Components of the atmosphere composed of solid or liquid matter. Particles may be both released from the earth's surface, such as dust or smoke, or formed in the atmosphere, as in rain or ice particles or sulfate aerosol. The particles in the atmosphere are usually defined in terms of their size, or diameter. Particles less than 100 μm in diameter are referred to as aerosol particles. These are aerodynamically stable and settle out only slowly (strictly speaking, the term aerosol refers to the gas–particle colloidal system, not just the particulate phase). The aerosol is usually divided into three modes: the Aitken mode (diameter less than 0. 5 μm), the accumulation mode (0. 5–2. 0 μm), and the coarse mode (greater than 2 μm). The Aitken and accumulation modes are collectively referred to as fine particles. The larger particles of the coarse mode compose clouds and hydrometeors such as rain and sleet, which precipitate out. See also particle.