Implementation and initial deployment of a field instrument for measurement of OH and HO₂ in the troposphere by laser-induced fluorescence

Abstract

An instrument to detect atmospheric concentrations of the hydroxyl (OH) and hydroperoxyl (HO₂) radicals has been developed using the FAGE (fluorescence assay by gas expansion) technique. The instrument is housed in a mobile laboratory and monitors the OH radical via on-resonance laser-induced fluorescence (LIF) spectroscopy of the A²Σ⁺ (v′=0)–X²Π_i (v^″=0) transition at 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 fluorescence being detected by gated photon counting. HO₂ is monitored by chemical conversion to OH by the addition of NO, with subsequent detection using LIF. Following laboratory and field calibrations to characterise the instrument sensitivity, detection limits of 1.8×10⁶ and 2.1×10⁷ molecule cm^-3 were determined for OH and HO₂ respectively, for a signal-to-noise ratio, S/N, of 1 with 150 s signal integration time. The instrument was deployed for the first time during the ACSOE field campaign at Mace Head, Eire, for which illustrative results are given.

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