1：Experimental Design and Method Selection:

The PeRCEAS instrument uses chemical amplification to convert peroxy radicals to NO2 via a chain reaction with NO and CO in a reactor, followed by detection using continuous-wave cavity ring-down spectroscopy (cw-CRDS) with an extended cavity diode laser at 408.9 nm. Optical feedback from a V-shaped resonator is employed to enhance sensitivity and stability.

2：9 nm. Optical feedback from a V-shaped resonator is employed to enhance sensitivity and stability. Sample Selection and Data Sources:

2. Sample Selection and Data Sources: Ambient air is sampled, and known peroxy radical mixtures (HO2 and CH3O2) are generated in the laboratory using a UV lamp photolysis source for calibration purposes.

3：List of Experimental Equipment and Materials:

Includes mass flow controllers, pressure regulators, gas cylinders (e.g., CO, N2, NO in N2, NO2 in synthetic air), Te?on-coated reactors, CRDS detectors with V-resonators, ECDL lasers, photodiode detectors, data acquisition systems (PXI computer), and various sensors for temperature, pressure, and humidity.

4：Experimental Procedures and Operational Workflow:

Air is sampled through an inlet, pressure-stabilized, mixed with NO and CO in reactors to amplify peroxy radical conversion to NO2, and then analyzed by CRDS detectors. The system alternates between amplification and background modes to measure NO2 differences. Calibrations involve generating known peroxy radical concentrations and measuring eCL.

5：Data Analysis Methods:

Ring-down times from CRDS are used to calculate absorption coefficients and NO2 concentrations via Eq. (4) and (5). Effective chain lengths (eCL) are determined from linear fits of ΔNO2 vs. peroxy radical concentrations. Allan variance analysis assesses detection limits.