5. GLOBAL ENVIRONMENTAL PROBLEMS
91
Motor traffic is considered one of the major sources of nitrogen
oxides which directly affect the air quality in cities (Figure 5.17).
Nitrogen (N
2
)
Nitrogen oxide (NO)
Combustion
Nitrogen oxide (NO)
and
Acid rain
Oxygen atom (O)
Nitrogen oxide (NO)
and
Various
pollutants
Hydrocarbons
Oxygen (O
2
)
Oxygen (O
2
)
Oxygen (O
2
)
Ozone (O
3
)
Nitrogen dioxide (NO
2
)
UV rays
Water vapour
(H
2
O)
Peroxyacyl nitrates
Nitric acid (HNO
3
)
Volatile
hydrocarbons
5.3.3. Dust and aerosols
Atmo spheric air contains particles of every sort, size and com-
po sition. Their sizes may vary from micron to millimetric parts. The
finer particles (size <10 µm) are called aerosols, whereas the coarser
are called dust. Liquid micro-droplets (haze) are also counted
as aerosols. The main sources of dust and aerosol for ma tion are
various natural processes – soil and its constituent mi ne ral particles
entering the atmo sphere with wind, dust storms (Figure 5.18),
volcanic eruptions, forest fires, and evaporation from the sea and
ocean surfaces (forming sea salt aerosols). Admittedly, to day human
activity causes the discharge of many aerosols and dust into the
air. The main anthropogenic sources are
such processes as the
generation of energy, the production of building materials, mining,
agriculture, air transport and others.
Figure 5.17.
Role of
nitrogen oxides emitted
with motor vehicle
engine exhaust in the
reactions
of formation
of smog, acid rain and
ozone
92
ENVIRONMENT, POLLUTION, DEVELOPMENT: THE CASE OF UZBEKISTAN
Aerosols can be formed and the composition of particles in the
air can vary as a consequence of different chemical reactions. Sulphur
and nitrogen oxides play a considerable role in these processes.
Since the airborne solid particles form the condensation centres for
water vapour, all these reactions take place in the atmo sphere in the
presence of water. Acid rain formation is a typical example.
As aerosols and dust particles have a relatively large surface, they
can sorb atmo spheric gases, chemicals, micro-organisms and water
vapour, and this sorption process determines
the rather complex
composition of aerosols.
Atmo spheric aerosols and dust can significantly affect the Earth’s
climate, as evidenced by long-term climate change analysis and the
increasing effect of dust from volcanic eruptions on the climate.
An important group of aerosols are those of organic composition.
Their source can be both natural processes (plants, forest fires,
organic matter decay) and anthropogenic processes. The presence
of metal compounds (Pb, Hg, Cu, Ni, Be), radioactive isotopes and
organic substances in the composition of aerosols can particularly
adversely affect the human health. The exposure to aerosols is
most hazardous in case of inhaling such particles that are retained
in the lungs (coarser dust particles are captured
in the nasal cavity
and upper airways, whereas finer particles can be exhaled). The
aerosols containing such particles are called respirable aerosols.
Their particle sizes are often smaller than 2.5 µm. These aerosols
are designated PM
2.5
(PM stands for Particulate Matter). The effects
of these aerosols can be as follows:
1) the substances adsorbed on the surfaces of aerosol particles may
desorb and enter the circulatory and lymphatic systems. Such
Figure 5.18.
Sand storm
in the Sahara
5. GLOBAL ENVIRONMENTAL PROBLEMS
93
an effect is typical of various combustion products. Combustion
product aerosols contain carbon, and
on the surface of these
aerosols, usually there is quite a high concentration of different
organic substances originated from incomplete combustion or
in the process of thermosynthesis;
2) ultrafine particles, such as asbestos fibres, can penetrate the
blood or lymphatic system from the lungs;
3) aerosol particles can remain in the lungs and calcify,
causing constant irritation. The aerosol effect of this type is
characteristic of occupational diseases – asbestosis and silicosis,
caused by inhaling the air polluted with mineral particles;
4) micro-organisms, bacteria or fungi
residing on the aerosol and
dust particles may cause allergic reactions as well as illness.
The dust of pollen, wood, flour and other organic substances can
be considered a special group of aerosols. Since these substances are
organic, their composition differs significantly from the traditional
dust composition; yet, these aerosols can cause allergies.
Since the effects of aerosols on humans, animals, plants
and buildings can be detrimental, and aerosols and dust can
be considered, by their mass, the largest group of atmo spheric
pollutants, it is crucial to limit their emission
in the main sectors
responsible for this kind of pollution, namely the production and
energy industry.
Do'stlaringiz bilan baham: 
