The Content of the Atmosphere
Aerosols are liquid or solid particles generally uniformly distributed in the atmosphere [6,13,24—43]. Aerosol particles play an important role in the precipitation process, providing the nuclei upon which condensation and freezing take place. The particles participate in chemical processes and influence the electrical properties of the atmosphere.
Actual aerosol particles range in diameter from a few nanometers to about a few micrometers. When smaller particles are in suspension, the system begins to acquire the properties of a real aerosol structure. For larger particles, the settling rate is usually so rapid that they may not properly be called real aerosols. Nevertheless, the term is commonly employed, especially in the case of fog or cloud droplets and dust particles, which can have diameters of over 100 pm. In general, aerosols composed of particles larger than about 50 pm are unstable unless the air turbulence is extreme, as in a severe thunderstorm (see details in References 5—7).
From all classifications of atmospheric aerosols, the most commonly used one is according to size. General classification suggests three modes of aerosols : (1) a nuclei mode, which is generated by spontaneous nucleation of the gaseous material for particles less than 0.1 pm in diameter, (2) the accumulation mode for particles between 0.1 and 1 pm in diameter, mainly resulting from coagulation and cloud processes, and (3) the coarse mode for particles larger than 1.0 pm in diameter originating from the Earth’s surface, land, and ocean. The classification is quite similar to Junge’s designation , referred to as Aitken, large, and giant particles. The particles vary not only in chemical composition and size but also in shape (spheres, ellipsoids, rods, etc.).
Aerosol concentrations and properties depend on the intensity of the sources, on the atmospheric processes that affect them, and on the particle transport from one region to another. The size distribution of the atmospheric aerosol is one of its core physical parameters. It determines the various properties such as mass and number density, or optical scattering, as a function of particle radius. For atmospheric aerosols, this size range covers more than five orders of magnitude, from about 10 nm to several hundred micrometers. This particle size range is very effective for scattering of radiation at ultraviolet (UV), visible, and infrared (IR) wavelengths. The aerosol size distribution varies according to location, altitude, and time. In a first attempt to identify geographically distinct atmospheric aerosols, Junge classified aerosols depending on their location in space and sources into background, maritime, remote continental, and rural . This classification later was expanded and quantified [25—30].
From the optical standpoint, to underline scattering properties of the atmosphere the term haze aerosol was introduced [31,32]. Hazes are polydisperse aerosols in which the size range of particles extends from about 0.01 to 10 pm. Haze is a condition wherein the scattering property of the atmosphere is greater than that attributable to the gas molecules but is less than that of fog. Haze can include all types of aerosols. Cosmic dust, volcanic ash, foliage exudations, combustion products, bits of sea salt are all found to varying degrees in haze.