Laser-induced fluorescence based detection system for measurement of tropospheric OH using 308 nm excitation at low pressure

A detection system for the measurement of tropospheric OH radicals by laser-induced fluorescence has been developed. Ambient air is expanded through a nozzle into a fluorescence cell and is irradiated at low pressure by a pulsed frequency-doubled dye laser. The laser wavelength is tuned to selectively excite the OH radicals on a single rovibronic transition at 308 nm. The OH-resonance fluorescence, emitted mostly between 307 and 311 nm, is detected by a gated photomultiplier/photon counter assembly. This excitation/detection method reduces interferences due to laser generated OH efficiently far below the projected limit of detection. Calibration of our present system yields a detection limit (SNR = 2) of 8.2 x 10 exp 6 OH/cu cm for a 5-min on-resonance and 5-min off-resonance signal integration period at a laser pulse repetition rate of 20 Hz. A considerable improvement of the detection limit to 3.7 x 10 exp 5 OH/cu cm is anticipated by replacing the currently available laser system by a copper-vapor laser pumped dye laser allowing a higher repetition rate of 10 kHz. This would allow useful in situ OH measurements for testing current tropospheric chemistry models.

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