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Vacuum ultraviolet/photofragmentation laser-induced fluorescence has been demonstrated to be a highly specific and sensitive method for the quantitative measurement of atmospheric ammonia (NH3). The fluorescence detected in this approach results from the two 193-nm photon photofragmentation step NH3 ⇒ NH2 ⇒ NH(b1∑+) followed by the excitation of the NH(b1∑+) ⇒ NH(c1Π) transition via a 450-nm photon with final emission being observed from the NH(c1Π) ⇒ NH(a1Δ) transition at 325 nm. Limits of detection for the instrument presented here are <10 pptv and <4 pptv for 1- and 5-min integration periods, respectively, in ambient sampling conditions. The technique is free from interferences and system performance does not significantly degrade in adverse sampling conditions (i.e., rain, fog, clouds, haze, etc.). Spectroscopic selectivity in the NH(b1∑+) ⇒ NH(c1Π) transition is sufficient to resolve 15NH3 and 14NH3 contributions for use in atmospheric tracer studies. Average ammonia measurements at Stone Mountain, GA, ranged from ≃110 pptv for air temperatures <5°C to ≃240 pptv for air temperatures ≥5°C over the period from Dec. 1987 to the end of Apr. 1988.
© 1990 Optical Society of America
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