Laser-Induced Fluorescence Instrument for the Detection of Tropospheric OH Radicals

An instrument for the measurement of troposphric OH radical concentrations using a laser-induced fluorescence technique has been developed. Ambient air is expanded through a pinhole into a low-pressure fluorescence cell and irradiated by the second harmonic of a dye laser at a high repetition rate of 1 kHz. The OH radicals are electronically excited using A 2 Σ + ( ν ′ = 0) ← X 2 Π ( ν ′′ = 0) transitions around 308 nm. The fluorescence from OH radicals is collected by a lens system and detected by a photomultiplier. The photoelectron pulses from the photomultiplier are processed by a photon-counting system. The dynode gate of the photomultiplier and counting gate systems are made to minimize the detection of the chamber and Rayleigh scattering signal. The sensitivity of the developed instrument is calibrated with two methods: one is a long-path absorption technique and the other is a titration technique with simultaneous photolysis of water vapor and oxygen. It is found that the background signal is mainly produced by after-pulse effects in the photomultiplier for the fluorescence detection with a dynode gate system. The minimum detection limit is determined to be 7.0 × 10 5 molecules cm − 3 for OH radicals with a signal-to-noise ratio of 2 and a signal integration time of 60 s.