An absolute calibration for gas-phase hydroxyl measurements

A hydroxyl radical detection system using gas expansion and fast gating laser-induced fluorescence techniques

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 8.5kHz. The OH radical is both excited and detected at 308nm 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 307.82-308.2nm 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 9.4×10⁵molecules/cm³, and the sensitivity is 9.5×10^-7cps/(OH·cm^-3), with a signal-to-noise ratio of 2 and an integration time of 60sec, while OH detection limit and the detection sensitivity using MCP is calculated to be 1.6×10⁵molecules/cm³ and 2.3×10^-6cps/(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.