text: Atmospheric pollutant concentrations expressed as mass per unit volume of atmospheric air (e.g., mg/m, µg/m, 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:
  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:
  As an example, given a concentration of 260 mg/m at sea level, calculate the equivalent concentration at an altitude of 1,800 meters:
  C = 260 × 0.9877 = 208 mg/m at 1,800 meters altitude