Particulates in Air Pollution

The black smoke released into the air by a diesel truck is often the most obvious form of pollution that we routinely encounter. The smoke is composed largely of particulate matter. Particulates are tiny solid or liquid particles—other than those of pure water—that are temporarily suspended in air and that are usually individually invisible to the naked eye. Collectively, however, such particles often form a haze that restricts visibility. Indeed, on many summer days the sky over North American and European cities is milky white rarher than blue. More importantly, breathing air that contains particulates is known to be hazardous to human health. In the material that follows, we investigate the wide range of sizes of the suspended particles and their origins.

The particles that are suspended in a given mass of air are neither all of the same size or shape nor do they all have the same chemical composition. The smallest suspended particles are about 0.002 jxm (i.e., 2 nm) in their dimensions; by contrast, the length of typical gaseous molecules is 0.0001 to 0.001 fxm (0.1 to 1 nm). The upper limit for suspended particles corresponds to dimensions of about 100 yum (i.e., 0.1 mm). When atmospheric water droplets coalesce to form particles bigger than this, they are raindrops and fall out of the air so quickly they are not considered to be "suspended." The ranges of particle sizes for common types of suspended particulates is illustrated in Figure 3-14-

Although few of the particles suspended in air are exactly spherical in shape, it is convenient and conventional to speak of all particles as if they were so. Indeed, the diameter of particulates is their most important property. Qualitatively, individual particles are classified as coarse or as fine depending upon whether their diameters are greater or less than 2.5 /nm, respectively. (About 100 million particles of diameter 2.5 fim would be required to cover the surface of a small coin.)

There are many common names for atmospheric particles: dust and soot refer to solids, whereas mist and fog refer to liquids, the latter denoting a high

Coarse particles

Fine particles

Rain Mist/Fog/Clouds

Atmospheric dust

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Agriculture sprays




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