Enhanced laser-induced fluorescence instrument based on chemical modulation for OH radical measurement: High-sensitivity detection and interference evaluation

The hydroxyl (OH) radical serves as the core driver of atmospheric oxidation processes. However, its low concentration and high reactivity pose substantial challenges to accurate measurement in complex atmospheric environments. This study explores a high-sensitivity Laser-Induced Fluorescence (LIF) detection approach based on chemical modulation. An efficient Chemical Titration Cell (CTC) was developed, and key parameters including OH scavenger concentration and flow rate were systematically optimized, resulting in an OH removal efficiency of over 99% and a transmission efficiency of 89%. A high-sensitivity detection system for atmospheric OH radicals based on chemical modulation (CM-LIF) is proposed herein. By optimizing the fluorescence cell, Sampling structure, and minimizing background laser scattering, the system’s measurement accuracy and detection sensitivity are improved. The detection limit reaches (1.78 ± 0.17) × 10⁵ cm^-3 with an integration time of 30 s. A comprehensive set of field observation experiaments and comparative analyses were carried out. Measurement results obtained via chemical modulation and laser wavelength modulation analyses show excellent consistency (slope = 0.95, R² = 0.89). Moreover, in environments with high ozone levels and elevated alkene concentrations, no unknown interferences were detected other than the well-quantified ozone laser photolysis interference. This study demonstrates that the CM-LIF technique offers a reliable solution for the precise measurement of OH radicals in complex atmospheres. This achievement holds significant scientific value, as it enables quantitative assessment of atmospheric oxidation capacity and facilitates the investigation of secondary pollution transformation mechanisms.