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NCAR Atmospheric Oxygen Instrument | AO2

There is great value in measuring O2 concentrations in carbon cycle studies as relationships between CO2 and O2 provide clues to what is affecting CO2 concentations. For example, photosynthesis produces O2 and consumes CO2, while fossil-fuel burning does the opposite with a slightly different ratio. Because the atmospheric concentration of O2 is so high (~ 21%), measuring the small changes that trees and power plants make is very difficult.

Atmospheric OxygenThe 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. AO2 flew on the Wyoming King-Air during the ACME-2007 campaign and on the NCAR G-V during the START-08 and HIPPO-1 campaigns. 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 106 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 HIPPO operational 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 AO2 system consists of a pump module, a cylinder module, an instrument module, and a dewar.

Reference: Stephens, B.B., R.F. Keeling, and W.J. Paplawsky, Shipboard measurements of atmospheric oxygen using a vacuum-ultraviolet absorption technique, Tellus Series B, 55, 857-878, 2003.
Contact Name: Britton Stephens, NCAR,

Dimensions: 19" rack wide X 30" tall X 22" deep plus a 6" diameter X 14" tall dewar
Instrument Weight: 200 lbs.
Power: 110 AC
Hazmat: 4 kg of solid CO2 (dry ice)
Inlet: Dedicated aft facing 1/4" inlet, unheated stainless steel or Synflex
Number of Pperators: 1
Altitude Requirements: max. alt 60,000 feet
Ground Requirements: 30 minute preflight




HIPPO is a landmark study for many reasons, not the least of which is it is the first time scientists have systematically mapped global distribution of carbon dioxide and other greenhouse gases in the atmosphere, covering the full troposphere in all seasons and multiple years.

  • HIPPO I :: 8 January-30 January 2009
  • HIPPO II :: 31 October-22 November 2009
  • HIPPO III :: 24 March-16 April 2010
  • HIPPO IV :: 14 June-11 July 2011
  • HIPPO V :: 9 August-9 September 2011