Correcting concentrations for altitude

Correcting concentrations for altitude

Atmospheric pollutant concentrations expressed as mass per unit volume of atmospheric air (e.g., mg/m³, µg/m³, etc.) at sea level will decrease with increasing altitude because the atmospheric pressure decreases with increasing altitude.
The change of atmospheric pressure with altitude can be obtained from this equation:^[2]

Given an atmospheric pollutant concentration at an atmospheric pressure of 1 atmosphere (i.e., at sea level altitude), the concentration at other altitudes can be obtained from this equation:

where:
	= altitude, in hundreds of meters
	= atmospheric pressure at altitude , in atmospheres
	= Concentration at sea level altitude, in mass per unit volume
	= Concentration at altitude , in mass per unit volume

As an example, given a concentration of 260 mg/m³ at sea level, calculate the equivalent concentration at an altitude of 1,800 meters:
C_a = 260 × 0.9877 ¹⁸ = 208 mg/m³ at 1,800 meters altitude