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

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.

Sea ice and oxygen

“…Regional data deficiencies make isolation of the potential drivers very difficult (Mathis et al 2010;Gregg et al 2014;Roden et al 2016;Bushinksy et al 2017;Hammond et al 2017;Rosso et al 2017;Frey et al 2018;Bai et al 2019). Satellite, aircraft and float observations of some key variables (such as plankton density, sea ice algal productivity, oxygen and carbon dioxide fluxes) are as yet very sparse in high latitudes (Rysgaard et al 2011;Liang et al 2017;Gray et al 2018;Bushinsky et al 2017Bushinsky et al , 2019Pinkerton & Hayward 2021;Stephens et al 2021) and remote sensing may not quantify under-ice productivity (Tremblay et al 2012).…”

Spatiotemporal variations of the δ(O2/N2), CO2 and δ(APO) in the troposphere over the Western North Pacific

“…Aircraft observations of the O2/N2 ratio have been conducted in the past (e.g. Sturm et al, 2005;Ishidoya et al, 2008aIshidoya et al, , 2012van der Laan et al, 2014;Morgan et al, 2019;Birner et al, 2020;Stephens et al, 2018Stephens et al, , 2021. Sturm et al (2005) observed a vertical gradient and seasonal cycle in the O2/N2 ratio in the height range of 0.8-3.1 km over Perthshire, United Kingdom, for the period [2003][2004].…”

“…Kawamura et al, 2006;Ishidoya et al, 2013), however Ishidoya et al (2012) and van der Laan et al (2014) did not observe them. and Stephens et al (2021) observed O2/N2 and Ar/N2 simultaneously, and were able to correct for thermally-diffusive artificial fractionation on O2/N2, by using coefficients of 3.54 and 3.77 of Ar/O2, respectively. By using the corrected O2/N2 ratio, were able to observe spatiotemporal variations in the O2/N2 ratio from the surface to the middle troposphere over the Western North Pacific around Japan, on monthly scheduled flights, for the period May 2012 to April 2013.…”

“…By using the corrected O2/N2 ratio, were able to observe spatiotemporal variations in the O2/N2 ratio from the surface to the middle troposphere over the Western North Pacific around Japan, on monthly scheduled flights, for the period May 2012 to April 2013. Similarly, Stephens et al (2021) were able to provide a better picture of much wider-area distributions of O2/N2 ratio, from 0-14 km and 87° N to 85° S, using measurements from a series of aircraft campaigns such as five HIAPER Pole-to-Pole Observations campaigns (HIPPO) in 2009-2011 (Wofsy et al, 2011), and the O2/N2 Ratio and CO2 Airborne Southern Ocean (ORCAS) study in 2016 (Stephens et al, 2018). Stephens et al (2021) also conducted continuous observations of O2 mole fraction using a vacuum ultraviolet (VUV) absorption detector (Stephens et al, 2003).…”

“…Similarly, Stephens et al (2021) were able to provide a better picture of much wider-area distributions of O2/N2 ratio, from 0-14 km and 87° N to 85° S, using measurements from a series of aircraft campaigns such as five HIAPER Pole-to-Pole Observations campaigns (HIPPO) in 2009-2011 (Wofsy et al, 2011), and the O2/N2 Ratio and CO2 Airborne Southern Ocean (ORCAS) study in 2016 (Stephens et al, 2018). Stephens et al (2021) also conducted continuous observations of O2 mole fraction using a vacuum ultraviolet (VUV) absorption detector (Stephens et al, 2003). They adjusted the continuous O2 data to the simultaneously observed flask-based O2/N2 ratio corrected for the artificial fractionation using Ar/N2 ratio, since the artificial fractionations for the continuous observations were more significant than that for the flask sampling due to the lower flow rate.…”