Global oxygen distributions at the Earth's surface

Area of Science:

• Atmospheric chemistry and physics
• Earth system science
• Climate science

Background:

• Oxygen (O2) partial pressure is critical for aerobic processes but is influenced by atmospheric water vapor.
• Dalton's law dictates that increasing water vapor pressure decreases dry air partial pressure and thus O2 partial pressure.
• Previous studies often neglected water vapor, leading to misrepresentations of O2 abundances and gradients.

Purpose of the Study:

• To quantify the impact of humidity on global oxygen partial pressure (pO2) distribution.
• To re-evaluate atmospheric O2 cycling by accounting for water vapor's influence.
• To improve atmospheric transport models by incorporating hydrological cycle effects.

Main Methods:

• Utilized ERA5 reanalysis data to estimate molar fractions of dry air (χd).
• Mapped global pO2 gradients, considering water vapor pressure (e) and dry air pressure (pd).
• Analyzed meridional gradients from poles to tropics and cross-equatorial exchanges.

Main Results:

• Tropical regions exhibit significantly lower pO2 due to high humidity, creating strong meridional gradients.
• Oxygen's molar fraction (χO2) drops by thousands of ppm in the tropics, contrasting with dry air fraction variations.
• Humidity's control over pd and χd clarifies O2 cycling, air-sea fluxes, and gas transport mechanisms.

Conclusions:

• Accurate atmospheric O2 assessment requires accounting for water vapor's influence on partial pressures.
• The hydrological cycle plays a vital role in transporting O2 and other gases, challenging diffusion-only models.
• This research provides a more accurate understanding of atmospheric gas transport and its drivers.