An instrument to detect atmospheric concentrations of the hydroxyl (OH) and hydroperoxyl radicals has been developed (HO 2 ) using the FAGE (Ñuorescence assay by gas expansion) technique. The instrument is housed in a mobile laboratory and monitors the OH radical via on-resonance laser-induced Ñuorescence (LIF) spectroscopy of the A 2&`(vA \ 0) transition at (v@ \ 0)ÈX 2% i ca. 308 nm. Ambient air is expanded through a 1 mm nozzle to low pressure where it is irradiated by the laser pulse at a repetition rate of 7 kHz, with the resultant Ñuorescence being detected by gated photon counting.is monitored by chemical HO 2 conversion to OH by the addition of NO, with subsequent detection using LIF. Following laboratory and Ðeld calibrations to characterise the instrument sensitivity, detection limits of 1.8 ] 106 and 2.1 ] 107 molecule cm~3 were determined for OH and respectively, for a signal-to-noise ratio, S/N, of 1 with 150 s signal integration time. The instrument was deployed for the HO 2 Ðrst time during the ACSOE Ðeld campaign at Mace Head, Eire, for which illustrative results are given.

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A normally ''on'' linear-focused 14-stage end-window photomultiplier tube ͑PMT͒ ͑Electron Tubes Limited 9893Q/100B͒, designed for fast photon counting, has been gated through control of the voltage applied to the first dynode. The gating circuit reduces the gain of the PMT during a laser pulse, in order to discriminate against the detection of scattered light, and then increases the gain promptly to observe extremely low levels of laser-induced fluorescence ͑LIF͒. An extinction factor for the laser scattered photons of Ͼ10 5 was observed, and has enabled count rates for photons due to LIF as low as 1 Hz to be measured for a laser pulse-repetition frequency of 7 kHz. The rise of the PMT gain is monitored directly by observation of the fluorescence using time-resolved photon counting, and the PMT turn-on time is 30 ns. No significant distortion of the temporal profile of the fluorescence was observed during PMT turn-on. The system, which can also be operated in ungated mode, is rugged and reliable, and has been employed outdoors during atmospheric field measurements of the hydroxyl radical using LIF. The gating circuit is suitable for the rejection of laser scattered light in applications with extremely low levels of fluorescence on the submicrosecond timescale.