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

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 delta(O-2/N-2) and CO2 amount fraction. Observations were corrected for significant artificial fractionation of O-2 and N-2 caused by thermal diffusion during the air sample collection using the simultaneously measured delta(Ar/N-2). The observed seasonal cycles of the delta(O-2/N-2) and atmospheric potential oxygen (delta(APO)) varied nearly in opposite phase to the cycle of the CO2 amount fraction at all latitudes and altitudes. Seasonal amplitudes of delta(APO) decreased with latitude from 34 to 25 degrees 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 delta(APO) and CO2 amount fraction values generated by an atmospheric transport model, we found that the seasonal delta(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 delta(APO) underestimated the observed interannual variation in delta(APO) significantly, probably due to the interannual variation in the annual mean air-sea O-2 flux. Interannual variation in delta(APO) driven by the net marine biological activities, obtained by subtracting the assumed solubility-driven component of delta(APO) from the total variation, indicated a clear influence on annual net sea-air marine biological O-2 flux during El Nino and net air-sea flux during La Nina. By analyzing the observed secular trends of delta(O-2/N-2) 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.