Abstract An instrument for the measurement of tropospheric OH radical concentrations by laser-induced fluorescence spectroscopy has been developed. Ambient air is expanded through a nozzle into a low-pressure fluorescence cell and is irradiated by a frequency-doubled dye laser, which is pulsed with a high repetition rate of 8.5 kHz. The laser wavelength is tunable to selectively excite single rovibronic transitions of the OH radicals at 308 nm [A2∑+(ν′ = 0) ← X2 Π (ν″=0)]. The OH resonance fluorescence, emitted mostly between 307 and 311 nm, is detected by gated photon counting. From laboratory calibrations and ambient air measurements the authors infer a detection limit (S/N = 2) of 8 × 105 OH cm−3 for 1-minute data integration time. First tests of the new instrument in ambient air revealed the existence of an interference problem due to generation of OH by a dark reaction of ozone inside the detection cell. Improvements of the instrument reduced the spurious OH signal to a level corresponding to an ambi...