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 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 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-820) 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.
The system as used during HIPPO flights on the GV is shown in the following photograph:
Four high pressure reference cylinders are carried inside the instrument for calibration. High span and low span is analyzed every ~50 minutes for calibration. O2 and CO2 values on established scales are assigned to these field cylinders by the NCAR O2 / CO2 Calibration Facility.
As currently configured, AO2 flew on the ACME-07, START-08, and all 5 HIPPO campaigns. Mid-way through START-08, the non-pressure-controlled 1/4" Synflex inlet tube was replaced with a pressure-controlled 1/8" stainless tube, which fixed a pressure-dependent O2 inlet surface effect problem. The earlier data have been corrected but have greater uncertainty.
