Info

" As of 1992 reported by Cochran et al. (1992) except for United States, 1997, standards; 1-h short-term standards reported unless otherwise indicated.

6 WHO = World Health Organization Standards. ' For Victoria.

components: a concentration and a time. For example, the U.S. NAAQS for CO is 35 ppm for 1 h. This means that to avoid deleterious effects, the CO concentration should not exceed 35 ppm for more than 1 h. The data in Table 2.7 are those for short exposure times, although standards also exist for longer times in many cases (see Cochran et al., 1992).

Table 2.8 illustrates the severity of air pollution problems in 20 so-called "megacities" around the world. Megacities were defined in this study as urban areas currently having, or anticipated to have by the year 2000, populations of > 10 million. In most cases, the World Health Organization guidelines (Tables 2.7) are exceeded by more than a factor of two for at least one of the air pollutants shown (Mage et al., 1996).

The United States sets two types of NAAQS, primary and secondary. They are based on information contained in air quality criteria documents that contain a wealth of information on all aspects of the criteria pollutants, as do the documents by the World Health Organization. These should be consulted for detailed information and references regarding pollutant sources, ambient levels, chemical transformations, effects, and so on.

Note that the definition of a U.S. primary ambient air quality standard is one designed "to protect the public health"—with "an adequate margin of safety." These standards are set to protect even the most susceptible groups in the population, including those with cardiac and respiratory disease and newborn infants whose defense systems are not well developed.

Secondary NAAQS are set to protect "public welfare." This includes economic losses due to damage to agricultural crops, forests, and materials as well as aesthetic effects, including visibility degradation.

In addition to the criteria pollutants, a wide variety of trace gaseous and particulate species are present in the polluted troposphere (Finlayson-Pitts and Pitts, 1997). Table 2.9 shows some of these gaseous noncrite-ria pollutants identified in photochemical air pollution and gives typical concentrations under conditions ranging from those in remote areas to severely polluted urban air (see also Chapter 11).

Although their peak concentrations are usually only in the ppb, or even ppt, range, taken together they can form a substantial fraction of the concentration of their copollutant ozone, a criteria pollutant for which an air quality standard has been set and which, in southern California, is used as the basis for smog alerts. For example, Fig. 2.22 shows the maximum concentrations of some of these secondary pollutants determined using FT-IR spectroscopy over a kilometer path length dur-

TABLE 2.8 Air Pollution Problems in Some Megacities"

Pollutant

TABLE 2.8 Air Pollution Problems in Some Megacities"

Pollutant

City

so2

0 0

Post a comment