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[1] We examine the latitudinal distribution of the tracer “atmospheric potential oxygen” APO = O2 + 1.1 × CO2 between 40°S and 50°N using new atmospheric CO2 and O2 measurements from flask samples collected onboard cargo ships between Japan and the United States, and Japan and Australia (or New Zealand) during the period from December 2001 to August 2004. Because APO is unaltered during photosynthesis and respiration by land vegetation, its atmospheric distribution is tightly linked to the air-sea gas exchange of O2 and CO2 and the underlying processes. We find that the seasonal amplitude of APO increases towards high latitudes in both hemispheres and its minimum is located approximately 10-degree north of the equator. The latitudinal distribution of annual mean APO shows a maximum in the tropics, confirming the recent coupled ocean-atmosphere model predictions for this region.