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

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.