How does oxygen partial pressure affect dissolved oxygen levels?
Oxygen in water obeys Henry’s law rather well; the solubility is roughly proportional to the partial pressure of oxygen in the air: pO2 = KO2 xO2 where pO2 is the partial pressure of oxygen in Torr, xO2 is the mole fraction of oxygen in oxygen-saturated water, and KO2 is the Henry’s law constant for oxygen in water (about 3.30 × 107 K/Torr for at 298 K [2]). Higher air pressure means higher partial pressure of oxygen, so waters at sea level can contain dissolve slightly more oxygen than mountain streams at the same temperature. High humidity very slightly lowers the fraction of oxygen in the air, and so lowers saturated dissolved oxygen levels slightly. Many empirical equations are available to accurately estimate oxygen solubility as a function of temperature, pressure, and humidity. The more accuracy you require, the more complex the equations are. Here are some very simple empirical equations that give the saturated dissolved O2 concentration (DO) in mg O2/L water.
