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

• Main Authors: B. B. Stephens, E. J. Morgan, J. D. Bent, R. F. Keeling, A. S. Watt, S. R. Shertz, B. C. Daube
• Format: Article
• Language: English
• Published: Copernicus Publications 2021-04-01
• Series: Atmospheric Measurement Techniques
• Online Access: https://amt.copernicus.org/articles/14/2543/2021/amt-14-2543-2021.pdf

<p>We have developed in situ and flask sampling systems for airborne measurements of variations in the <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M1" display="inline" overflow="scroll" dspmath="mathml"><mrow class="chem"><msub><mi mathvariant="normal">O</mi><mn mathvariant="normal">2</mn></msub><mo>/</mo><msub><mi mathvariant="normal">N</mi><mn mathvariant="normal">2</mn></msub></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="34pt" height="14pt" class="svg-formula" dspmath="mathimg" md5hash="1de51542faa86ab36b34a562b3d106f9"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="amt-14-2543-2021-ie00001.svg" width="34pt" height="14pt" src="amt-14-2543-2021-ie00001.png"/></svg:svg></span></span> ratio at the part per million level. We have deployed these instruments on a series of aircraft campaigns to measure the distribution of atmospheric O<span class="inline-formula">₂</span> from 0–14 km and 87<span class="inline-formula">^∘</span> N to 86<span class="inline-formula">^∘</span> S throughout the seasonal cycle. The National Center for Atmospheric Research (NCAR) airborne oxygen instrument (AO2) uses a vacuum ultraviolet (VUV) absorption detector for O<span class="inline-formula">₂</span> and also includes an infrared CO<span class="inline-formula">₂</span> sensor. The VUV detector has a precision in 5 s of <span class="inline-formula">±1.25</span> per meg (1<span class="inline-formula"><i>σ</i></span>) <span class="inline-formula"><i>δ</i></span>(<span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M10" display="inline" overflow="scroll" dspmath="mathml"><mrow class="chem"><msub><mi mathvariant="normal">O</mi><mn mathvariant="normal">2</mn></msub><mo>/</mo><msub><mi mathvariant="normal">N</mi><mn mathvariant="normal">2</mn></msub></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="34pt" height="14pt" class="svg-formula" dspmath="mathimg" md5hash="6f7587bd10895708ba7d22e72927ea31"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="amt-14-2543-2021-ie00002.svg" width="34pt" height="14pt" src="amt-14-2543-2021-ie00002.png"/></svg:svg></span></span>), but thermal fractionation and motion effects increase this to <span class="inline-formula">±2.5</span>–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 O<span class="inline-formula">₂</span> 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, <span class="inline-formula">O₂:CO₂</span> 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 <span class="inline-formula"><i>δ</i></span>(<span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M15" display="inline" overflow="scroll" dspmath="mathml"><mrow class="chem"><msub><mi mathvariant="normal">O</mi><mn mathvariant="normal">2</mn></msub><mo>/</mo><msub><mi mathvariant="normal">N</mi><mn mathvariant="normal">2</mn></msub></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="34pt" height="14pt" class="svg-formula" dspmath="mathimg" md5hash="b1374199d4ad746bc8cf74082403b011"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="amt-14-2543-2021-ie00003.svg" width="34pt" height="14pt" src="amt-14-2543-2021-ie00003.png"/></svg:svg></span></span>) measurements for fractionation during sampling or analysis with the use of the concurrent <span class="inline-formula"><i>δ</i></span>(<span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M17" display="inline" overflow="scroll" dspmath="mathml"><mrow class="chem"><mi mathvariant="normal">Ar</mi><mo>/</mo><msub><mi mathvariant="normal">N</mi><mn mathvariant="normal">2</mn></msub></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="32pt" height="14pt" class="svg-formula" dspmath="mathimg" md5hash="9acc7455db2d95cbdef9198818ac924b"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="amt-14-2543-2021-ie00004.svg" width="32pt" height="14pt" src="amt-14-2543-2021-ie00004.png"/></svg:svg></span></span>) measurements. We have also corrected the in situ <span class="inline-formula"><i>δ</i></span>(<span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M19" display="inline" overflow="scroll" dspmath="mathml"><mrow class="chem"><msub><mi mathvariant="normal">O</mi><mn mathvariant="normal">2</mn></msub><mo>/</mo><msub><mi mathvariant="normal">N</mi><mn mathvariant="normal">2</mn></msub></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="34pt" height="14pt" class="svg-formula" dspmath="mathimg" md5hash="0e8fa2039bc0f930a8484ff4c503d83e"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="amt-14-2543-2021-ie00005.svg" width="34pt" height="14pt" src="amt-14-2543-2021-ie00005.png"/></svg:svg></span></span>) measurements for inlet fractionation and humidity effects by comparison to the corrected flask values. A comparison of <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M20" display="inline" overflow="scroll" dspmath="mathml"><mrow class="chem"><mi mathvariant="normal">Ar</mi><mo>/</mo><msub><mi mathvariant="normal">N</mi><mn mathvariant="normal">2</mn></msub></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="32pt" height="14pt" class="svg-formula" dspmath="mathimg" md5hash="209cc7642aa921daec7bca4f1cdda5d0"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="amt-14-2543-2021-ie00006.svg" width="32pt" height="14pt" src="amt-14-2543-2021-ie00006.png"/></svg:svg></span></span>-corrected Medusa flask <span class="inline-formula"><i>δ</i></span>(<span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M22" display="inline" overflow="scroll" dspmath="mathml"><mrow class="chem"><msub><mi mathvariant="normal">O</mi><mn mathvariant="normal">2</mn></msub><mo>/</mo><msub><mi mathvariant="normal">N</mi><mn mathvariant="normal">2</mn></msub></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="34pt" height="14pt" class="svg-formula" dspmath="mathimg" md5hash="e068a2fa009057cf0e914203ac265a77"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="amt-14-2543-2021-ie00007.svg" width="34pt" height="14pt" src="amt-14-2543-2021-ie00007.png"/></svg:svg></span></span>) measurements to regional Scripps O<span class="inline-formula">₂</span> Program station observations shows no systematic biases over 10 recent campaigns (<span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M24" display="inline" overflow="scroll" dspmath="mathml"><mrow><mo>+</mo><mn mathvariant="normal">0.2</mn><mo>±</mo><mn mathvariant="normal">8.2</mn></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="52pt" height="10pt" class="svg-formula" dspmath="mathimg" md5hash="9bc995d4cb01bf6c9dff83c54d3a82bd"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="amt-14-2543-2021-ie00008.svg" width="52pt" height="10pt" src="amt-14-2543-2021-ie00008.png"/></svg:svg></span></span> per meg, mean and standard deviation, <span class="inline-formula"><i>n</i>=86</span>). For AO2, after resolving sample drying and inlet fractionation biases previously on the order of 10–100 per meg, independent AO2 <span class="inline-formula"><i>δ</i></span>(<span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M27" display="inline" overflow="scroll" dspmath="mathml"><mrow class="chem"><msub><mi mathvariant="normal">O</mi><mn mathvariant="normal">2</mn></msub><mo>/</mo><msub><mi mathvariant="normal">N</mi><mn mathvariant="normal">2</mn></msub></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="34pt" height="14pt" class="svg-formula" dspmath="mathimg" md5hash="0a7a2a38cf6ff85c3de0dbdd03ca455f"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="amt-14-2543-2021-ie00009.svg" width="34pt" height="14pt" src="amt-14-2543-2021-ie00009.png"/></svg:svg></span></span>) measurements over six more recent campaigns differ from coincident Medusa flask measurements by <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M28" display="inline" overflow="scroll" dspmath="mathml"><mrow><mo>-</mo><mn mathvariant="normal">0.3</mn><mo>±</mo><mn mathvariant="normal">7.2</mn></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="52pt" height="10pt" class="svg-formula" dspmath="mathimg" md5hash="905c70aa53681caf1a68e896ff598bb8"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="amt-14-2543-2021-ie00010.svg" width="52pt" height="10pt" src="amt-14-2543-2021-ie00010.png"/></svg:svg></span></span> per meg (mean and standard deviation, <span class="inline-formula"><i>n</i>=1361</span>) with campaign-specific means ranging from <span class="inline-formula">−5</span> to <span class="inline-formula">+5</span> per meg.</p>