Page 484 - Environment: The Science Behind the Stories

The thick and blinding “killer smog” of 1952 in London and Los Angeles also have sunny climates. As a result, such
occurred when weather conditions trapped emissions from cities suffer from a different type of smog. Photochemical
the coal used to fire the city’s industries and heat people’s smog forms when sunlight drives chemical reactions between
homes. Sulfur dioxide and particulate matter caused most primary pollutants and normal atmospheric compounds, pro-
of the 4000–12,000 deaths from that episode. In the wake of ducing a mix of over 100 different chemicals, tropospheric
this catastrophe and others, the governments of developed ozone often being the most abundant (Figure 17.22a). Because
nations began regulating industrial emissions and have greatly it also includes NO photochemical smog generally appears
2,
reduced industrial smog. However, in industrializing regions as a brownish haze (Figure 17.22b).
such as China, India, and eastern Europe, coal burning and lax Hot, sunny, windless days in urban areas provide perfect
pollution control result in industrial smog that poses signifi- conditions for the formation of photochemical smog. On a
cant health risks. typical weekday, exhaust from morning traffic releases NO
As we’ve seen, weather and topography play roles in smog and VOCs into a city’s air. Sunlight then promotes the produc-
formation. Four years before London’s killer smog, a similar tion of ozone and other constituents of photochemical smog.
event occurred in Pennsylvania in a small town named Donora Levels of photochemical pollutants in urban areas typically
(Figure 17.21b). Donora is located in a mountain valley, and peak in midafternoon and can irritate people’s eyes, noses,
one day after air had cooled during the night, the morning sun and throats.
did not reach the valley floor to warm and disperse the cold
air. The resulting thermal inversion trapped smog containing We can take steps to reduce smog
particulate matter emissions from a steel and wire factory.
Twenty-one people died, and over 6000 people—nearly half Los Angeles’s struggle with air pollution began in 1943,
the town—became ill. when the city’s first major smog episode cut visibility to three
For most urban areas today, however, pollution results blocks. Since then, L.A. residents have dealt with headaches,
largely from automobile exhaust. In Mexico City, vehicles eye irritation, asthma, lung damage, and related illnesses.
contribute 31% of volatile organic compounds, 50% of sulfur However, Los Angeles confronted its problem and has made
dioxide, and 82% of nitrogen oxides. Cities like Mexico City great progress in clearing the air since the 1970s.

Nitrogen (N )
2
Oxygen (O 2 )
Burning

Nitric oxide (NO)

Oxygen (O 2 )

Nitrogen dioxide (NO 2 )
(b) Photochemical smog over Mexico City 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

UV radiation
Water vapor Hydrocarbons
(H 2 O)
Nitric oxide (NO) Nitric oxide (NO)
Peroxyacyl
and and nitrates
(PANs)
Oxygen atom (O) Nitric acid (HNO )
3
Oxygen (O 2 )

Ozone (O ) Acid rain Figure 17.22 Photochemical smog results when pollutants
3
from automobile exhaust react with sunlight. Nitric oxide
Volatile organic can start a chemical chain reaction (a) that produces compounds
compounds (VOCs) including nitrogen dioxide, nitric acid, ozone, and peroxyacyl
nitrates (PANs). PANs can induce further reactions that damage liv-
Various pollutants ing tissues. Photochemical smog is common over Mexico City (b)
and many other urban areas, especially those with hilly topography
(a) Formation of photochemical smog or frequent inversion layers. 483

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