements of atmospheric ?OH total reactivity, which is crucial for understanding ozone formation and secondary organic aerosols. The pump?probe technique represents a vital method for time-resolved ?OH measurements. This technique employs a 266 nm UV photolysis laser to generate ?OH and initiate its chemical reaction with reactants while synchronously detecting ?OH in another optical path. Using an optical multi-pass cell (MPC) to increase the overlap path length between the detection optical path and the UV photolysis beam effectively enhances pump?probe detection sensitivity. Several research groups have implemented Herriott-type multi-pass cells for pump?probe applications. Although these multi-pass cells provide powerful tools for pump?probe technology, their effective utilization efficiencies remain relatively low compared to designed path lengths, limiting further improvements in detection sensitivity. This study developes a high-efficiency Herriott pump?probe cell and constructed a pump?probe system based on the cell for time-resolved ?OH measurements.

The spot distribution pattern of the Herriott cell is investigated. A pump?probe MPC with an optical path utilization efficiency of 75.4% is developed. Based on the cell, a Faraday rotation spectroscopy system for time-resolved measurement of ?OH is constructed (Fig. 5). ?OH radicals are generated through the photolysis of O₃ and H₂O at 266 nm. The system uses a 2.8 μm continuous-wave distributed feedback (cw-DFB) laser as the probe light source. The Q(1.5e) line of ?OH at 3568.523 cm^-1 is selected as the detection absorption line, with a line intensity of S=9.023×10^-20 cm^-1/(molecule·cm^-2). By measuring the beam waist position of the laser (Fig. 4) and matching it with the multi-pass cell, the problem of beam divergence is solved. The stability and detection precision are evaluated by Allan deviation analysis. The kinetic rate constant for the reaction between ?OH and CH₄ is measured. The dynamic monitoring performance of the system is tested in a photochemical smog chamber. Additionally, the system is applied to real atmospheric field observation.

The distributions of the reflection spots on the mirror surface and at the cell center position are simulated under reflection angles of 50.4°, 79.2°, 122.4°, and 158.4°, respectively (Fig. 2). When the reflection angle is set to 158.4°, the system achieved an effective absorption path length of 28.5 m, with an overlapping efficiency of 75.4%. The red light test demonstrates that positioning the laser beam waist outside the multi-pass cell results in significant beam dispersion after several reflections, preventing the formation of a clear and complete spot pattern (Fig. 3). When the laser beam waist matches the cell center, a distribution of 25 reflection spots, including the light-through hole, is obtained on the mirror surface with relatively uniform spot sizes. The Allan deviation analysis (Fig. 7) of zero air measurement indicates a measurement precision of 0.22 s^-1 with an acquisition time of 60 s, improving to 0.14 s^-1 and 0.11 s^-1 at averaging times of 180 s and 300 s, respectively. The statistical histogram exhibits a normal distribution, indicating system stability without obvious drift. The measured reaction rate constant for ?OH+CH₄ is 6.49(-1.1, +1.3)×10^-15 cm³ molecule^-1 s^-1 (Fig. 8). The time series of kOH' monitored in the smoke chamber correlate well with calculated values from measured CO particle concentration (Fig. 9), demonstrating good agreement in numerical values and change trends with a slope of 0.95 and a linear correlation coefficient of R²=0.97. The daily variation of atmospheric kOH' is measured in the Shouxian area in May 2024 (Fig. 10). The daily average value of kOH' is 18.4 s^-1, with peaks of 19.6 s^-1 at 06:00 and 21.1 s^-1 at 19:00, respectively. A trough of 15.8 s^-1 occurres at 14:00.

A pump?probe MPC with an optical base length of 77.2 cm achieves an overlap efficiency of 75.4%. The ray propagation in the cell is confirmed using red light. Through precise alignment of the incident laser beam’s waist position with the cell center, the beam maintains consistent propagation during multiple reflections, producing 25 uniformly distributed spots on the mirror surface. The beam waist position of the 2.8 μm cw-DFB laser is determined and aligned with the cell center. A Faraday rotation spectroscopy system is established for time-resolved ?OH measurements. Allan deviation analysis reveals a measurement precision of the ?OH decay rate at 100 mbar of 0.22 s^-1 (1σ, 60 s). The measured reaction rate constant for ?OH+CH₄ demonstrates strong agreement with the recommended values from the International Association of Pure and Applied Chemistry (IUPAC). The system’s deviation from dynamic measurements of kOH' in the smog chamber remains below 5%. The daily variation of atmospheric kOH' is monitored in the Shouxian area in May 2025.