The atmospheric aerosol is a suspension of solid and liquid particles in the air that displays a degree of stability with respect to gravitational settling. Aerosol particles originate from a large number of sources whose influences can change dramatically over time scales of minutes to hours or can remain relatively constant for years. Although the term aerosol technically applies to both the solid and liquid particles and the gases in which they are suspended, common usage allows aerosol to refer to the particles alone, a practice that will be followed here. Increasing interest in the sources and composition of aerosols has resulted from a growing awareness of their linkages to meteorology, climate, and global change and from a better appreciation of the roles these particles play in biogeochemical cycles.

Aerosols range in size from clusters of molecules (< 0.001 |im radius) to ultra-giant particles with radii of 100 (im or more. In one commonly used scheme (Junge, 1963), the particle size spectrum for the aerosol is separated into three classes: (l) Aitken particles (< 0.1 (im radius); (2) large particles (0.1 to l .0 |im radius); and giant particles (> l (im radius). In another scheme (Whitby, 1978) particles with radii, r < 0.1 (im, which are formed by homogeneous condensation, are referred to as the nucleation mode; particles in the 0.1 to l -(im size range are referred to as the accumulation mode because they are formed from the accumulation of nucleation mode particles and the deposition of gases. Often aerosols < l |im radius are simply referred to as fine particles (e.g., Heintzenberg, 1989), while larger aerosols, with a peak in the mass distribution at r — 2 to 5 (im, compose the coarse mode.

The various schemes for characterizing aerosol size distributions generally rely on the concept of an aerodynamic equivalent size, which is a normalization based on

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the behavior of a spherical particle of unit density (1 g/cm3). Most solid aerosol particles, however, are not spherical and few are of unit density. Some aerosol particles, such as sea salt under low relative humidity, are crystalline while others, including many composed of mineral matter, are angular. Aerosol particles also take the shape of rods or flat plates, and both natural and anthropogenic aerosols can be aggregates of smaller particles, which can be roughly spherical or in some cases can form chains. Biological aerosols, especially spores and pollen, often display complex geometries that have evolved to favor dispersal over long distances.

Populations of aerosol particles can be classified according to various criteria in addition to size: natural versus anthropogenic, organic versus inorganic, internally mixed versus externally mixed, mechanically generated (primary particles) versus products of gaseous reactions (secondary), etc. These various classification schemes often serve specific purposes, and the diversity in the different types of aerosols can hardly be overemphasized. This chapter will serve as an introduction to the sources and composition of the particles that compose the atmospheric aerosol. It will also summarize information regarding the sources and composition of aerosols and introduce some of the ways in which the biogeochemical cycles of aerosols are linked to those of other atmospheric constituents.

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