We have developed in situ and flask sampling systems for airborne measurements of variations in the inline-formula
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ratio at the part per million level. We have deployed these instruments on a series of aircraft campaigns to measure the distribution of atmospheric Oinline-formula2 from 0–14 km and 87inline-formula∘ N to 86inline-formula∘ S throughout the seasonal cycle. The National Center for Atmospheric Research (NCAR) airborne oxygen instrument (AO2) uses a vacuum ultraviolet (VUV) absorption detector for Oinline-formula2 and also includes an infrared COinline-formula2 sensor. The VUV detector has a precision in 5 s of inline-formula±1.25 per meg (1inline-formulaσ) inline-formulaδ(inline-formula
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), but thermal fractionation and motion effects increase this to inline-formula±2.5–4.0 per meg when sampling ambient air in flight. The NCAR/Scripps airborne flask sampler (Medusa) collects 32 cryogenically dried air samples per flight under actively controlled flow and pressure conditions. For in situ or flask Oinline-formula2 measurements, fractionation and surface effects can be important at the required high levels of relative precision. We describe our sampling and measurement techniques and efforts to reduce potential biases. We also present a selection of observational results highlighting the individual and combined instrument performance. These include vertical profiles, inline-formulaO2:CO2 correlations, and latitudinal cross sections reflecting the distinct influences of terrestrial photosynthesis, air–sea gas exchange, burning of various fuels, and stratospheric dynamics. When present, we have corrected the flask inline-formulaδ(inline-formula
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) measurements for fractionation during sampling or analysis with the use of the concurrent inline-formulaδ(inline-formula
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) measurements. We have also corrected the in situ inline-formulaδ(inline-formula
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) measurements for inlet fractionation and humidity effects by comparison to the corrected flask values. A comparison of inline-formula
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-corrected Medusa flask inline-formulaδ(inline-formula
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) measurements to regional Scripps Oinline-formula2 Program station observations shows no systematic biases over 10 recent campaigns (inline-formula
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per meg, mean and standard deviation, inline-formulan=86). For AO2, after resolving sample drying and inlet fractionation biases previously on the order of 10–100 per meg, independent AO2 inline-formulaδ(inline-formula
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) measurements over six more recent campaigns differ from coincident Medusa flask measurements by inline-formula
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per meg (mean and standard deviation, inline-formulan=1361) with campaign-specific means ranging from inline-formula−5 to inline-formula+5 per meg.