Preprints
https://doi.org/10.5194/amt-2020-294
https://doi.org/10.5194/amt-2020-294

  28 Aug 2020

28 Aug 2020

Review status: a revised version of this preprint was accepted for the journal AMT and is expected to appear here in due course.

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

Britton B. Stephens1, Eric J. Morgan2, Jonathan D. Bent2,1,a, Ralph F. Keeling2, Andrew S. Watt1, Stephen R. Shertz1, and Bruce C. Daube3 Britton B. Stephens et al.
  • 1National Center for Atmospheric Research, Boulder, Colorado, USA
  • 2Scripps Institution of Oceanography, La Jolla, California, USA
  • 3Harvard University, Cambridge, Massachusetts, USA
  • anow at: Picarro, Inc., Santa Clara, California, USA

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 85° S throughout the seasonal cycle. The 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 seconds 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 Network 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 6 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.

Britton B. Stephens et al.

 
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Britton B. Stephens et al.

Data sets

START-08 Airborne Oxygen Instrument B. Stephens, R. Keeling, J. Bent, A. Watt, S. Shertz, and W. Paplawsky https://doi.org/10.5065/D6DJ5CZ5

HIPPO-1 Airborne Oxygen Instrument B. Stephens, R. Keeling, J. Bent, A. Watt, S. Shertz, and W. Paplawsky https://doi.org/10.5065/D6J38QVV

HIPPO-2 Airborne Oxygen Instrument B. Stephens, R. Keeling, J. Bent, A. Watt, S. Shertz, and W. Paplawsky https://doi.org/10.5065/D67H1GXJ

HIPPO-3 Airborne Oxygen Instrument B. Stephens, R. Keeling, J. Bent, A. Watt, S. Shertz, and W. Paplawsky https://doi.org/10.5065/D65Q4TF0

HIPPO-4 Airborne Oxygen Instrument B. Stephens, R. Keeling, J. Bent, A. Watt, S. Shertz, and W. Paplawsky https://doi.org/10.5065/D679431D

HIPPO-5 Airborne Oxygen Instrument B. Stephens, R. Keeling, J. Bent, A. Watt, S. Shertz, and W. Paplawsky https://doi.org/10.5065/D6WW7G0D

HIPPO-1 Multiple Enclosure Device for Unfractionated Sampling of Air (MEDUSA) Flask Data B. Stephens, R. Keeling, J. Bent, A. Watt, S. Shertz, and W. Paplawsky https://doi.org/10.26023/J0VT-J67P-330R

HIPPO-1 Multiple Enclosure Device for Unfractionated Sampling of Air (MEDUSA) Kernel Data B. Stephens, R. Keeling, J. Bent, A. Watt, S. Shertz, and W. Paplawsky https://doi.org/10.26023/4NM6-3MPG-WC14

HIPPO-2 Multiple Enclosure Device for Unfractionated Sampling of Air (MEDUSA) Flask Data B. Stephens, R. Keeling, J. Bent, A. Watt, S. Shertz, and W. Paplawsky https://doi.org/10.26023/30T9-FZ21-4G04

HIPPO-2 Multiple Enclosure Device for Unfractionated Sampling of Air (MEDUSA) Kernel Data B. Stephens, R. Keeling, J. Bent, A. Watt, S. Shertz, and W. Paplawsky https://doi.org/10.26023/P4PE-KKYS-FZ07

HIPPO-3 Multiple Enclosure Device for Unfractionated Sampling of Air (MEDUSA) Flask Data B. Stephens, R. Keeling, J. Bent, A. Watt, S. Shertz, and W. Paplawsky https://doi.org/10.26023/MYW6-DQQ6-PZ0R

HIPPO-3 Multiple Enclosure Device for Unfractionated Sampling of Air (MEDUSA) Kernel Data B. Stephens, R. Keeling, J. Bent, A. Watt, S. Shertz, and W. Paplawsky https://doi.org/10.26023/GA02-K0FR-C10M

HIPPO-4 Multiple Enclosure Device for Unfractionated Sampling of Air (MEDUSA) Flask Data B. Stephens, R. Keeling, J. Bent, A. Watt, S. Shertz, and W. Paplawsky https://doi.org/10.26023/XQW5-YHPP-XG0M

HIPPO-4 Multiple Enclosure Device for Unfractionated Sampling of Air (MEDUSA) Kernel Data B. Stephens, R. Keeling, J. Bent, A. Watt, S. Shertz, and W. Paplawsky https://doi.org/10.26023/FF65-2RZM-ZB00

HIPPO-5 Multiple Enclosure Device for Unfractionated Sampling of Air (MEDUSA) Flask Data B. Stephens, R. Keeling, J. Bent, A. Watt, S. Shertz, and W. Paplawsky https://doi.org/10.26023/R8JN-Z3TG-2E0N

HIPPO-5 Multiple Enclosure Device for Unfractionated Sampling of Air (MEDUSA) Kernel Data B. Stephens, R. Keeling, J. Bent, A. Watt, S. Shertz, and W. Paplawsky https://doi.org/10.26023/X9KY-CK34-VR10

ORCAS Airborne Oxygen Instrument B. Stephens, J. Bent, A. Watt, R. Keeling, E. Morgan, and S. Afshar https://doi.org/10.5065/D6N29VC6

ORCAS Medusa Flask Sampler Flask Data B. Stephens, J. Bent, A. Watt, R. Keeling, E. Morgan, S. Afshar, and W. Paplawsky https://doi.org/10.5065/D6H130FW

ORCAS Medusa Flask Sampler Kernel Data B. Stephens, J. Bent, A. Watt, R. Keeling, E. Morgan, S. Afshar, and W. Paplawsky https://doi.org/10.5065/D6MS3R6C

ATom: L2 In Situ Measurements from the NCAR Airborne Oxygen Instrument (AO2) B. Stephens, E. Morgan, A. Watt, J. Bent, S. Afshar, R. Keeling, and W. Paplawsky https://doi.org/10.3334/ORNLDAAC/1704

ATom: L2 Measurements from Medusa Whole Air Sampler (Medusa) E. Morgan, B. Stephens, J. Bent, A. Watt, S. Afshar, W. Paplawsky, and R. Keeling https://doi.org/10.3334/ORNLDAAC/1729

Britton B. Stephens et al.

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Short summary
We describe methods used to make high-precision global-scale airborne measurements of atmospheric oxygen concentrations over a period of 20 years, in order to study the global carbon cycle. Our techniques include an in situ vacuum ultraviolet absorption instrument and a pressure and flow-controlled, cryogenically-dried, glass flask sampler. We have deployed these instruments in 15 airborne research campaigns spanning from the Earth’s surface to the lower stratosphere and from pole to pole.