1：Experimental Design and Method Selection:

The LFP-FRS instrument consists of an FRS spectrometer for real-time in-situ measurement of OH radicals and a LFP device for generating pulses of high concentrations of OH radicals. OH radicals were generated by photolysis of O3 at 266 nm in the presence of H2O. The decay of the OH signal was directly measured using a time-resolved FRS spectrometer at

2：2O. The decay of the OH signal was directly measured using a time-resolved FRS spectrometer at 8 μm. Sample Selection and Data Sources:

2.8 μm. 2. Sample Selection and Data Sources: A small amount of the zero air was bypassed through an ozone generator before being added to the sample to obtain an O3 concentration of ~

3：5 × 1012 molecule/cm3 in the reactor. List of Experimental Equipment and Materials:

The probe laser of the FRS spectrometer was a

4：8 μm continuous-wave (CW) distributed feedback (DFB) diode laser controlled by a LDC501 controller. A Herriott-type optical multipass cell was used as the reactor for flash photolysis. The flash laser beam (266 nm, 25 mJ, 8 ns pulse duration) from a compact pulsed Nd:

YAG laser was expanded to a diameter of 30 mm with a beam expander.

5：Experimental Procedures and Operational Workflow:

The pulse repetition rate of the flash photolysis laser ranged from 1 to 5 Hz. The signal acquired between the two rising edges was the background signal without photolysis. When the pulsed laser was triggered, O3 was photolyzed and reacted rapidly with H2O to produce OH radicals. The FRS signal increased rapidly. The OH decay was fitted with a single-exponential equation to get the total reactivity k’OH and the reaction rate associated with the OH radical.

6：Data Analysis Methods:

The WMS 2f signal (B = 0) of OH absorption was obtained using the same method as FRS 2f signal. H2O with known concentration was used as the reference for the cross-calibration of the OH concentration.