New Atmospheric O2/N2 Ratio Measurements over the Western North Pacific Using a Cargo Aircraft C-130H

Air samples collected over the western North Pacific using a cargo aircraft C-130H have been analyzed for O2/N2 ratio (δ(O2/N2)), Ar/N2 ratio (δ(Ar/N2)), δ15N of N2, δ18O of O2 and δ40Ar since May 2012. The relationships of δ(Ar/N2), δ18O and δ40Ar with δ15N indicate a significant artificial fractionation due to thermal diffusion during the air sample collection. The observed δ(O2/N2) and the atmospheric potential oxygen (APO = O2 + 1.1 × CO2), corrected for the artificial fractionation, show clear seasonal cycles at all altitudes over Minamitorishima, with amplitudes decreasing with height. Both the seasonal amplitude of the mid-tropospheric APO and its ratio to the surface value are found to decrease significantly from the mid-latitude to the subtropical region. The amplitude of the mid-tropospheric CO2 seasonal cycle does not change significantly meridionally in a latitude band.