Some Definitions

Particles, or particulate matter, may be solid or liquid, with diameters between ~ 0.002 and ~100 /jlm. The lower end of the size range is not sharply defined because there is no accepted criterion at which a cluster of molecules becomes a particle. The upper end corresponds to the size of fine drizzle or very fine sand; these particles are so large that they quickly fall out of the atmosphere and hence do not remain suspended for significant periods of time. There are, of course, larger particles produced in the atmosphere (e.g., raindrops, ~1 mm, and hail, ~l-20 mm), but their rapid fallout precludes, for all practical purposes, their inclusion in the definition of atmospheric particles. As we shall see, the most important particles with respect to atmospheric chemistry and physics are in the 0.002- to 10-/j,m range.

Aerosols are defined as relatively stable suspensions of solid or liquid particles in a gas. Thus aerosols differ from particles in that an aerosol includes both the particles and the gas in which they are suspended. However, while this is the rigorous definition of aerosols, one should note that the term is often used in the atmospheric chemistry literature to denote just the particles.

Particles may be either directly emitted into the atmosphere or formed there by chemical reactions; we refer to these as primary and secondary particles, respectively. The relative importance of primary and secondary particles will clearly depend on the phenomena examined, the geographical location with its particular mix of emissions, and the atmospheric chemistry.

There are a number of properties of particles that are important for their role in atmospheric processes. These include, in addition to their number concentration, their mass, size, chemical composition, and aerodynamic and optical properties. Of these, size is the most important; it is related not only to the source of the particles (see later) but also to their effects on health, visibility, and climate.

Atmospheric particles are usually referred to as having a radius or a diameter, implying they are spherical. However, many particles in the atmosphere have quite

(1 nm) Particle Diameter (|xm)

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