NASA - National Aeronautics and Space Administration

Medusa collects 32 cryogenically dried, flow and pressure controlled samples per flight. The samples are collected by an automated sampler into 1.5 L glass flasks that integrate over 25-s (1 e-fold) periods. Medusa provides discretely-sampled comparisons for onboard in situ O2/N2 ratio and CO2 measurements and unique measurements of Ar/N2 and ¹³C, ¹⁴C, and ¹⁸O isotopologues of CO2. The complementary measurements allow ground-truthing of onboard instrument measurements in a laboratory setting, where analysis conditions can often be more stringently controlled and carefully monitored. Isotope and argon measurements can provide additional information about land and ocean controls over the carbon cycle, and about the age and source of the air sampled.

Medusa consists of an onboard computer, two pressure controllers, two
 pumps, three multi-position selector
valves, and a host of other hardware that
control and direct the air samples. All air
is dried by passing it through traps
immersed in a -78 C dry ice bath, adjusted to match atmospheric pressure
at sea level, and then automatically isolated in a flask. Medusa flasks are analyzed on a sector-magnet mass spectrometer and a LiCor non-dispersive infrared CO2 analyzer by the Scripps O2 Program at Scripps Institution of Oceanography.

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.