Page 469 - Environment: The Science Behind the Stories

minibuses were replaced with fuel-efficient buses that over 75%, and the air meets health standards on one of every
moved rapidly in designated lanes. Over 450,000 peo- two days.
ple use these buses each day, and the new system has Plenty of hurdles remain. Rampant development is chal-
reduced carbon dioxide emissions by an estimated 80,000 lenging the city’s efforts to plan for sustainable growth. New
tons per year. highway segments under construction may induce more
In 2010 Ebrard introduced a bicycle-sharing program in people to drive. The typical driver still spends three hours a day
order to free short-distance commuters from dependence stuck in traffic that averages just 13 mph. Worst of all, smog
on cars. With 1000 bikes at rental stations throughout the still contributes to an estimated 4000 deaths each year. But
city, people can rent a bike cheaply at one location and drop Mexico City is following in the footsteps of Los Angeles, mak-
it off at another. In Ebrard’s most popular initiative, every ing steady progress toward cleaner air.
Sunday morning the city’s main boulevard, the Paseo de la Indeed, L.A. and Mexico City today typify cities of
Reforma, is closed to car traffic, creating a safe and pleas- developed nations and those of developing nations. Nations
ant community space for pedestrians, bikers, joggers, and that are industrializing as they try to build wealth for their citizens
skateboarders. are confronting the same air quality challenges that plagued
Mexico City residents were so happy with the progress the United States and other wealthy nations a generation and
that in 2012, when Ebrard stepped down because of term lim- more ago. Los Angeles has 24 other sister cities, many of which
its, voters rewarded his political party by electing his ally Miguel struggle with severe air pollution. Athens, Greece; Jakarta,
Ángel Mancera by a 44-point landslide. Mancera announced Indonesia; Mumbai, India; Guangzhou, China; Taipei, Taiwan;
plans to expand the bicycle program to 4000 bikes and to San Salvador, El Salvador; and Tehran, Iran all experience poor
put electric buses and taxis on the roads. He also rolled out a air quality on a regular basis as new emissions from increased
car-sharing program that officials hope can eventually remove auto exhaust and fossil fuel combustion are added to more
40,000 vehicles from circulation. Private entrepreneurs are traditional pollution sources.
now getting in on the act; recent university graduates have Most of these cities are taking steps to improve their air
launched companies providing car-sharing and carpooling ser- quality, just as Los Angeles and other American cities have
vices, some using electric vehicles. done before them. We will examine the solutions sought in Los
All these changes are paying off with cleaner air. In 1991, Angeles, Mexico City, and elsewhere throughout this chapter
the city’s air was deemed hazardous to breathe on all but 8 as we learn about Earth’s atmosphere and how to reduce the
days of the year. Today, most pollutants have been slashed by pollutants we release into it.

The Atmosphere Argon (Ar): 0.93%

Other permanent gases
Every breath we take reaffirms our connection to the Neon (Ne): 0.0018%
atmosphere, the thin layer of gases that envelops our planet. Helium (He): 0.0005%
The atmosphere moderates our climate, provides us oxygen, Nitrogen (N ) Hydrogen (H ): trace
2
2
absorbs hazardous solar radiation, burns up incoming mete- (78.08%) Oxygen (O ) Xenon (Xe): trace
ors, and transports and recycles water and nutrients. (20.95%) 2
Earth’s atmosphere consists of 78% nitrogen (N ) and Variable gases
2
21% oxygen (O ). The remaining 1% is composed of argon Water vapor (H O): 0–4%
2
2
(Ar) and minute concentrations of water vapor and other gases Carbon dioxide (CO ): 0.039%
2
(Figure 17.1). These include permanent gases, which remain Methane (CH ): 0.00018%
4
2
at stable concentrations over centuries or millennia, and vari- Nitrous oxide (N O): trace
Ozone (O ): trace
3
able gases, which can vary in concentration on shorter time Chlorofluorocarbons
scales or from place to place as a result of natural processes (CFCs): trace
or human activities.
Over our planet’s long history, the atmosphere’s compo- Figure 17.1 Earth’s atmosphere consists of nitrogen,
sition has changed. Long ago our atmosphere was dominated oxygen, argon, and a mix of gases at dilute concentrations.
by carbon dioxide (CO ), nitrogen, carbon monoxide (CO), Permanent gases are fixed in concentration over long periods of
2
and hydrogen (H ), but about 2.7 billion years ago, oxygen time, whereas variable gases vary in concentration. Data from Ahrens,
2
began to build up with the emergence of autotrophic microbes C.D., 2007. Meteorology today, 8th ed. Belmont, CA: Brooks/Cole.
that emitted oxygen by photosynthesis (p. 50). Today, human
activity is altering the quantities of some atmospheric gases,
such as carbon dioxide, methane (CH ), and ozone (O ). The atmosphere is layered
3
4
Before exploring how our pollutants change the air we breathe
and how we strive to control pollution, we will begin with an The atmosphere that stretches so high above us and seems
468 overview of Earth’s atmosphere. so vast is actually just a thin coating about 1/100 of Earth’s

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