Airborne measurements of oxygen concentration from the surface to the lower stratosphere and pole to pole

We have developed in situ and flask sampling systems for airborne measurements of variations in the inline-formula $M1inlinescrollmathmlchem{\mathrm{normal\; O}}_{\mathrm{normal\; 2}}/{\mathrm{normal\; N}}_{\mathrm{normal\; 2}}$ 34pt14ptsvg-formulamathimg1de51542faa86ab36b34a562b3d106f9 amt-14-2543-2021-ie00001.svg34pt14ptamt-14-2543-2021-ie00001.png 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 $M10inlinescrollmathmlchem{\mathrm{normal\; O}}_{\mathrm{normal\; 2}}/{\mathrm{normal\; N}}_{\mathrm{normal\; 2}}$ 34pt14ptsvg-formulamathimg6f7587bd10895708ba7d22e72927ea31 amt-14-2543-2021-ie00002.svg34pt14ptamt-14-2543-2021-ie00002.png ), 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 $M15inlinescrollmathmlchem{\mathrm{normal\; O}}_{\mathrm{normal\; 2}}/{\mathrm{normal\; N}}_{\mathrm{normal\; 2}}$ 34pt14ptsvg-formulamathimgb1374199d4ad746bc8cf74082403b011 amt-14-2543-2021-ie00003.svg34pt14ptamt-14-2543-2021-ie00003.png ) measurements for fractionation during sampling or analysis with the use of the concurrent inline-formulaδ(inline-formula $M17inlinescrollmathmlchem\mathrm{normal\; Ar}/{\mathrm{normal\; N}}_{\mathrm{normal\; 2}}$ 32pt14ptsvg-formulamathimg9acc7455db2d95cbdef9198818ac924b amt-14-2543-2021-ie00004.svg32pt14ptamt-14-2543-2021-ie00004.png ) measurements. We have also corrected the in situ inline-formulaδ(inline-formula $M19inlinescrollmathmlchem{\mathrm{normal\; O}}_{\mathrm{normal\; 2}}/{\mathrm{normal\; N}}_{\mathrm{normal\; 2}}$ 34pt14ptsvg-formulamathimg0e8fa2039bc0f930a8484ff4c503d83e amt-14-2543-2021-ie00005.svg34pt14ptamt-14-2543-2021-ie00005.png ) measurements for inlet fractionation and humidity effects by comparison to the corrected flask values. A comparison of inline-formula $M20inlinescrollmathmlchem\mathrm{normal\; Ar}/{\mathrm{normal\; N}}_{\mathrm{normal\; 2}}$ 32pt14ptsvg-formulamathimg209cc7642aa921daec7bca4f1cdda5d0 amt-14-2543-2021-ie00006.svg32pt14ptamt-14-2543-2021-ie00006.png -corrected Medusa flask inline-formulaδ(inline-formula $M22inlinescrollmathmlchem{\mathrm{normal\; O}}_{\mathrm{normal\; 2}}/{\mathrm{normal\; N}}_{\mathrm{normal\; 2}}$ 34pt14ptsvg-formulamathimge068a2fa009057cf0e914203ac265a77 amt-14-2543-2021-ie00007.svg34pt14ptamt-14-2543-2021-ie00007.png ) measurements to regional Scripps Oinline-formula₂ Program station observations shows no systematic biases over 10 recent campaigns (inline-formula $M24inlinescrollmathml+\mathrm{normal\; 0.2}\pm \mathrm{normal\; 8.2}$ 52pt10ptsvg-formulamathimg9bc995d4cb01bf6c9dff83c54d3a82bd amt-14-2543-2021-ie00008.svg52pt10ptamt-14-2543-2021-ie00008.png 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 $M27inlinescrollmathmlchem{\mathrm{normal\; O}}_{\mathrm{normal\; 2}}/{\mathrm{normal\; N}}_{\mathrm{normal\; 2}}$ 34pt14ptsvg-formulamathimg0a7a2a38cf6ff85c3de0dbdd03ca455f amt-14-2543-2021-ie00009.svg34pt14ptamt-14-2543-2021-ie00009.png ) measurements over six more recent campaigns differ from coincident Medusa flask measurements by inline-formula $M28inlinescrollmathml-\mathrm{normal\; 0.3}\pm \mathrm{normal\; 7.2}$ 52pt10ptsvg-formulamathimg905c70aa53681caf1a68e896ff598bb8 amt-14-2543-2021-ie00010.svg52pt10ptamt-14-2543-2021-ie00010.png per meg (mean and standard deviation, inline-formulan=1361) with campaign-specific means ranging from inline-formula−5 to inline-formula+5 per meg.