Prognosis for a mid-infrared magnetic rotation spectrometer for the in situ detection of atmospheric free radicals.

Mid-infrared magnetic rotation spectroscopy (MRS) experiments on nitric oxide (NO) are quantitatively modeled by theoretical calculations. The verified theory is used to specify an instrument that can make in situ measurements on NO and NO(2) in the Earth's atmosphere at a sensitivity level of a few parts in 10(12) by volume per second. The prototype instrument used in the experiments has an extrapolated detection limit for NO of 30 parts in 10(9) for a 1-s integration time over a 12-cm path length. The detection limit is an extrapolation of experimental results to a signal-to-noise ratio of one, where the noise is considered to be one-half the peak-to-peak baseline noise. Also discussed are the various factors that can limit the sensitivity of a MRS spectrometer that uses liquid-nitrogen-cooled lead-salt diode lasers and photovoltaic detectors.