Global oxygen distributions at the Earth's surface

Available oxygen (O₂), described by its partial pressure ( [Formula: see text] ), governs key aerobic processes like dissolution, respiration, and oxidation. It makes up a practically constant fraction of the partial pressure of dry air (p_d), which declines at constant pressure (p) with rising water vapor pressure (e) according to Dalton's law (p = p_d + e). Since sea level p does not increase equatorward away from temperate zones, tropical humidity-with e reaching several percent of p-suppresses both p_d and [Formula: see text] by equal proportions. Oxygen's molar fraction ( [Formula: see text] ) reflects this, dropping by thousands of ppm in the tropics, unlike the humidity-invariant dry-air fraction ( [Formula: see text] ), which varies inversely with carbon dioxide over mere dozens of ppm. Past O₂ studies eliminated water vapor and analyzed dry air expressing [Formula: see text] , which was useful in constraining the carbon cycle but misrepresented O₂ abundances and gradients. Using ERA5 reanalysis data, we estimate molar fractions of dry air (χ_d) to map global [Formula: see text] , revealing strong meridional gradients from the highly aerobic winter pole to the relatively hypoxic tropics. Reckoning with humidity's control over p_d and χ_d clarifies key features of atmospheric O₂ cycling such as cross-equatorial exchanges, air-sea fluxes, and the physical mechanisms of gas transport. This challenges transport models based exclusively on diffusion and highlights the hydrological cycle's role in propelling air away from water vapor sources, significantly conveying not only O₂, but also greenhouse gases like methane and nitrous oxide.