- Direct effects on humans and other animals. These are usually respiratory
effects, and asthmatics are especially vulnerable. - Effects upon plants and fish (especially fish fingerlings) in acidified lake water
where the lake is not in contact with minerals, particularly CaCO 3 , capable of
neutralizing acid. - Damage to materials. Stone (especially acid-soluble limestone and marble)
and metal used in building can be corroded and etched by acidic precipitation.
Electrical equipment, particularly relay contacts and springs can be corroded
by acidic precipitation.
Some measures can be taken to mitigate the effects of acid rain, although these
are very limited once the pollutant has formed. Some success has been achieved with
treating acidified lakes with pulverized limestone to neutralize acid. Corrosion-resistant
materials can be used in applications where exposure to acid rain is likely. Protective
coatings, such as corrosion-resisting paint primers on metals, can be applied to materials
likely to be exposed to acidic precipitation. But the best protection is to prevent formation
and release of SO 2 and NOx gases leading to acid rain formation by measures described
in the preceding section.
8.8. Miscellaneous Gases in the Atmosphere
There are several inorganic gases other than oxides that can be significant atmospheric
pollutants. One of the most common of these is ammonia, NH 3. In addition to industrial
pollution, such as from heating coal to make coke for steel making, ammonia can be
added to the atmosphere by bacterial sources, from sewage treatment, and from the
decay of animal wastes. Accidental releases can occur from liquid anhydrous ammonia
used as an agricultural nitrogen fertilizer.
Ammonia is strongly attracted to water, so it is normally present in the atmosphere
in water droplets. It is the only significant gaseous base in the atmosphere, so that it
reacts with atmospheric acids to produce corrosive ammonium salts as shown by the
following reactions:
NH 3 + H 2 SO 4 → NH 4 HSO 4 (8.8.1)NH 3 + HNO 3 → NH 4 NO 3 (8.8.2)Gaseous chlorine, fluorine, and volatile fluorides are uncommon, but serious,
air pollutants. Elemental chlorine, Cl 2 , is widely produced and distributed as a water
disinfectant, bleach, and industrial chemical. It is very reactive and so toxic that it was
the first poisonous gas used as a military poison in World War I. Most toxic exposures of
chlorine occur as the result of transportation accidents leading to its release.
Hydrogen chloride, HCl, can get into the atmosphere from accidental releases of
the gas, from reaction with atmospheric water of reactive chlorine-containing chemicals,
such as SiCl 4 ,
212 Green Chemistry, 2nd ed