Partial Pressure / Absolute Pressure

As discussed in our article on atmospheric pressure, atmospheric air is a mixture of several kinds of gases. According to Dalton’s Law, the atmospheric pressure is the sum of the vapor pressures of each individual type of gas. The pressure contribution of each kind of gas toward the total atmospheric pressure is called the partial pressure of that gas.

Dry atmospheric air contains 78.1% nitrogen (N₂) and 20.95% oxygen (O₂) by volume. It follows that 78.1% of the atmospheric pressure can be attributed to the partial pressure of N₂ and 20.95% to the partial pressure of O₂.

At an atmospheric pressure of 101.3 kPa at sea level (1 atm), oxygen exerts a partial pressure P_oxygen of 21.22 kPa, and nitrogen exerts a partial pressure P_nitrogen of 79.12 kPa. The partial pressure of oxygen can be calculated by:

where X_oxygen = 0.2095 (20.95% by volume of O₂ molecules in air) and P_air is the total absolute air pressure.

The absolute atmospheric pressure depends on altitude. At sea level the absolute pressure is 101.3 kPa. The pressure decreases approximately 0.2 kPa per every 100 m increase in altitude. This relationship is defined by the barometric-altitude equation:

where P_a is the absolute atmospheric pressure (unit: kPa) at altitude “a,” P_s is the atmospheric pressure at sea level (101.3 kPa), a is the altitude (unit: km) above sea level, and c is a constant equal to 18.4 km. Some absolute pressure values for various altitudes are shown in the table.