EFFECTS OF SMOG

13.8. EFFECTS OF SMOG

The harmful effects of smog occur mainly in the areas of (1) human health and comfort, (2) damage to materials, (3) effects on the atmosphere, and (4) toxicity to plants. The exact degree to which exposure to smog affects human health is not known, although substantial adverse effects are suspected. Pungent-smelling, smog- produced ozone is known to be toxic. Ozone at 0.15 ppm causes coughing, wheezing, bronchial constriction, and irritation to the respiratory mucous system in healthy, exercising individuals. Largely because of potential health effects, the U.S. Environmental Protection Agency proposed in July 1997 to phase in revised national ambient air quality standards for ground-level ozone that would phase out the existing one-hour ozone standard and set a new 8-hour standard of 0.08 ppm ozone. 9 In addition to ozone, oxidant peroxyacyl nitrates and aldehydes found in smog are eye irritants.

Materials are adversely affected by some smog components. Rubber has a high affinity for ozone and is cracked and aged by it. Indeed, the cracking of rubber used to be employed as a test for the presence of ozone. Ozone attacks natural rubber and similar materials by oxidizing and breaking double bonds in the polymer according to the following reaction:

Rubber polymer

C OH + H 3 C C R' (13.8.1) This oxidative scission type of reaction causes bonds in the polymer structure to

break and results in deterioration of the polymer. Aerosol particles that reduce visibility are formed by the polymerization of the smaller moleules produced in smog-forming reactions. Since these reactions largely involve the oxidation of hydrocarbons, it is not suprising that oxygen-containing organics make up the bulk of the particulate matter produced from smog. Ether- soluble aerosols collected from the Los Angeles atmosphere have shown an

empirical formula of approximately CH 2 O. Among the specific kinds of compounds identified in organic smog aerosols are alcohols, aldehydes, ketones, organic acids, esters, and organic nitrates. Hydrocarbons of plant origin are prominent among the precursors to particle formation in photochemical smog. 10

Smog aerosols likely form by condensation on existing nuclei rather than by self-nucleation of smog reaction product molecules. In support of this view are electron micrographs of these aerosols showing that smog aerosol particles in the micrometer-size region consist of liquid droplets with an inorganic electron-opaque core ( Figure 13.6 ). Thus, particulate matter from a source other than smog may have Smog aerosols likely form by condensation on existing nuclei rather than by self-nucleation of smog reaction product molecules. In support of this view are electron micrographs of these aerosols showing that smog aerosol particles in the micrometer-size region consist of liquid droplets with an inorganic electron-opaque core ( Figure 13.6 ). Thus, particulate matter from a source other than smog may have

smoggy atmosphere. The three major oxidants involved are ozone, PAN, and nitro-

Figure 13.6. Representation of an electron micrograph of smog aerosol particles collected by a jet inertial impactor, showing electron-opaque nuclei in the centers of the impacted droplets.

gen oxides. Of these, PAN has the highest toxicity to plants, attacking younger leaves and causing “bronzing” and “glazing” of their surfaces. Exposure for several hours to an atmosphere containing PAN at a level of only 0.02-0.05 ppm will damage vegetation. The sulfhydryl group of proteins in organisms is susceptible to damage by PAN, which reacts with such groups as both an oxidizing agent and an acetylating agent. Fortunately, PAN is usually present at only low levels. Nitrogen oxides occur at relatively high concentrations during smoggy conditions, but their toxicity to plants is relatively low.

Short-chain alkyl hydroperoxides, which were mentioned in Section 13.4, occur at low levels under smoggy conditions, and even in remote atmospheres. It is possible that these species can oxidize DNA bases, causing adverse genetic effects. Alkyl hydroperoxides are formed under smoggy conditions by the reaction of alkyl

peroxy radicals with hydroperoxy radical, HO 2 • , as shown for the formation of methyl hydroperoxide below:

H 3 CO 2 • + HO 2 • → H 3 COOH + O 2 (13.8.2) Ames assays of methyl, ethyl, n-propyl, and n-butyl hydroperoxides (see Chapter

22) have shown some tendency toward mutagenicity on select strains of Salmonella typhimurium , 11 although any conclusions drawn from such studies on human health should be made with caution. The low toxicity of nitrogen oxides and the usually low levels of PAN, hydroperoxides, and other oxidants present in smog leave ozone as the greatest smog-produced threat to plant life. Some plant species, including sword-leaf lettuce, black nightshade, quickweed, and double-fortune tomato, are so susceptible to the effects of ozone and other photochemical oxidants that they are used as bioindicators

of the presence of smog. 12 Typical of the phytotoxicity of O 3 , ozone damage to a of the presence of smog. 12 Typical of the phytotoxicity of O 3 , ozone damage to a

Figure 13.7. Representation of ozone damage to a lemon leaf. In color, the spots appear as yellow chlorotic stippling on the green upper surface caused by ozone exposure.

Brief exposure to approximately 0.06 ppm of ozone may temporarily cut photosynthesis rates in some plants in half. Crop damage from ozone and other photochemical air pollutants in California alone is estimated to cost millions of dollars each year. The geographic distribution of damage to plants in California is illustrated in Figure 13.8 .

San Francisco

Los Angeles

Figure 13.8. Geographic distribution of plant damage from smog in California.