Environmental Engineering Reference
In-Depth Information
A pH of 7 is the neutral point, that is, the pH of pure water, which contains an
equal number of positive and negative ions. Liquids with a pH below 7 are acid and
those above 7 are nonacid, or basic. The pH of acid rain precipitation in the United
States and Sweden is typically in the range of 4-5.
The two major acids in acid rain are sulfuric (H
2
SO
4
) and nitric (HNO
3
). These
acids are formed in the atmosphere from sulfur dioxide (SO
2
) and oxides of nitro-
gen (NO
x
). The products of fossil fuel combustion, SO
2
and NO
x
, can be carried
by wind to distances of up to 1,000 km from the point of emission. This causes acid
rain far from the primary source of pollution, thus making it a regional problem,
which frequently crosses national borders. SO
2
and NO
x
cause damage through
two mechanisms:
- Dry deposition, damaging vegetation and structure, or
- Wet deposition, when dissolved in rain, cloud water, or atmospheric water vapor.
The chemistry of the production process is only partly understood at present. It ap-
pears that a variety of mechanisms can cause acids to form and that the dominant
chemical reactions depend on location and weather conditions as well as on the
chemical composition of the local atmosphere. Sunlight, soot, and trace metals may
also expedite the process of acid formation under certain circumstances.
There is a natural flux of S and N oxides due to volcanic emissions, biomass
burning, lightning, and so on, but the natural flux, which is evenly spread out, does
not give deposition fluxes higher than about 0.28 g of sulfur per square meter per
year. What is really significant, however, is the flux of deposition of S or N of an-
thropogenic origin, because it is concentrated in a few industrial regions.
These are environmental problems that transcend national boundaries such as the
emissions of carbon dioxide (CO
2
) from fossil fuels burning and deforestation of
native forests, and methane emissions (CH
4
) and other gases responsible for the
greenhouse effect.
Other global environmental problems indirectly related to energy include ozone
emissions, deforestation for the production of fuel wood and charcoal as well as
for agricultural expansion; accumulation of heavy metals (such as mercury, which
is emitted by coal thermopower plants that then enters the food chain) in living or-
ganisms; human-made toxic compounds (such as PCBs—polychlorinated biphen-
