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NCAR Airborne Oxygen Instrument (AO2)


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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 1-sigma precision of
± 2 per meg on a 5 second measurement.
 For comparison, this is equivalent to detecting the removal of one O2 molecule
 from 2.5 million molecules of air. At typical
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. To minimize inlet
surface effects, the pressure in the inlet line
is actively controlled at the aircraft bulkhead.
The sample air is cryogenically dried in a
series of electropolished stainless steel traps immersed in a dry ice Fluorinert slurry. The
 AO2 system consists of a pump module, a cylinder module, an instrument module, and a dewar.

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