The NCAR Airborne Oxygen Instrument measures O₂ concentration using a vacuum-ultraviolet absorption technique. AO2 is based on earlier ship-board and laboratory instruments using the same technique, but has been designed specifically for airborne use to minimize motion and thermal sensitivity and with a pressure and flow controlled inlet system. To achieve the high-levels of precision needed, AO2 switches between sample gas and air from a high-pressure reference cylinder every 2.5 seconds. Atmospheric O₂ concentrations are typically reported in units of one part in 1,000,000 relative deviations in the O₂/N₂ ratio, which are referred to as "per meg."  AO2 has a precision of +/- 2.5 per meg on a 5 second measurement. For comparison, this is equivalent to detecting the removal of one O₂ molecule from 2 million molecules of air. At typical GV flight speeds of 300 kts or climb/descent rates of 1500 fpm, 5-seconds correspond to a horizontal resolution of 750 m and a vertical resolution of 40 m. The instrument includes an internal single-cell CO₂ sensor (LI-840) which is used to correct the O₂ measurements for dilution by CO₂ and for scientific purposes.  The AO2 system consists of a pump module, a cylinder module, an instrument module, and a dewar.

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• The system as used during HIPPO flights on the GV is shown in the following photograph:

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