Page 35 - Air pollution and greenhouse gases from basic concepts to engineering applications for air emission control

8 1 Air Emissions

1.3.2.2 Acid Rain
There are wet forms of acidic pollutants found in rain, snow, fog, and cloud vapor.
Most acid rain forms when NO x and SO 2 are converted through oxidation and
dissolution to nitric acid (HNO 3 ) and sulfuric acid (H 2 SO 4 ), respectively, and also
+
when ammonia gas (NH 3 ) is converted into ammonium (NH 4 ). Emissions of sulfur
and nitrogen oxides to the atmosphere have increased since the Industrial Revo-
lution and have recently become more prevalent in China, Eastern Europe, and
Russia, where sulfur-containing coal is the major energy source.
Acid rain was first discovered in 1852 and has been extensively studied since the
late 1960s [2, 19]. The most important gas that leads to acidification is sulfur
dioxide. Emissions of nitrogen oxides are also of increasing importance, due to their
contribution to ozone and nitric acid.
Acid rain has an adverse impact on ecology. It falls on forests, soils, and bodies of
freshwater, killing off insect and aquatic life forms. Acid rain can seriously damage
soil biology. While some microbes can quickly consume acids, others cannot tol-
erate much of it. The enzymes of the latter are changed by the acid and lose their
capabilities. In addition, acid rain also removes nutrients and minerals, which are
necessary to maintain a flourishing ecosystem. The depletion of minerals in the soil
can slow the growth of food crops, plants, and forests. When the pH values are
extremely low, the acid rain can cause a part of or an entire forest to die, mostly
because the trees are weakened and become susceptible to harsh environments.
Acid rain can also corrode building materials and historical monuments.
Sculptures commonly lose their sheen, because acid rain chemically reacts with the
calcium compounds in the stones.

1.3.3 Greenhouse Gas Effects

Under normal conditions, solar radiation reaches the surfaces of the Earth after
penetrating through its atmosphere. Some of the solar energy (heat) is absorbed,
reflected or reradiated within the Earth’s atmosphere. With the increases of
greenhouse gases (GHGs) in the atmosphere, long-wave infrared (IR) heat radiation
is reradiated and trapped within the Earth’s atmosphere, resulting in the warming of
the Earth’s surface. Readers are referred to heat transfer textbooks for in-depth
knowledge of solar radiation. Related courses are also offered to undergraduate
students in most engineering schools.
The stratosphere of the Earth contains a layer of ozone gas (O 3 ) that normally
absorbs most of the harmful, shortwave ultraviolet radiation (UVB) from the sun,
protecting Earth’s living organisms. However, this protective shield has become
thinner and thinner. Recent data shows a trend of 3.4 % decrease per decade in the