Visualizing Environmental Science

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of mountains (the side toward which the wind blows)

are prime candidates for temperature inversions. The

Los Angeles Basin, for example, lies between the Pacific

Ocean on the west and mountains to the north and east.

During the summer the sunny climate produces a layer of

warm dry air at upper elevations. A region of upwelling

occurs just off the Pacific coast, bringing cold ocean water

to the surface and cooling the ocean air. As this cool air

blows inland over the basin, the mountains block its move-

ment further. Thus, a layer of warm, dry air overlies cool

air at the surface, producing a temperature inversion.

Air Pollution in Los Angeles Los Angeles, California,

has some of the worst smog in the world. Its location, in

combination with its sunny climate, is conducive to the

formation of stable temperature inversions that trap pho-

tochemical smog near the ground, sometimes for long

periods (Figure 8.10). Passenger vehicles, heavy-duty

trucks, and buses are the source of more than half of the

smog-producing emissions in Los Angeles.

In 1969 California became the first state to enforce

emissions standards on motor vehicles, largely because

of the air pollution problems in Los Angeles. Today

Los Angeles has stringent smog controls that regulate

everything from low-emission alternative fuels (such

as compressed natural gas) for buses to lawn mower

emissions to paint vapors. Using the cleanest emission-

reduction equipment available significantly reduces

emissions from large industrial and manufacturing

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

Peak ozone concentration in southern



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Peak ozone is the highest level of ozone recorded on any single

day during the year. Average daily ozone, number of days above

federal and state standards, and other measures show similar

patterns. Air quality has improved steadily over the past half

century but still presents a health threat.

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Ozone is a major constituent of the photochemical smog

depicted here. Photographed in Los Angeles, California, on a day

when air pollution exceeded federal air quality standards.

will come in part from requiring auto manufacturers to

sell ultra-low-emission cars.

After several decades devoted to improving its air

quality, Los Angeles now has the cleanest skies it has had

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 120

days above the federal ozone standard in 2008, down from

203 days in 1977.

Urban Heat Islands and Dust Domes

Streets, rooftops, and parking lots in areas of high

population density absorb solar radiation during the

day and radiate heat into the at-

mosphere at night. Heat from

human activities such as fuel com-

bustion is also highly concentrated

in cities. The air in urban areas

therefore forms urban heat islands in the surrounding

suburban and rural areas (Figure 8.12).

urban heat island

Local heat buildup

in an area of high

population.

1960

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

1970 1980 1990 2000 2010

Year

Ozone level (parts per million)

South Coast Air Quality Management District

How do these ozone levels compare

to where you live? (Search the EPA website for data.)

Think Critically