Table 1.

Commonly Used Terminology to Describe Characteristics of Aerosols

Parameter	Abbreviation	Comments
Aerodynamic diameter	AD	The AD is the diameter of a fictitious sphere of unit density (1 g cm−3) that has the same gravitational (settling) velocity in the same gas as the actual particle.
		AD = d (sg)1/2; where sg = ρparticle/ρwater.
		For particles of unit density, the AD is the same as the physical diameter.
Mass median aerodynamic diameter	MMAD	The MMAD divides the aerosol size distribution in half by mass. It is the diameter at which half the mass of the aerosol particles is contained in particles with larger diameter and the other half in particles with smaller diameter.
Geometric SD	GSD	GSD is a measure of dispersion of particle sizes within an aerosol. The GSD is the ratio of the median diameter to the diameter at ±1 SD from the median diameter. In a cumulative distribution plot of the AD and mass of particles, the GSD is calculated as the ratio of the median diameter to the diameter at 15.9% of the probability scale, or the ratio of the diameter at 84.1% on the probability scale to the median diameter. Aerosols with GSD ≥ 1.22 are considered polydisperse.