Geoscience Reference
In-Depth Information
Figure 8.13. Cathedral, National Palace, and Zocalo (main square) through the smog of Mexico City.
C
Alexandre Fagundes/Dreamstime.com.
Vehicles are the largest source of air pollution in
Canada. The country has about 560 vehicles for every
1,000 people, the seventh highest penetration in the
world after the United States, Luxembourg, Iceland,
Australia, Puerto Rico, and Italy.
Canada has been concerned with air pollution since
at least the late 1960s. In 1969, it initiated an air pollu-
tion monitoring network called the National Air Pollu-
tion Surveillance (NAPS) Network. This network con-
tained about 150 air quality monitoring stations, located
mostly in urban areas. In 1970, Canada passed a Clean
Air Act to regulate emissions of lead, asbestos, mer-
cury, and vinyl chloride.
In 1999, Canada enacted the Canadian Environ-
mental Protection Act (CEPA) , under which the gov-
ernment obtained new powers to control emissions, pri-
marily of toxic substances. PM 10 was declared toxic,
allowing it to be regulated under the act. The act
reduced the allowable level of sulfur in gasoline and
diesel, reduced the allowable level of benzene in gaso-
line, and doubled funding for outdoor air pollution
monitoring.
Canadian on-road vehicle emission standards were
first set under the Motor Vehicle Safety Act of 1971 .
The standards became aligned closely with those in
the United States starting in 1988. Under CEPA, the
on-road regulations were tightened according to U.S.
standards in 2003, and nonroad emissions were first
regulated based on U.S. standards in 1999.
8.2.13. Mexico
In the 1940s, Mexico City's air was relatively clean,
resulting in visibility of about 100 km. From 1940 to
2011, the population of Mexico City and its surround-
ings grew from 1.8 million to about 20 million, making
it the most populated metropolitan area in the world.
Mexico has 209 vehicles per 1,000 people, suggesting
that about 4.1 million vehicles operate in the Mexico
City metropolitan area. Due to vehicle and factory emis-
sions, as well as emissions from cooking, heating, and
electricity generation, Mexico City's air is among the
most polluted in the world (Figure 8.13).
Like Los Angeles, Mexico City sits in a basin sur-
rounded by mountains, and pollution is frequently
trapped beneath the Pacific high-pressure system.
Unlike Los Angeles, Mexico City is not bounded by
an ocean on one side. The low-latitude, relatively thin
stratospheric ozone layer above it (because the ozone
layer becomes thinner with decreasing latitude - Chap-
ter 11) and high elevation also enhance the UV radiation
penetration to Mexico City, increasing ozone formation
there.
Air pollution regulations in Mexico City were first
enacted in 1971, with a Decree to Prevent and Con-
trol Environmental Pollution , under which monitors
were required to measure outdoor pollution. A full
set of monitors was implemented only by 1986, at
which time ozone was found to be the main pollutant.
 
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