In Situ Measurement of Tropospheric OH Radicals by Laser-Induced Fluorescence—A Description of the KFA Instrument

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 ambient OH concentration of 3 × 10⁵ cm⁻³, thus being within the detection limit of the instrument. During a sunny and clear period in May—June 1994 the instrument was tested in the field. For the first time it was possible to record excitation spectra of tropospheric OH comprising the Q₁(3), Q₂₁(3), and P₁(1) rotational lines. The analysis of the data yielded hydroxyl radical concentrations of up to 4×10⁶ molecules cm⁻³. These spectra unambiguously identify the OH radical and demonstrate the high detection sensitivity and selectivity of this new instrument.