Abstract
Abstract. We have developed in situ and flask sampling systems for airborne measurements of variations in the O2/N2 ratio at the part per million level. We have deployed these instruments on a series of aircraft campaigns to measure the distribution of atmospheric O2 from 0–14 km and 87∘ N to 86∘ S throughout the seasonal cycle. The National Center for Atmospheric Research (NCAR) airborne oxygen instrument (AO2) uses a vacuum ultraviolet (VUV) absorption detector for O2 and also includes an infrared CO2 sensor. The VUV detector has a precision in 5 s of ±1.25 per meg (1σ) δ(O2/N2), but thermal fractionation and motion effects increase this to ±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 O2 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, O2: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 δ(O2/N2) measurements for fractionation during sampling or analysis with the use of the concurrent δ(Ar/N2) measurements. We have also corrected the in situ δ(O2/N2) measurements for inlet fractionation and humidity effects by comparison to the corrected flask values. A comparison of Ar/N2-corrected Medusa flask δ(O2/N2) measurements to regional Scripps O2 Program station observations shows no systematic biases over 10 recent campaigns (+0.2±8.2 per meg, mean and standard deviation, n=86). For AO2, after resolving sample drying and inlet fractionation biases previously on the order of 10–100 per meg, independent AO2 δ(O2/N2) measurements over six more recent campaigns differ from coincident Medusa flask measurements by -0.3±7.2 per meg (mean and standard deviation, n=1361) with campaign-specific means ranging from −5 to +5 per meg.
Highlights
Atmospheric O2 observations can be a powerful tool for elucidating carbon cycle processes on multiple time and space scales because of unique relationships between O2 and CO2 surface exchange (e.g., Keeling and Shertz, 1992; Stephens et al, 1998; Keeling and Manning, 2014; Ishidoya et al, 2013a; Nevison et al, 2015; Morgan et al, 2019b)
For AO2, after resolving sample drying and inlet fractionation biases previously on the order of 10–100 per meg, independent AO2 δ(O2/N2) measurements over 6 more recent campaigns differ from coincident Medusa flask measurements by -0.3 ± 7.2 per meg, with campaign-specific means ranging from -5 to +5 per meg
We focus on data from recent campaigns on the NSF/NCAR High-performance Instrumented Airborne Platform for Environmental Research (HIAPER) Gulfstream V (GV) 30 aircraft (UCAR/NCAR - Earth Observing Laboratory, 2005): the Stratosphere-Troposphere Analyses of Regional Transport campaign (START-08; Pan et al, 2010), five HIAPER Pole-to-Pole Observations campaigns (HIPPO 1–5, 2009-2011; Wofsy et al, 2011), and the 2016 O2/N2 Ratio and CO2 Airborne Southern Ocean (ORCAS) study (Stephens et al, 2018); as well as the Airborne Research Instrumentation Testing Opportunity (ARISTO-2015) campaign on the NSF/NCAR C-130 (UCAR/NCAR - Earth Observing Laboratory, 1994) and four Atmospheric Tomography Mission (ATom 1–4, 2016-2018) campaigns on the NASA DC-8
Summary
Atmospheric O2 observations can be a powerful tool for elucidating carbon cycle processes on multiple time and space scales because of unique relationships between O2 and CO2 surface exchange (e.g., Keeling and Shertz, 1992; Stephens et al, 1998; Keeling and Manning, 2014; Ishidoya et al, 2013a; Nevison et al, 2015; Morgan et al, 2019b). AO2 first made research quality measurements on the University of Wyoming King Air during the 2007 Airborne Carbon in the Mountains Experiment (ACME-07; Desai et al, 2011), The NCAR / Scripps Medusa airborne flask sampler was designed to collect cryogenically-dried air samples under controlled pressure and flow conditions. An earlier 16 flask version of the sampler flew on the University of North Dakota Citation II aircraft during the CO2 Budget and Rectification and Airbone Study (COBRA-1999test, COBRA-2000 and COBRA-2003; Stephens et al, 2000; Kort et al, 2008) and during 20 IDEAS-1 This version flew on the NSF/NCAR C-130 during ACME-04, but collected smaller samples for 13C of CO2 and not O2 measurements (Sun et al, 2010). We present a selection of measured vertical profiles, O2:CO2 correlations, and latitude / altitude cross-sections (Sect. 5) that 30 highlight the resolution of the measurements and their ability to distinguish the influences of specific processes
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