Airborne Oxygen Instrument

Short Name or Variable Name

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 an in-flight precision of +/- 2.5-4.0 per meg on a 5 second measurement, dependent on thermal and motion conditions. 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.

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


Measurements Provided: Concentration of atmospheric oxygen relative to atmospheric nitrogen, in units of "per meg" (or 0.0001% or 1/1000000) deviation from a reference value, and concentration of atmospheric carbon dioxide as mole fraction in dry air in ppm.

Measurement Characteristics: 

  • Overall estimate of uncertainty: +/- 5 per meg
  • Response time: 0.4 Hz (raw data sampled at 10 Hz) for sensor, but mixing in inlet and trap volumes results in a 20 s smoothing window.
  • Precision: 2.5-4.0 per meg (5 s integrated measurement) dependent on aircraft motion and thermal conditions.
  • Other measurement characteristics (comments on signal/noise, bias limits, etc):
    • O2 measurements are relative to the Scripps O2 Program O2 scale, with zero relative to a stored reference sample of air.
    • CO2 measurements are available both on the Scripps O2 Program CO2 scale or the WMO CO2 scale.

History of Significant Changes: AO2 flew on ACME-07, START-08, all 5 HIPPO campaigns, ARISTO-2015, ORCAS, and all 4 ATom 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 inlet surface effect problem.  The earlier data have been corrected but have greater uncertainty.

Examples of Measurement:




AO2 is described in:

Stephens, B. B., Morgan, E. J., Bent, J. D., Keeling, R. F., Watt, A. S., Shertz, S. R., and Daube, B. C.: Airborne measurements of oxygen concentration from the surface to the lower stratosphere and pole to pole, Atmos. Meas. Tech., 14, 2543–2574,, 2021.

Calibration Methods

Four high pressure reference cylinders are carried inside the instrument for calibration. High span and low span gases are 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.

Lead Contact
Britt Stephens
Typical Sampling Rates
2.5 seconds