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.