Applications

Concentrations of gaseous pollutants in the air: units of measurement

The concentrations of gaseous pollutants present in the air are generally given in units of mass (milligrams, micrograms, nanograms, or picograms) per unit of air volume (cubic meter, cubic centimeter, or liter). However, concentrations can also be expressed in parts per million (ppm) or parts per billion (ppb) using a conversion factor. This factor is determined based on the molar mass of the chemical compound and also depends on temperature and atmospheric pressure. As a general rule, conversions are performed assuming a pressure of 1 atmosphere and a temperature of 25°C.

Method

Under these conditions, the equation used to convert a given concentration from parts per million to milligrams per cubic meter (\(mg/m^3\)) is as follows:

Concentration (\(mg/m^3\)) = 0.0409 × concentration (\(ppm\)) × molar mass (\(g/mol\))

The conversion from (\(mg/m^3\)) to \(ppm\) is done using the following equation:

Concentration (\(ppm\)) = 24.45 × concentration (\(mg/m^3\)) ÷ molar mass (\(g/mol\))

The same equation can be used to convert micrograms per cubic meter (µg/m³) to parts per billion (\(ppb\)) and vice versa:

Concentration (\(µg/m^3\)) = 0.0409 × concentration (\(ppb\)) × molar mass (\(g/mol\))

Concentration (\(ppb\))= 24.45 × concentration (\(µg/m^3\)) ÷ molar mass (\(g/mol\))

Reminder

Numbers:

Million = 1,000,000

Billion = 1,000,000,000

Trillion = 1,000,000,000,000

Masses:

1 kilogram (kg) = 1,000 grams = 1.10³ grams

1 milligram (mg) = 1/1,000 gram = 0.001 gram = 1.10⁻³ gram

1 microgram (µg) = 1/1,000,000 gram = 0.000001 gram = 1.10⁻⁶ gram

1 nanogram (ng) = 1/1,000,000,000 gram = 0.000000001 gram = 1.10⁻⁹ gram

1 picogram (pg) = 1/1,000,000,000,000 gram = 0.000000000001 gram = 1.10⁻¹² grams

28 grams ≈ 1 ounce

Volumes:

1 liter (L) = 1,000 milliliters (mL) = 1,000 cubic centimeters (cm³)

1 cubic meter (m³) = 1,000 liters (L)