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FIGURE 1.2 Concentrations of S02 and "smoke" as well as the death rate during the 1952 smog episode (adapted from Wilkins, 1954).

FIGURE 1.2 Concentrations of S02 and "smoke" as well as the death rate during the 1952 smog episode (adapted from Wilkins, 1954).

3. "Los Angeles" Smog: Ozone and Photochemical Oxidants a. Historical

In the late 1940's, a remarkable air pollution phenomenon began to impact the Los Angeles area. In sharp contrast to "London" smog, the ambient air contained strongly oxidizing, eye-watering and plant-killing pollutants—and occurred on hot days with bright sunshine. Plant pathologists at the University of California, Riverside, observed a unique type of damage to agricultural crops in areas of the Los Angeles basin impacted by this "plague" and reported it to be an entirely new form of air pollution—Los Angeles smog (Middleton et al., 1950).

Shortly thereafter, in a classic series of papers in the early 1950's, Arie Haagen-Smit and co-workers reported that these plant damage symptoms observed outdoors in ambient air could be duplicated in the laboratory by irradiating plants with sunlight and concurrently exposing them to synthetic polluted air containing alkenes and nitrogen dioxide:

Similar effects were observed with sunlight-irradiated, diluted auto exhaust, which contains NOx (NOx = NO + N02) and a variety of hydrocarbons (HC)

(Haagen-Smit, 1952; Haagen-Smit et al., 1952, 1953; Haagen-Smit and Fox, 1954, 1955, 1956).

Since then, high ozone levels have also been measured throughout the world, e.g., in Athens, Greece, and in regions downwind from Sydney, Australia. In Mexico City, ozone levels over 400 ppb have been measured. Thus, although photochemical air pollution was first recognized in the Los Angeles area, it is now recognized to be a world-wide problem in areas where volatile organic compounds (VOC) and NOx emissions from major mobile and stationary sources are "trapped" by thermal inversions and irradiated by sunlight during transport to downwind regions. Encouragingly, in southern California, ozone peaks have declined significantly since around 1980 due to increasingly tight controls on VOC and NOx.

Table 1.2 summarizes some aspects of London and photochemical air pollution that have traditionally been considered to differentiate the two. However, as we shall see, it has become clear since the mid-1970's that these two, along with such phenomena as the fate of airborne toxic chemicals, are inextricably linked through their atmospheric chemistry. It is this common linkage that forms the central core of this book.

b. Photochemical Air Pollution

Today in many major urban areas around the world, air pollution is characterized more by the formation of ozone and other oxidants rather than by S02, particles, and sulfuric acid. In these regions, the primary pollutants are NOx (mainly NO) and volatile organic compounds (VOC), which undergo photochemical reactions in sunlight to form a host of secondary pollutants, the most prominent of which is 03. Some of these are

TABLE 1.2 Historical Aspects of Sulfurous (London) and Photochemical (Los Angeles) Air Pollution

Characteristics

Sulfurous (London)

Photochemical (Los Angeles)

First recognized Primary pollutants Secondary pollutants

Temperature Relative humidity

Type of inversion

Time air pollution peaks

Centuries ago S02, soot particles H2S04, sulfate aerosols, etc.

Radiation (ground)

Early morning

Mid-1940s voc, noa 03, PAN, HN03, aldehydes, particulate nitrate and sulfate, etc. Hot (> 75°F)

Low, usually hot and dry

Subsidence

(overhead) Noon to evening criteria pollutants for which air quality standards have been set, such as 03, S02, CO, N02, PMI(), and PM25 (ambient particulate matter less than 10 or 2.5 /¿m in diameter, respectively). Others are so-called "trace" noncriteria pollutants, e.g., gaseous peroxyacetyl nitrate (CH3C(0)00N02, PAN), nitric acid (HN03 or H0N02), formaldehyde (HCHO), and formic acid (HCOOH). The overall reaction is now written as

Certain reproducible features of time-concentration profiles for pollutants are observed in "smoggy" ambient air. Figure 1.3, a classic example of historical interest, shows such profiles for NO, N02, and total oxidant (mainly 03) in Pasadena, California, during a severe photochemical air pollution episode in July 1973. Reproducible features include the following:

• In the early morning, the concentration of NO rises and reaches a maximum at a time that approximately coincides with the maximum emissions of NO, in this case, peak automobile traffic;

• Subsequently, NOz rises to a maximum;

• Oxidant (e.g., 03) levels, which are relatively low in the early morning, increase significantly about noon when the NO concentration drops to a low value.

In this instance, near an urban center, the 03 reaches a maximum after N02 peaks. Downwind from urban

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