Abstract

We analyzed air samples collected onboard a cargo aircraft C-130 over the western North Pacific from May 2012 to March 2020 for atmospheric δ(O2/N2) and CO2 amount fraction. We corrected for significant artificial fractionation of O2 and N2 caused by thermal diffusion during the air sample collection by using the simultaneously-measured δ(Ar/N2). The observed seasonal cycles of the δ(O2/N2) and atmospheric potential oxygen (δ(APO)) varied nearly in opposite phase to that of the CO2 amount fraction at all latitudes and altitudes. Seasonal amplitudes of δ(APO) decreased with latitude from 34 to 25° N, as well as with increasing altitude from the surface to 6 km by 50–70 %, while those of CO2 amount fraction decreased by less than 20 %. By comparing the observed values with the simulated δ(APO) and CO2 amount fraction values generated by an atmospheric transport model, we found that the seasonal δ(APO) cycle in the middle troposphere was modified significantly by a superposition of the northern and southern hemispheric seasonal cycles due to the inter-hemispheric mixing of air. The simulated δ(APO) underestimated the observed interannual variation in δ(APO) significantly, probably due to the interannual variation in the annual mean air-sea O2 flux. Interannual variation in δ(APO) driven by the net marine biological activities, obtained by subtracting the assumed solubility-driven component of δ(APO) from the total variation, indicated a clear evidence of influence on annual sea-to-air (air-to-sea) marine biological O2 flux during El Niño (La Niña). By analyzing the observed secular trends of δ(O2/N2) and CO2 amount fraction, global average terrestrial biospheric and oceanic CO2 uptakes for the period 2012–2019 were estimated to be (1.8 ± 0.9) and (2.8 ± 0.6) Pg a−1 (C equivalents), respectively.

Highlights

  • Atmospheric O2/N2 ratio has been observed since the early 1990s, for separating out terrestrial biospheric and oceanic CO2 uptakes, and for evaluating marine biospheric activities (Keeling and Shertz, 1992)

  • We have presented the analytical results of the air samples for CO2 amount fraction, d(O2/N2), d(Ar/N2), d(15N) of N2, d(18O) of O2 and d(40Ar) for the period May 2012 – March 2020

  • The dcor.(O2/N2) values, corrected for the artificial fractionation by using d(Ar/N2), and the d(APO) values derived from dcor.(O2/N2) were shown to have clear seasonal cycles nearly in opposite phase to that of the CO2 amount fraction from the surface to 6 km along the latitudinal path from 25.5 to 33.5° N

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Summary

Introduction

Atmospheric O2/N2 ratio has been observed since the early 1990s, for separating out terrestrial biospheric and oceanic CO2 uptakes, and for evaluating marine biospheric activities (Keeling and Shertz, 1992). For this purpose, observations of the O2/N2 ratio have been carried out at many surface stations and on commercial cargo ships (e.g., Bender et 30 al., 2005; Manning and Keeling, 2006; Tohjima et al, 2008, 2019; Goto et al, 2017). With CO2 amount fraction due to the terrestrial biospheric activities and fossil fuel combustion, of which respective O2:CO2 exchange ratios (oxidative ratio (OR) = -Dy(O2)Dy(CO2)-1 mol mol-1) are about 1.1 and 1.4, respectively (Keeling, 1988; Severinghaus, 1995), where y stands for the dry amount fraction of gas, as recommended by the IUPAC Green Book (2007).

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