Info

0.1 1 10 100 1000 Fuel Consumption (106tons yr"1)

10,000

FIGURE 2.8 S02 emissions in many different regions as a function of rate of fuel consumption. Data for Europe and the United States are for 1980, and those for Asia are for 1987 (from Kato and Akimoto, 1992).

Biogenic processes, however, emit reduced forms of sulfur, including dimethyl sulfide and hydrogen sulfide, with lesser amounts of carbon disulfide (CS2), dimethyl disulfide (CH3SSCH3), carbonyl sulfide (COS), and methyl mercaptan (CH3SH). These reduced sulfur compounds are then oxidized in the atmosphere as described in detail in Chapter 8.E.

One estimate of the global emissions of sulfur com pounds from both anthropogenic and natural sources is given in Table 2.3 (Spiro et al., 1992).

5. Total Suspended Particles (TSP), PM10, and PM2.5

Air quality standards for particulate matter in the United States were expressed some years ago in terms of the mass of total suspended particulate matter (TSP).

Fuel Combustion -// ""N. Industrial

Fuel Combustion -Electric Utility 67%

Fuel Combustion -// ""N. Industrial

Fuel Combustion -Electric Utility 67%

FIGURE 2.9 Contribution of various sources to total anthropogenic S02 emissions in the United States in 1996 (from EPA, 1997).

Fuel Combustion - Other 4%

All Other 7%

Non-Road Engines and Vehicles 2% Metals Processing 3%

FIGURE 2.9 Contribution of various sources to total anthropogenic S02 emissions in the United States in 1996 (from EPA, 1997).

Year

FIGURE 2.10 S02 emissions in million tons of equivalent S02 for the period 1970 to 1986 for Asia, Europe, North America, and the USSR (from Hameed and Dignon, 1992).

Year

FIGURE 2.10 S02 emissions in million tons of equivalent S02 for the period 1970 to 1986 for Asia, Europe, North America, and the USSR (from Hameed and Dignon, 1992).

The standard was then changed to mass of suspended particulate matter less than 10 /¿m in size, commonly called PM10 or PMI(), and more recently was modified to include particulate matter less than 2.5 /¿m in diameter, PM2.5 or PM25.

The rationale for basing air quality standards on smaller particles is evident from an examination of Fig. 2.12, a diagram of the human respiratory tract. Larger particles that are inhaled are removed in the head or upper respiratory tract. The respiratory system from the nose through the tracheobronchial region is covered with a layer of mucus that is continuously moved upward by the motion of small hairlike projections called cilia. Large particles deposit on the mucus, are moved up, and are ultimately swallowed.

On the other hand, particles from fossil fuel combustion and gas-to-particle conversion are generally much smaller (< 2.5-/j,m diameter) and fall in the respirable size range. These particles can reach the alveolar region where gas exchange occurs. This region is not coated with a protective mucus layer, and here the clearance time for deposited particles is much greater than in the upper respiratory tract; hence the potential for health effects is much greater (Phalen, 1984).

Figure 2.13 shows the deposition of particles in various regions of the respiratory tract as a function of particle diameter (Phalen, 1984; Phalen et al., 1991; Yeh et al., 1996). The deposition fraction of PM10 in the pulmonary and tracheobronchial regions can be quite large, so it is not surprising that health effects could be associated with these particles. Deposition in the upper portions of the respiratory system is dominated primarily by the large particles, which are readily taken out in the nose and upper airways.

The deposition curves shown in Fig. 2.13 can be understood in terms of the major mechanisms of removal of particles in the respiratory tract: diffusion, sedimentation, and inertial impaction (see Chapter 9). The smallest particles undergo rapid Brownian diffusion, which carries them to the lung surface where they can be taken up; this is responsible for the large deposition in the pulmonary region seen in Fig. 2.13 for particle sizes below about 0.5 /¿m. Gravitational settling, i.e., sedimentation, is also an important mechanism of deposition both in the pulmonary region and in the tracheobronchial region. In both cases, the airways are relatively small so that the particle does not have large distances to travel before reaching a surface. The third mechanism, impaction, occurs when the airstream in which the particle is suspended changes direction due to a bifurcation in the lung, but the inertia of the particle carries it forward to impact the lung surface.

There has been great interest in airborne particulate matter recently due to the results of a number of epidemiological studies showing a correlation between increased mortality and levels of airborne particles. Figure 2.14 shows one such correlation reported by Dockery et al. (1993). A clear relationship between mortality rates and the concentration of fine particles PM25, as well as with particle sulfate, is seen. Since sulfate is found primarily in fine particles, these observations are not independent. Schwartz et al. (1996) report a 1.5% increase in total daily mortality with an increase of 10 /jlg m~3 in PM25. Deaths due to chronic obstructive pulmonary disease increased by 3.3% and those to ischemic heart disease by 2.1%.

What is somewhat puzzling, but certainly intriguing, is that the cities included in the studies in Fig. 2.14, as well as more recent ones where these findings have been corroborated, are quite disparate in terms of location and the types of air pollutants that would be expected to predominate in each region, yet a consistent relationship is found. Most such epidemiological studies to date are consistent with this finding. This suggests either that the health effects associated with particles are independent of their chemical composition or that there is some common chemical component. In addition, recent analysis of these studies also finds a correlation with other air pollutants as well and suggests that more than one pollutant may be involved (Lipfert and Wyzga, 1995). This issue is a fascinating one that clearly requires much more research on the formation, chemical composition, and effects of particles and associated air pollutants (e.g., see Phalen and McClellan, 1995; Dockery and Schwartz, 1995; Bascom et al., 1996a,b; Wilson and Suh, 1997; and the review by Vedal, 1997).

FIGURE 2.11 Patterns of 1987 annual emission flux of S02 in Asia (in units of millimoles as S per m2 per year) (from Akimoto and Narita, 1994).

Interestingly, these fine particles not only are of great concern from the point of view of health effects but also are responsible for most of the light scattering,

TABLE 2.3 Estimates of Global Emissions of Sulfur Compounds in 1980 (Tg yr ~ 1 as S)"

Source Sulfur emissions (Tg yr ~1)

Fuel combustion/industrial activities 77.6

Biomass burning 2.3

Volcanic eruptions 9.6

Marine biosphere 11.9

Terrestrial biosphere 0.9

Total 102.2

that is, visibility reduction. Thus an improvement in visibility in areas impacted by air pollution may be accompanied by a reduction in the total particle mass deposition in the alveolar region of the respiratory system as well.

In the United States in 1996, the total emissions of PMI0 were 31 x 106 short tons per year, or 28 Tg per year (EPA, 1997). Fugitive dust sources such as un-paved roads make up ~ 90% of the total PM10 emissions. Figure 2.15 shows sources of PM10 in the United States in 1996 split into (a) nonfugitive dust sources (~ 10% of the total) and (b) fugitive dust sources (EPA, 1997).

Globally, anthropogenic emissions of PMH) have been estimated to be 345 Tg yr-1 without including secondary nitrate and organics (Wolf and Hidy, 1997)

Turbinates Vestibule

Turbinates Vestibule

Nasopharynx Oral Pharynx Epiglottis

Lung

FIGURE 2.12 Schematic diagram of human respiratory tract. (From Hinds, W. C. Aerosol Technology. Copyright © 1982 John Wiley & Sons, Inc. Reprinted by permission of John Wiley & Sons, Inc.)

Nasopharynx Oral Pharynx Epiglottis

Lung

Conducting Bronchiole Terminal Bronchiole Respiratory Bronchiole Alveolar Duct

Alveolar Sac Alveolus

Sulfate (jig m"3)

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