Other Pollution Derived Aerosols

Aerosols, both primary and secondary, are generated by a variety of pollution sources in addition to biomass burning; these include industrial processes, electric utilities, transportation, construction, and other fuel combustion. While large-scale urban air pollution is a consequence of modern industrial and technological development, smoke produced by indoor fires was perhaps the earliest form of air pollution (Brimblecombe, 1995). It is remarkable that the emissions from many anthropogenic sources are known with greater certainty than are those from natural sources. Even so, there are major gaps in our understanding of anthropogenic aerosol sources on a global scale (Graedel et al., 1993). These gaps include limited information on the geographical distribution of sources, inadequate measurements of the sizes of the particles emitted by the various sources, a lack of knowledge concerning the transformations of the particles as they age, and only a recent appreciation of the complex ways in which the entire mix of atmospheric constituents, especially nitrogen oxides, volatile organic compounds (VOCs), and ozone, affect the formation and composition of aerosols (Meng et al., 1997).

Pollution emissions are, of course, subject to many of the same processes discussed above for natural aerosol sources, such as new particle formation via gas-to-particle conversion or the condensation of volatile materials in plumes emitted by high-temperature sources. In addition studies of semivolatile organic compounds, including pollution-derived PAHs, have shown that the partitioning of these compounds between the gas phase and particles is largely determined by their subcooled liquid-vapor pressures (Bidleman and Foreman, 1987). Calculations by these authors based on the approach of Yamasaki et al. (1984) indicated that for typical, urban, suspended-particle loads, 11 to 55% of the total mass of a substance with a vapor pressure of 10 6 Torr should be partitioned in the particulate phase. Other factors besides the total suspended particle load that can influence the vapor/particle partitioning of organic substance include relative humidity (Pankow et al., 1993), temperature, and the radiative flux (Kamens et al., 1988). Gas-particle partitioning studies of organics have shown that it is important to determine whether the gas-phase species are adsorbed to a solid particle's surface or absorbed into a liquid phase (Pankow, 1994). This same issue is certainly relevant to the gas-particle partitioning of inorganic species, specifically with respect to wetted aerosols and liquid droplets. Partitioning among the gas, liquid, and solid phases in the atmosphere is especially relevant for the chemistry of sulfur and nitrogen oxides and acid deposition, but further discussion of this topic is beyond the scope of this chapter.

Some quantitative estimates of trace element emissions from anthropogenic sources were produced in the 1970s and 1980s (e.g., Lantzy and Mackenzie, 1979; Nriagu, 1989; Pacyna, 1986; Nriagu and Pacyna, 1988). These emission estimates clearly show that the biogeochemical cycles of a number of trace elements have been severely perturbed by human activities (Table 2). However, of the trace elements it is atmospheric Pb that has been subject to the greatest perturbation, and while anthropogenic Pb has been spread throughout Earth, largely as a result of atmospheric transport (e.g., Murozumi et al., 1969; Patterson, 1987), the concentrations of Pb in the atmosphere have started to decline in response to the phase out of leaded gasolines (Huang et al., 1996).

There are examples of aerosol pollution even more extreme than Pb, and these involve the atmospheric releases of substances that exist purely as a result of human activities. Examples of the substances involved include synthetic organic chemicals, such as polychlorinated biphenyls (PCBs) and various types of pesticides. Radio-

TABLE 2 Percent of Total Atmospherlc Emissions from Natural Sources3

Element

Percent from Natural Sources

Antimony

41

Arsenic

39

Cadmium

15

Chromium

59

Copper

44

Lead

4

Manganese

89

Mercury

41

Molybdenum

48

Nickel

35

Selenium

58

Vanadium

25

Zinc

34

"Data from Nriagu (1989).

"Data from Nriagu (1989).

active nuclides produced by nuclear weapons testing and by nuclear reactors also have been released into the environment, and these man-made nuclides also make up a component of the contemporary aerosol. The dispersal of these and other pollutant aerosols to the most remote parts of the globe is a measure of the efficiency with which atmospheric transport operates.

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