Atmospheric O2/N2 measurements at two Japanese sites: estimation of global oceanic and land biotic carbon sinks and analysis of the variations in atmospheric potential oxygen (APO)

We present measurements of atmosphericO2/N2 and CO2 mixing ratios taken at Hateruma Island (HAT) and Cape Ochi- Ishi (COI) in Japan. Global carbon sinks are estimated from the tracer atmospheric potential oxygen (APO) calculated as the weighted sums of the observed O2/N2 and CO2, and the global CO2 data from the NOAA/ESRL GMD flask sampling network. The oceanic and land biotic sinks are 2.4 ± 0.7 and 0.5 ± 0.9 Pg C yr−1, respectively, for the 7-yr period (July 1998–July 2005) and 2.1 ± 0.7 and 1.0 ± 0.9 Pg C yr−1, respectively, for the 6-yr period (July 1999–July 2005). The former 7-yr estimates are based on the data from HAT only, while the latter 6-yr estimates are obtained using data from both HAT and COI. These estimations include an ocean outgassing correction of 0.48 Pg C yr−1. The instantaneous rates of change in the APO trends show large interannual variability with peak-to-peak amplitudes of about 30 per meg yr−1. The winter anomalies in the APO trend are the major contributor to the interannual variability, and the oceanic O2 influx associated with winter ventilation may be a significant cause of the variability in APO