Spatiotemporal variations of the <i>δ</i>(O<sub>2</sub> ∕ N<sub>2</sub>), CO<sub>2</sub> and <i>δ</i>(APO) in the troposphere over the western North Pacific

Abstract. We analyzed air samples collected on board a C-130 cargo aircraft over the western North Pacific from May 2012 to March 2020 for atmospheric δ(O2 / N2) and CO2 amount fraction. Observations were corrected for significant artificial fractionation of O2 and N2 caused by thermal diffusion during the air sample collection 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 the cycle 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 the 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 combination of the northern and southern hemispheric seasonal cycles due to the interhemispheric 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 influence on annual net sea–air marine biological O2 flux during El Niño and net air–sea flux during La Niña. By analyzing the observed secular trends of δ(O2 / N2) and the 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.