Biology 12

528 MHR • Unit 5 Population Dynamics

thinner, particularly in certain parts of the world.

This damage, or ozone depletion, is thought to be

chiefly caused by chlorofluorocarbons (CFCs).

When CFCs are released into the lower atmosphere,

they move slowly upward and decompose,

producing chlorine atoms that react with the ozone

to produce oxygen. In 1987, even before current

levels of damage appeared, an international

agreement was signed promoting the phasing out of

CFC production in most industrialized countries.

Public awareness leading to the decreased use of

aerosol sprays also had a significant impact on

chlorofluorocarbon use. Since it takes 10 to 20 years

for these pollutants to reach the upper atmosphere,

there will be a delay before any benefits of reducing

CFC production are realized. Figure 15.19 illustrates

how ozone depletion occurs.

Acid Precipitation

The deleterious effects of acid precipitation were

convincingly demonstrated first by Canadian

scientists, including Dr. David Schindler and

Harold Harvey. Dr. Schindler’s group performed

experiments in the Experimental Lakes Area, a

study site located in northwestern Ontario. The

major sources of atmospheric acid are sulfur

dioxide (SO 2 ), released into the air as a result of

burning high-sulfur coal and oil, and nitrogen

oxides (NOx) contained in automobile exhaust.

These gases combine with water vapour in the air

and dissolve in raindrops, which then fall as acid

precipitation(rain or snow). As its name suggests,

this precipitation has a pH of 4 to 5, much more

acidic than that of normal rain (about 5.6). Acid

precipitation corrodes metal and stone and harms

trees. Because it makes lakes acidic, it alters

aquatic ecosystems, killing some organisms and

harming many others. Acid precipitation can also

harm water supplies by causing contaminants to

leach from surrounding soils and by dissolving

metals, such as copper and lead, in water pipes.

The effects of acid precipitation are most obvious

in the northeastern United States, southeastern

Canada, and Europe. Recently, sulfur dioxide

emissions have been reduced by the use of

low-sulfur fuels and the installation of scrubbers

Your Electronic Learning Partner has animation clips that

will enhance your understanding of ozone depletion and its

effect on Earth.

ELECTRONIC LEARNING PARTNER

Figure 15.19Ozone depletion is also referred to as the thinning of the ozone layer.

CI (chlorine)

CI

Earth

(3)

CI + (2)

O 2

O 3

O 3

O 3

O 3 O^3 O^3

O 3

O 2

O 2