ABSTRACT
When quantifying the level of a pollutant in an environment, it is customary to express it in terms of the amount of some property of the pollutant per unit volume of the air in which it is dispersed. This is called the concentration
of the contaminant. The relevant property for a gaseous contaminant is typically its mass or volume, whereas for an aerosol, additional properties, such as the number of particles or the surface area of the particles, can be expressed as a concentration. The units of concentration can therefore be mass/volume (e.g.,
µ
g/m
), volume/volume (e.g., ppm), number/volume (e.g., particles per cubic centimeter, ppcc), or surface area/volume (e.g.,
µ
m
/m
).
