Concentration - is the measure of how much of a given substance there is mixed with another substance. This can apply to any sort of chemical mixture, but most frequently the concept is limited to homogeneous solutions. For scientific or technical applications, a qualitative account of concentration is almost never sufficient; therefore quantitative measures are needed to describe concentration. There are a number of different ways to quantitatively express concentration. They are based on mass, volume, or both. Depending on what they are based on it is not always trivial to convert one measure to the other, because knowledge of the density might be needed to do so. At times this information may not be available, particularly if the temperature varies.

Mixing ratio - The ratio of the mass of a variable atmospheric constituent to the mass of dry air.

Size distribution - The amounts of different size particles of solids or liquids that are suspended in air as an aerosol. Particle size affects scattering of sunlight (see Rayleigh scattering and Mie scattering) that makes blue skies, white clouds, and hazy smog, and that affects visibility. Size affects the nucleation capability of particles to form cloud droplets due to both the curvature effect and the solute effect. Relative amounts of different particle sizes can be used as a tracer for an air mass, such as indicating whether it originated over continents, oceans, urban areas, or rural areas. The total abundance of particles is often proportional to the total number density of cloud droplets, which affects the size to which these droplets can grow and their resulting evolution.

Composition - Type and quantity of dissolved or suspended compounds in aerosol volume.