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An OH radical measurement instrument based on Fluorescence Assay by Gas Expansion (FAGE) has been developed in our laboratory.Ambient air is introduced into a low-pressure fluorescence cell through a pinhole aperture and irradiated by a dye laser at a high repetition rate of 8.5 kHz.The OH radical is both excited and detected at 308 nm using A-X(0,0) band.To satisfy the high efficiency needs of fluorescence collection and detection,a 4-lens optical system and a self-designed gated photomultiplier (PMT) is used,and gating is actualized by switching the voltage applied on the PMT dynodes.A micro channel photomultiplier (MCP) is also prepared for fluorescence detection.Then the weak signal is accumulated by a photon counter in a specific timing.The OH radical excitation spectrum range in the wavelength of 307.82-308.2 nm is detected and the excited line for OH detection is determined to be Q1(2) line.The calibration of the FAGE system is researched by using simultaneous photolysis of H2O and O2.The minimum detection limit of the instrument using gated PMT is determined to be 9.4 × 105 molecules/cm3,and the sensitivity is 9.5 × 10-7 cps/ (OH·cm-3),with a signal-to-noise ratio of 2 and an integration time of 60 sec,while OH detection limit and the detection sensitivity using MCP is calculated to be 1.6 × 105 molecules/ cm3 and 2.3 × 10-6 cps/(OH·cm-3).The laboratory OH radical measurement is carried out and results show that the proposed system can be used for atmospheric OH radical measurement.