Airborne open path ir laser spectrometer for atmospheric trace gas measurements

Summary form only given. The oxides of nitrogen play important roles in the chemistry of the Earth's troposphere and stratosphere, affecting both the production and loss of ozone, as well as influencing the partitioning of the halogen and hydrogen oxide compounds. HNO/sub 3/ is the major reservoir species of the nitrogen oxide family. Similarly, NO/sub 2/ is a major chemically active species of this family. Both molecules present difficulties to closed sampling systems, as the molecules tend to stick to inlet surfaces and also be produced on surfaces, either from other molecules or from particles. Optical probing of the freestream air outside of the aircraft eliminates these sampling problems. This paper describes OPTIMA (Open Path Tunable Infrared Monitor of the Atmosphere), an instrument built to fly on the NASA DC-8 to measure HNO/sub 3/ and NO/sub 2/. Freestream air is sampled by an open path Herriott resonator, created by two externally mounted mirrors: one mirror is mounted on the inboard engine pylon, the other in an open cavity located in a fuselage window opening. Two IR probe beams and one visible guide beam make 64 traverses between these mirrors for a pathlength of 384 meters. The optical system is actively aligned by servo-controlling 5 degrees of freedom, using a dedicated control computer with 3 DSP processors. OPTIMA employs high frequency wavelength modulation to detect the weak absorption signatures of HNO/sub 3/ and NO/sub 2/. The IR lasers are wavelength modulated at about 500 kHz (1f), and the resultant amplitude modulation of the laser power is phase-sensitively detected at twice that frequency (2f). Fractional absorption sensitivities of about 1-2E-5 are thus achieved in a 60 second integration period.