Airborne particles are known collectively as aerosols. Large particles fall to the surface fairly quickly, usually within minutes. Small ones fall so slowly that they remain airborne for longer, but even they seldom remain in the air for more than a matter of hours. They are constantly replenished, however, and while they remain in the air they both scatter and absorb radiation.
Aerosols include mineral grains blown into the air by desert dust storms and by plowing dry soils, volcanic ash, smoke, and salt crystals left in the air when drops of sea spray evaporate. Some gases released from factory chimneys and from burning fossil fuels react in the air to form a variety of solid particles. There are also aerosols of biological origin, such as pollen grains, fungal and bacterial spores, and dimethyl chloride released by marine algae. Terpenes, which are chemicals released by plants, especially coniferous trees, form the particles that produce a blue haze, for example over the Smoky Mountains, in Tennessee.
Whether particular particles will scatter or absorb radiation depends on their size and color. Black smoke particles absorb radiation, for example, sulfate particles reflect it, and mineral dust may do either, depending on its composition. It also depends on the amount of particulate matter present. Particles are removed from the air by fallout, impaction, rainout, and washout. Fallout is the removal of particles by gravity. As the term suggests, they fall to the surface. Impaction removes particles that collide with surfaces and adhere to them. Rainout removes particles onto which water vapor condenses to form droplets that fall as precipitation. Washout removes particles that are gathered by raindrops and snowflakes as they fall through the air.
The concentration of aerosols varies widely. A period without rain allows them to accumulate, often sufficiently to reduce visibility. A shower then washes them to the surface. This is why the sunshine seems so much brighter after a shower and distant objects are more sharply delineated. Nevertheless, the quantity of aerosols is surprisingly large. On average, air over land contains approximately 2.5-65.5 million per cubic inch (150,000-4 million cm-3).
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