(the side toward which the wind blows) are prime candi- Peak ozone concentration in southern

dates for temperature inversions. The Los Angeles Basin, California, 1960–2010 • Figure 8.11
for example, lies between the Pacific Ocean on the west
and mountains to the north and east. During the summer Peak ozone is the highest level of ozone recorded on any single day
the sunny climate produces a layer of warm dry air at up- during the year. Average daily ozone, number of days above federal
and state standards, and other measures show similar patterns. Air
per elevations. A region of upwelling occurs just off the
quality has improved steadily over the past half century but still
Pacific coast, bringing cold ocean water to the surface and
presents a health threat. Peak ozone in 2014 was 0.11 parts per
cooling the ocean air. As this cool air blows inland over the
million (ppm). It remains above the acceptable federal standard of
basin, the mountains block its movement further. Thus, a
0.7 ppm.
layer of warm, dry air overlies cool air at the surface, pro-
ducing a temperature inversion.
0.8
Air Pollution in Los Angeles Los Angeles, Cali-

Based on data from South Coast Air Quality Management District.

fornia, once had some of the worst smog in the world. 0.7

Ozone level (parts per million)

Its location, in combination with its sunny climate, is
0.6
conducive to the formation of stable temperature inver-
sions that trap photochemical smog near the ground, 0.5
sometimes for long periods (Figure 8.10). Major indus-
tries like oil refineries contribute much of the pollution, 0.4
but so do millions of vehicles and small businesses and 0.3
households.
In 1969 California became the first state to enforce 0.2
emissions standards on motor vehicles, largely because
0.1
of the air pollution problems in Los Angeles. Today
Los Angeles has stringent smog controls that regulate 0.0
everything from low-emission alternative fuels (such 1960 1970 1980 1990 2000 2010
Year
as compressed natural gas) for buses to lawn mower
emissions to paint vapors. Using the cleanest emission-
reduction equipment available significantly reduces
Th in k Cr it ica lly How do these ozone levels compare
emissions from large industrial and manufacturing to where you live? (Search the EPA website for data.)
sources, including oil refineries and power plants. Cali-
fornia has no coal-fired power plants; most of its power
plants burn natural gas. Future pollution reductions After several decades devoted to improving its air
will come in part from requiring auto manufacturers to quality, Los Angeles now has the cleanest skies it has had
sell ultra-low-emission cars. since the 1950s (Figure 8.11). Despite the impressive
progress, Los Angeles still exceeds federal air quality stan-
dards on more days than almost any other metropolitan
region in the United States. Los Angeles experienced
Photochemical smog • Figure 8.10
115 days above the federal ozone standard in 2014, down
Ozone is a major constituent of the photochemical smog depicted from 203 days in 1977.
here. Photographed in Los Angeles, California, on a day when air
pollution exceeded federal air quality standards.
Urban Heat Islands and Dust Domes
Justin Lambert/The Image Bank/Getty Images

Streets, rooftops, and parking lots in areas of high

population density absorb solar radiation during the day
and radiate heat into the atmosphere at night. Heat from
human activities such as fuel combustion is also highly
concentrated in cities. The air in urban areas therefore
forms urban heat islands in the
surrounding suburban and rural
urban heat island
areas (Figure 8.12). Urban heat
Local heat buildup
islands also contribute to the
in an area of high
buildup of pollutants, especially
population.
particulate matter, in the form

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