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A new method based on laser induced two photon ionization was developed for detection of low levels of NO concentration in the troposphere. A frequency-doubled pulsed dye laser operating at near 226 nm was used to photoionize NO by a (1+1) resonance enhanced two photon ionization via its A 2Σ←X 2Π(0,0) band. NO ions are detected by an electron multiplier. Air gas samples are injected through a pulsed nozzle into a vacuum chamber. Cooling of rovibrational levels during the expansion process allows better selectivity for NO detection from other nitrocompounds. Measurement conditions were optimized as a function of X/D for the nozzle (X is the distance from the nozzle orifice to the ionization area, and D is the diameter of the nozzle orifice), laser power, and electron multiplier voltage. For the first time, selectivities for NO measurement, which were obtained from ion intensity ratios of NO and other nitrocompounds such as NO2 at the same concentrations, are discussed. A selectivity of 45 for NO vs NO2 was obtained using a 44 μJ pulse energy and an X/D of 120 for the nozzle. The current detection limit determined for NO was 16 pptv with an integration time of 1 min and a pulse energy of 44 μJ. © 1997 American Institute of Physics.
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