A suspension of small particles in a gas. The particles may be solid or liquid or a mixture of both. Aerosols are formed by the conversion of gases to particles, the disintegration of liquids or solids, or the resuspension of powdered material. Aerosol formation from a gas results in much finer particles than disintegration processes ( except when condensation takes place directly on existing large particles). Dust, smoke, fume, haze, and mist are common terms for aerosols. Dust usually refers to solid particles produced by disintegration, while smoke and fume particles are generally smaller and

formed from the gas phase. Mists are composed of liquid droplets. These special terms are helpful but are difficult to define exactly.

Aerosol particles range in size from molecular clusters on the order of 1 nanometer to 100 micrometers. The stable clusters formed by homogeneous nucleation and the smallest solid particles that compose agglomerates have a significant fraction of their molecules in the surface layer.

Aerosols are important in the atmospheric sciences and air pollution; inhalation therapy and industrial hygiene; manufacture of pigments, fillers, and metal pow­ders; and fabrication of optical fibers. Atmospheric aerosols influence climate di­rectly and indirectly. They directly affect radiation transfer on global and regional scales. Indirect effects result from their role as cloud condensation nuclei in chang­ing droplet size distributions that affect the optical properties of clouds and pre­cipitation. There is evidence that the stratospheric aerosol is significant in ozone destruction.

The atmospheric aerosol consists of material emitted directly from sources (primary component) and material formed by gas-to-particle conversion in the atmosphere (sec­ondary component). The secondary component is usually the result of chemical re­actions which take place in either the gas or aerosol phases. Contributions to the atmospheric aerosol come from both natural and anthropogenic sources. The effects of the atmospheric aerosol are largely determined by the size and chemical composition of the individual particles and their morphology (shape or fractal character). For many applications, the aerosol can be characterized sufficiently by measuring the particle size distribution function and the average distribution of chemical components with respect to particle size. The chemical composition of the atmospheric aerosol can be used to resolve its sources, natural or anthropogenic, by a method based on chemical sig­natures. Particle-to-particle variations in chemical composition and particle structural characteristics can also be measured; they probably affect the biochemical behavior and nucleating properties of aerosols.

Aerosol optical properties depend on particle size distribution and refractive index, and the wavelength of the light. These are determining factors in atmospheric visibility and the radiation balance.

Effects of the atmospheric aerosol on human health have led to the establishment of ambient air-quality standards by the United States and other industrialized nations. Adverse health effects have stimulated many controlled studies of aerosol inhalation by humans and animals. There is much uncertainty concerning the chemical com­ponents of the atmospheric aerosol that produce adverse health effects detected in epidemiological studies.

Aerosols containing pharmaceutical agents have long been used in the treatment of lung diseases such as asthma. Current efforts are directed toward systemic delivery of drugs, such as aerosolized insulin, which are transported across the alveolar walls into the blood.

Aerosol processes are used routinely in the manufacture of fine particles. Aerosol reaction engineering refers to the design of such processes, with the goal of relat­ing product properties to the properties of the aerosol precursors and the process conditions. The most important large-scale commercial systems are flame reactors for production of pigments and powdered materials such as titania and fun:ed silica. Op­tical fibers are fabricated by an aerosol process in which a combustion-generated silica fume is deposited on the inside walls of a quartz tube a few centimeters in diameter, along with suitable dopant aerosols to control refractive index. Pyrolysis reactors are used in carbon black manufacture. Micrometer-size iron and nickel powders are pro­duced industrially by the thermal decomposition of their carbonyls. Large pilot-scale

aerosol reactors are operated using high-energy electron beams to irradiate flue gases from fossil fuel combustion. The goal is to convert sulfur oxides and nitrogen oxides to ammonium sulfate and nitrate that can be sold as a fertilizer.

Atmospheric aerosols and aerosols emitted from industrial sources are normally composed of mixtures of chemical compounds. Each chemical species is distributed with respect to particle size in a way that depends on its source and past history; hence, different substances tend to accumulate in different particle size ranges. This effect has been observed for emissions from pulverized coal combustion and municipal waste incinerators, and it undoubtedly occurs in emissions from other sources. Chemical segregation with respect to size has important implications for the effects of aerosols on public health and the environment, because particle transport and deposition depend strongly on particle size.

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