Measurement of total OH reactivity in the urban atmosphere by Chemical Perturbation using a Laser-Induced Fluorescence technique

A measurement system of total OH reactivity in the atmosphere has been newly developed by chemical perturbation using a laser-induced fluorescence method. Ambient air was introduced into a flow tube placed just above an OH detector. In the flow tube, a pulsed 266-nm laser (a fourth harmonic of Nd:YAG laser) with a low repetition rate (0.5 - 10 Hz) was irradiated. OH is generated in the photolysis of ozone by the UV laser. The OH reacts with trace species in the reaction tube and the concentrations of OH are decreased with time after irradiating the laser pulse. The OH decay in the flow tube is measured by laser-induced fluorescence. The air in the reaction tube was introduced into a low-pressure ( ∼ 300 Pa) fluorescence detection cell through an orifice (1-mm diameter), which was settled at the center of the radial cross section of the flow tube. The A-X(0,0) Q₁(2) line of OH (λ ∼ 308 nm) was excited by use of a tunable frequency-doubled dye laser pumped by a second harmonic of the Nd:YVO₄ laser with a repetition rate of 10 kHz. The resonant fluorescence was detected using a photomultiplier tube. When concentrations of the species which react with OH radicals are much higher than the OH radicals, it is expected that the OH decay rate is pseudo-first order. The time profile of the measured LIF signal was fit to single-exponential curve and then the decay rate was determined by use of the fitting expression. The decay rates of zero air with known amount of CO were measured and the obtained second-order rate coefficient agreed excellently with the previous reported value. Field observations were conducted in Tokyo Metropolitan University in July and August 2003. Simultaneous measurements of O₃, CO, SO₂, NO, NO₂, VOCs, OVOCs, temperature and relative humidity were made during the observations. The measured OH reactivities were usually higher than the calculated values using simultaneously measured concentrations of various trace species and the rate coefficients of OH and the trace gases. This disagreement would be due to existence of unknown species reacting with OH and/or the uncertainty of the OH + NO₂ reaction rate coefficient. OH uptake on particles was not confirmed during this observation.