CHAPTER 17 (Moran and Morgan, 1997) Human activity may adversely affect air quality when large quantities of gases or particulate matter are released into the atmosphere and weather conditions do not permit dilution of these pollutants. Typically, high pressure systems that have weak winds and have a vertical temperature profile that is conducive to stable atmospheric conditions tend to be favorable to development of an air pollution episode. Topics such as acid rain, urban weather and modification of the stratospheric ozone shield are addressed. STUDY NOTES CHAPTER 17 Figure 17.2 -- Take a moment to convince yourself that by increasing the wind speed from the top to the lower panel would increase the dilution of an atmospheric pollutant released at a fixed release rate. Figure 17.3 -- Note that by increasing the stack height often times increase the dilution of pollutants. Figures 17.4, 17.5 and 17.6 -- These figures show how the behavior of a plume from a stack will depend upon the atmospheric stability as determined by the vertical temperature profile in the atmosphere from the earth's surface up to several hundred meters above the top of the stack. In Figure 17.4, an unstable atmospheric profile (compare the graph of the vertical temperature profile along the dry adiabatic lapse rate appearing in this diagram with Figure 6.14), the plume can undergo rapid mixing. However, in Figure 17.5, where a temperature inversion extends upward to a level above the stack, the atmospheric layer would be stable and the plume would be "trapped" under the top of the inversion. In Figure 17.6, the development of an elevated inversion, such as shown with a sinking layer of air that forms a subsidence inversion aloft, the plume remains trapped under the inversion. How this case differs from Figure 17.5 is that the layer between the ground and the inversion, to include the top of the stack in this case, is well mixed, permitting the pollutant to be mixed downward toward the surface. Figures 17.7, 17.8 and 17.9 -- Inspect the following sequence of diagrams, which provide several examples of locations that can experience pollution episodes because of development of temperature inversions that tend to inhibit mixing. In Figure 17.7, note how the large scale prevailing winds passing across a mountain barrier can help produce a subsidence temperature inversion on the lee-slope of the mountains. In this case, Denver has a pollution problem because the metropolitan area is located to the east or downwind of the Rocky Mountains. In Figure 17.8, a large-scale wind flow is usually absent, but the cold air drainage into the valleys because of greater density of cold air can produce a temperature inversion. Such a situation occurs along with the development of a mountain breeze as depicted by the lower panel of Figure 12.14. Wintertime pollution episodes may develop around cities and industrial areas located in mountain valleys not only in the West, but also in the Appalachians of the East. In Figure 17.9, several factors combine to produce a stable layer that traps pollution in the Los Angeles Basin. Cool air from the Pacific Ocean flows inland, producing a marine layer, while above, sinking air associated with the large Pacific anticyclone is warmed to enhance the inversion. Table 17.3 -- Scan the pH scale, noting the range of pH values of various substances. In particular, note that the scale is centered upon a neutral substance (pure water) with a pH equal to 7.0. The range of values is from the most acidic (lowest pH values) to the most alkaline (highest value). Note that natural rain water has a slightly acidic pH, with a pH = 5.6. CHAPTER 17 (Moran and Morgan, 1997) AIR POLLUTION METEOROLOGY Here we are concerned primarily with the relationship between weather and air quality. We begin by examining the meteorological, topographic, and other factors that influence the rate of dilution of air pollutants and the conditions favorable for development of an air pollution episode. Wind speed and atmospheric stability are the principal meteorological variables that influence air pollution potential. The concentration of air pollutants is countered to some extent by natural cleansing processes including impaction, gravitational settling, and scavenging by precipitation. We follow this with a discussion of urban weather, acid deposition, and threats to the stratospheric ozone shield. CHAPTER OBJECTIVES After reading this chapter, the student should be able to: define air pollution and air pollution episode. explain the influence of wind speed and atmospheric stability on the rate of dilution of air pollutants. describe the various processes that lead to the development of a temperature inversion. explain how topography influences air pollution potential. identify the natural cleansing processes operating in the troposphere. describe the special characteristics of urban weather that could influence air quality. evaluate the impact of polluted air on cloud development and precipitation processes. describe the causes and environmental impact of acid deposition. identify the major threat to the stratospheric ozone shield and what has been done about that threat. explain the cause and significance of the Antarctic ozone hole. 17 Air Pollution Meteorology 402 Air Pollutants 403 Air Pollution Episodes 404 Air Pollution's Impact on Weather 412 The Ozone Shield 418 Conclusions 422 Special Topic: Principal Air Pollutants 406 Special Topic: Blue Haze and Tropospheric Ozone Abatement 414 Key Terms 422 Summary Statements 422 Review Questions 422 Questions for Critical Thinking 423 Selected Readings 423 2