152
ENVIRONMENT, POLLUTION, DEVELOPMENT: THE CASE OF UZBEKISTAN
It should be noted that acidification of precipitation is not
observed in Latvia. In some regions as a result of cement and
building material production significant amounts of alkaline
dust and aerosols are
released into the atmo sphere, therefore the
precipitation becomes alkaline. The analysed concentrations of
substances and their modules of fall in precipitation elsewhere in
Latvia also do not show significant local pollution sources or the
impact of cross-border pollution.
Today a question of water acidification is still topical, and it is
caused by the changes in atmo spheric precipitation pH level.
In natural water reservoirs several stages of acidification can be
distinguished. In the first phase, the inflow of acidic water leaves
the average pH largely unaffected. This is due to some natural water
buffering capacity, which determines the presence of carbonates in
the water. In the first phase of the acidification process the hydrogen
ions neutralize hydrogen carbonate ions.
If water alkalinity goes below 0.1 mol/l, the natural water
buffering capacity is exhausted. In such reservoirs at the period of
intense acidic water inflow (in autumn, but especially in the spring
during snow melt) a rapid pH drop is possible. These processes to
some extent can be offset by the increased dissolution of alkaline
rock. It is necessary to note that even a
short-term decrease in the
pH of water leads to serious ecological consequences. Of particular
note is the impact of acidification on benthic organisms, fish eggs
and other aquatic organisms in their early stages of development.
As the acidification processes continue, the water pH is settled
below 5.5. Such a body of water is denoted as moderately acidic. At
this acidification stage of the body of water substantial changes in
its ecosystem take place.
The concluding stage of freshwater acidification is characterized
by formation of a stable pH (pH <4.5), which persists even when
the atmo spheric precipitation acidity is lower. This effect can be
explained by the presence of humic substances, magnesium and
aluminium compounds in water. In general,
these substances, either
by connecting or separating the protons, stabilize the pH value.
At the reduced water pH the intensity of chemical processes
with aluminium participation increases. Naturally the aluminium
concentrations in water reservoirs are low, but insoluble aluminium
compounds – alumosilicates, which are the main components of clay,
make up a large part of the sediments in water bodies. As the acidity
of
the water reservoir increases, the intensive dissolution process
of these compounds takes place, and as a result the aluminium
compounds enter the water. Recently it has been proved that
aluminium is one of the key factors determining the toxic effects of
environ mental acidification.
7.
AIR POLLUTION
153
Atmo spheric acidification also directly affects various materials
used in building, but primarily those containing carbonates.
Materials containing limestone le in acidic conditions dissolve
(become weathered).
The harmful substances damage plants primarily at the
molecular level, and only after the changes at this level the overall
impact on metabolism process begins, affecting the entire organism.
Sulfur dioxide has an essential influence on plant development.
Sulfur dioxide released into the plante first of all impacts the cells,
which regulate the plant’s gas exchange with the environ ment. Even
a minor amount stimulates inflow channels in the gas stomata of
leaves and they remain constantly open.
Once in the plant cells, the
molecules of sulfur dioxide (in the form of sulfite ion) easily pass
through the cell membrane. Subsequently they affect mitochondria
and chloroplasts. These effects reduce the intensity of photosynthesis
process. In addition, SO
2
, SO
4
2–
and other sulfur dioxide
transformation products interact actively with enzymes within the
cell, affecting such processes as, for example, the ATP synthase in
mitochondria, –S–S– link splitting in enzymes, etc. The following
visual changes quite often occur in plants due to the activity of the
plants’ natural enemies – a variety of micro-organisms.
Different plant species have a different sensitivity to pollutants.
The pollution caused by sulfur dioxide particularly impacts lucerne,
wheat, yews, pines, while apricots, gladiolus, peaches, etc. are
sensitive to the pollution with fluorine compounds. The impact of
a specific air pollutant cause different symptoms in various plant
species. Atmo spheric acidification typically results in destruction of
coniferous trees (Figure 7.30). If a few decades ago the forest drying
due to acid precipitation was a major problem
in Central Europe and
Scandinavia, today this type of impact is observed in China, India,
and the United States.
Given the amount of pollution today, the effects of pollutants
on inorganic materials are specifically investigated. Atmo spheric
pollution with sulfur and nitrogen oxides has the most important
impact on inorganic materials. Iron, cast iron and steel corrosion
takes place in parallel with a variety of electrochemical processes,
which are initiated by micro-alloys in the metal products themselves
and also by aerosol and dust particles accumulated on the metal
surface. The main role in the course of this process is played by
sulfuric acid and
sulfates resulting from SO
2
oxidation.
In case of non-ferrous metals the air pollution by sulfur dioxide
causes direct corrosion of surface layer.
The influence of air pollution on architectural monuments is
Do'stlaringiz bilan baham: 
