Page 480 - Environment: The Science Behind the Stories

Figure 17.16 Scrubbers typically
5 Purified flue gas exits to stack
remove at least 90% of particu-
Clean air late matter and gases such as
sulfur dioxide. Scrubbers and other
Mist eliminator
pollution control devices come in
4 Excess mist condenses on screen many designs. In this spray-tower
Spray nozzles wet scrubber, polluted air rises
through a chamber while nozzles
spray a mist of water mixed with lime
or other active chemicals to capture
pollutants and wash them out of
Mist of chemically the air.
treated water
2 Gas rises
through
shower
of mist
1 Dirty flue 7 Water is reused in spray nozzles
gas enters

Piping
Polluted
air Settling tank

3 Mist captures Pollutants removed
pollutants and from water
brings them
to bottom
8 Sludge is disposed
Water of as hazardous
with pollutants 6 Dirty water is drained to waste
Dirty water is drained to
tank and cleansed
tank and cleansed

40% national ambient air quality standards for five of the six criteria
Percent difference from health and safety standard
pollutants, according to 2012 data, while people in four adjacent
southern California counties breathe air that violates four of the
20%
standards. All together, as of 2010, 124 million Americans lived
Ozone
Most recent in counties that violated the national ambient air quality stand-
0% ard for at least one criteria pollutant. Still, even L.A. and other
national standard
pollution-choked metropolises are making perceptible headway CHAPTER 17 • AT m os PHER i C sC i E n CE , Ai R Qu A li T y, A nd Poll u T i on Con TR ol
PM 2.5
-20% toward cleaner air for their citizens (Figure 17.18).
Although we tend to focus on pollution in cities, air
-40% CO quality is a rural issue as well. In rural areas, people suf-
fer from drift of airborne pesticides from farms, as well as
PM 10 industrial pollutants that drift far from cities, factories, and
-60%
power plants. Air pollution also emanates from feedlots
NO 2 (p. 269) where cattle, hogs, or chickens are raised. The huge
-80% numbers of animals densely concentrated at feedlots and the
SO 2
voluminous amounts of waste they produce generate dust
-100% as well as methane, hydrogen sulfide, and ammonia. These
1990 1995 2000 2005 2010 gases create objectionable odors, and ammonia contributes
Year to nitrogen deposition. Studies show that people working at
and living near feedlots have high rates of respiratory illness.
Figure 17.17 Concentrations of criteria pollutants in
ambient air across the United States have steadily fallen. Indeed, some of the worst air quality in the United States
All except tropospheric ozone now average well below their occurs in certain rural regions, including California’s Central
standards for health and safety set by the EPA. Lead is not Valley (the nation’s agricultural fruit basket) and areas near
shown but has declined from 450% of its standard in 1990. Data natural gas extraction sites, where fumes from extraction
from U.S. EPA. pollute the air. 479

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