Introduction

Smog, whether sulfur-based or photochemical, often has unpleasant odors due to some of its gaseous components. More seriously, the initial pollutants, intermediates, and final products of the reactions in smog affect human health and can cause damage to plants, animals, and some materials. In this chapter, we describe the detrimental effects on animals, plants, and materials of the gases and particles in polluted air—including the air we encounter indoors—and methods by which air pollution can be combated. Included in the discussions ate the environmental effects of acid rain, a phenomenon that results from polluted air.

Haze

The most obvious manifestation of photochemical smog is a yellowish-brownish-gray haze that is due to the presence in air of small water droplets containing products of chemical reactions that occur among pollutants in air. This haze, familiar to most of us who live in urban areas, now extends periodically to once-pristine areas such as the Grand Canyon in Arizona.

Particles whose diameter is about that of the wavelength of visible light, i.e., 0.4-0.8 /xm, can scatter light and interfere with its transmission, thereby reducing visual clarity, long-distance visibility, and the amount of sunlight reaching the ground. A high concentration in air of particles of diameters between 0.1 fim and 1 jam produces a haze. Indeed, one conventional technique of measuring the extent of particulate pollution in an air mass is to determine its haziness. The existence of smog in the air can often be determined by simply looking at buildings or hills in the distance and seeing if their appearance is partially masked by haze.

The widespread haze in the Arctic atmosphere in winter is due to sulfate aerosols that originate from the burning of coal, especially in Russia and Europe. The enhanced haziness in summertime over much of North America is due mainly to sulfate aerosols arising from industrialized areas in the United States and Canada. Fine particles also are largely responsible for the haze associated with Los Angeles and other locations subject to episodes of photochemical smog. The smog aerosols contain nitric acid that has been neutralized to salts. Also present in these aerosols are carbon-containing products that are intermediates in the photochemical smog reactions; however, intermediates formed from fuel molecules having short carbon chains usually have vapor pressures high enough that they exist as gases rather than condense onto particles. The typical composition of the fine component of an aerosol suspended over continental areas is illustrated in Figure 4-1.

Since most fine particles in urban air are secondary pollutants, their number can only be controlled by reducing emissions of the primary pollutant gases from which they are created. Thus governments have successively required more and more stringent emission controls on vehicles, power plants, etc., as discussed in Chapter 3. The switch to low-sulfur gasoline and diesel fuels should make catalytic converters on vehicles more efficient in reducing emissions.

Nitrate, 4%

Ammonium, 11%

Elementary carbon, 5%

Nitrate, 4%

Ammonium, 11%

Elementary carbon, 5%

FIGURE 4-1 Typical composition of fine continental aerosol. [Adapted from J. Heinlzenberg, Teilus 41 B (1989): 149-160.J
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