There are two ways to express the concentration of pollutants in the atmospheric environment:

(1) Mass concentration expression: that is, the mass number of pollutants contained in every cubic meter of air (mg/m³), the conversion relationship:

mg/m³ = μg/L
μg/m³ = ng/L
ng/m³ = pg/L

(2) Volume concentration expression: that is, the volume of pollutants contained in one million volumes of air (ppn), conversion relationship:

1 ppm (parts per million) = 1×10⁻⁶
1 ppb (parts per billion) = 1×10⁻⁹
1 ppt (parts per trillion) = 1×10⁻¹²

Using the mass concentration unit (mg/m³) as the expression method of air pollutant concentration can easily calculate the true amount of pollutants, but it is affected by temperature and pressure environmental conditions; the volume concentration (ppm/ppb) is the volume ratio, so there is no such problem.

Conversion formula between mass unit (μg/m³) and ppb:

Y(μg/m³) = X(ppb) × M/22.4/K
K = (273+T)/273

In 101325 Pa, at 23℃, the conversion of gas concentration is as follows: