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A variety of air pollutants have known or suspected harmful effects on human health and
the environment. In most areas of Europe, these pollutants are principally the products of
combustion from space heating, power generation or from motor vehicle traffic. Pollutants
from these sources may not only prove a problem in the immediate vicinity of these sources
but can travel long distances, chemically reacting in the atmosphere to produce secondary
pollutants such as acid rain or ozone.
Evolutionary Trends in Pollution Problems
In both developed and rapidly industrializing countries, the major historic air
pollution problem has typically been high levels of smoke and SO2 arising from
the combustion of sulfur-containing fossil fuels such as coal for domestic and industrial
purposes. Smog's resulting from the combined effects of black smoke, sulphate, acid aerosol
and fog have been seen throughout Northern European cities for centuries, and still occur
in many parts of the developing world.
In economically developed countries, however, this problem has diminished over recent
decades as a result of changing fuel-use patterns; the increasing use of cleaner fuels
such as natural gas, and the implementation of effective smoke and emission control
policies. These long-term changes in air pollution climate are also seen, often occurring
very rapidly, in many developing countries.
In both developed and developing countries, the major threat to clean air is now posed by
traffic emissions. Petrol- and diesel-engine motor vehicles emit a wide variety of
pollutants, principally carbon monoxide (CO), oxides of nitrogen (NOx),
volatile organic compounds (VOCs) and particulates, which have an increasing impact on
urban air quality.
In addition, photochemical reactions resulting from the action of sunlight on NO2 and VOCs
from vehicles leads to the formation of ozone, a secondary long-range pollutant, which
impacts in rural areas often far from the original emission site. Acid rain is another
long-range pollutant influenced by vehicle NOx emissions.
In all except worst-case situations, industrial and domestic pollutant sources, together
with their impact on air quality, tend to be steady-state or improving over time. However,
traffic pollution problems are worsening world-wide. The problem may be particularly acute
in developing countries with dramatically increasing vehicle fleets, infrastructure
limitations, poor engine/emission control technologies and limited provision for
maintenance or vehicle regulation.
Below is an introduction to the principal pollutants produced by
industrial, domestic and traffic sources.
Sulfur dioxide is a corrosive acid gas which combines with water vapor
atmosphere to produce acid rain. Both wet and dry deposition have been implicated in the
damage and destruction of vegetation and in the degradation of soils, building materials
and watercourses. SO2 in ambient air is also associated with asthma and chronic
The principal source of this gas is power stations burning fossil fuels which contain
sulphur. Major SO2 problems now only tend to occur in cities in which coal is
still widely used for domestic heating, in industry and in power stations. As some power
stations are now located away from urban areas, SO2 emissions may effect air
quality in both rural and urban areas. Since the decline in domestic coal burning in
cities and in power stations overall, SO2 emissions have diminished steadily
and, in most European countries, they are no longer considered to pose a significant
threat to health.
Of particular concern in the past was the combination of SO2 and black smoke
and particulate matter; current EC Directive Limit Values for SO2 are defined
in terms of accompanying black smoke levels, although these are likely to change.
AIRBORNE PARTICULATE MATTER
Airborne particulate matter varies widely in its physical and chemical composition,
source and particle size. PM10 particles (the fraction of particulates in air
of very small size (<10 ?m)) are of major current concern, as they are small enough to
penetrate deep into the lungs and so potentially pose significant health risks. Larger
particles meanwhile, are not readily inhaled, and are removed relatively efficiently from
the air by sedimentation. Particles are often classed as either primary (those emitted
directly into the atmosphere) or secondary (those formed or modified in the atmosphere
from condensation and growth).
A major source of fine primary particles are combustion processes, in particular diesel
combustion, where transport of hot exhaust vapor into a cooler tailpipe or stack can lead
to spontaneous nucleation of carbon particles before emission. Secondary
particles are typically formed when low volatility products are generated in the
atmosphere, for example the oxidation of sulphur dioxide to sulphuric acid. The
atmospheric lifetime of particulate matter is strongly related to particle size, but may
be as long as 10 days for particles of about 1mm in diameter.
The principal source of airborne PM10 matter in European cities is road
traffic emissions, particularly from diesel vehicles. As well as creating dirt,
visibility problems, PM10 particles are associated with health effects
including increased risk of heart and lung disease. In addition, they may carry
surface-absorbed carcinogenic compounds into the lungs.
Concern about the potential health impacts of PM10 has increased very rapidly
over recent years. Increasingly, attention has been turning towards monitoring of the
smaller particle fraction PM2.5 capable of penetrating deepest into the lungs,
or to even smaller size fractions or total particle numbers.
Carbon monoxide (CO) is a toxic gas which is emitted into the atmosphere as a result of
combustion processes, and is also formed by the oxidation of hydrocarbons and other
organic compounds. In European urban areas, CO is produced almost entirely (90%) from road
traffic emissions. CO at levels found in ambient air may reduce the oxygen-carrying
capacity of the blood. It survives in the atmosphere for a period of approximately 1 month
but is eventually oxidized to carbon dioxide (CO2).
Nitrogen oxides are formed during high temperature combustion processes from the
oxidation of nitrogen in the air or fuel. The principal source of nitrogen oxides - nitric
oxide (NO) and nitrogen dioxide (NO2), collectively known as NOx -
is road traffic, which is responsible for approximately half the emissions in Europe. NO
and NO2 concentrations are therefore greatest in urban areas where traffic is
heaviest. Other important sources are power stations, heating plants and industrial
Nitrogen oxides are released into the atmosphere mainly in the form of NO, which is then
readily oxidized to NO2 by reaction with ozone. Elevated levels of NOx
occur in urban environments under stable meteorological conditions, when the
air mass is
unable to disperse.
Nitrogen dioxide has a variety of environmental and health impacts. It is a respiratory
irritant, may exacerbate asthma and possibly increase susceptibility to infections. In the
presence of sunlight, it reacts with hydrocarbons to produce photochemical pollutants such
as ozone (see below). In addition, nitrogen oxides have a lifetime of approximately 1 day
with respect to conversion to nitric acid. This nitric acid is in turn removed from the
atmosphere by direct deposition to the ground, or transfer to aqueous droplets (e.g. cloud
or rainwater), thereby contributing to acid deposition.
Ground-level ozone (O3), unlike other primary pollutants mentioned above, is
not emitted directly into the atmosphere, but is a secondary pollutant produced by
reaction between nitrogen dioxide (NO2), hydrocarbons and sunlight. Ozone can
irritate the eyes and air passages causing breathing difficulties and may increase
susceptibility to infection. It is a highly reactive chemical, capable of attacking
surfaces, fabrics and rubber materials. Ozone is also toxic to some crops, vegetation and
Whereas nitrogen dioxide (NO2) participates in the formation of ozone, nitrogen
oxide (NO) destroys ozone to form oxygen (O2) and nitrogen dioxide (NO2).
For this reason, ozone levels are not as high in urban areas (where high levels of NO are
emitted from vehicles) as in rural areas. As the nitrogen oxides and hydrocarbons are
transported out of urban areas, the ozone-destroying NO is oxidized to NO2,
which participates in ozone formation.
Sunlight provides the energy to initiate ozone formation; near-ultra-violet radiation
dissociates stable molecules to form reactive species known as free radicals. In the
presence of nitrogen oxides these free radicals catalyze the oxidation of hydrocarbons to
carbon dioxide and water vapor. Partially oxidized organic species such as
ketones and carbon monoxide are intermediate products, with ozone being generated as a
Since ozone itself is photo dissociated (split up by sunlight) to form free radicals, it
promotes the oxidation chemistry, and so catalyses its own formation (ie. it is an
auto catalyst). Consequently, high levels of ozone are generally observed during hot, still
sunny, summertime weather in locations where the air mass has previously collected
emissions of hydrocarbons and nitrogen oxides (e.g. urban areas with traffic). Because of
the time required for chemical processing, ozone formation tends to be downwind of
pollution centers. The resulting ozone pollution or summertime smog" may
for several days and be transported over long distances.
There are two main groups of hydrocarbons of concern: volatile organic
compounds (VOCs) and polycyclic aromatic hydrocarbons (PAHs - see section below on
VOCs are released in vehicle exhaust gases either as unburned fuels or as combustion
products, and are also emitted by the evaporation of solvents and motor fuels. Benzene and
1,3-butadiene are of particular concern as they are known carcinogens. Other VOCs are
important because of the role they play in the photochemical formation of ozone in the
Benzene is an aromatic VOC which is a minor constituent of petrol (about 2% by
volume). The main sources of benzene in the atmosphere in Europe are the distribution and
combustion of petrol. Of these, combustion by petrol vehicles is the single biggest source
(70% of total emissions) whilst the refining, distribution and evaporation of petrol from
vehicles accounts for approximately a further 10% of total emissions. Benzene is emitted
in vehicle exhaust not only as unburnt fuel but also as a product of the decomposition of
other aromatic compounds. Benzene is a known human carcinogen.
1,3-butadiene, like benzene, is a VOC emitted into the atmosphere principally from
fuel combustion of petrol and diesel vehicles. Unlike benzene, however, it is not a
constituent of the fuel but is produced by the combustion of olefins. 1,3-butadiene is
also an important chemical in certain industrial processes, particularly the manufacture
of synthetic rubber. It is handled in bulk at a small number of industrial locations.
Other than in the vicinity of such locations, the dominant source of 1,3-butadiene in the
atmosphere is the motor vehicle. 1,3 Butadiene is also a known, potent, human carcinogen.
TOMPs (Toxic Organic Micropollutants
TOMPs (Toxic Organic Micro pollutants) are produced by the incomplete combustion of
fuels. They comprise a complex range of chemicals some of which, although they are emitted
in very small quantities, are highly toxic or carcinogenic. Compounds in this category
? PAHs (PolyAromatic Hydrocarbons)
? PCBs (PolyChlorinated Biphenyls)
LEAD & HEAVY METALS
Particulate metals in air result from activities such as fossil fuel combustion
(including vehicles), metal processing industries and waste incineration. T here are
currently no EC standards for metals other than lead, although several are under
development. Lead is a cumulative poison to the Central Nervous System, particularly
detrimental to the mental development of children.
Lead is the most widely used non-ferrous metal and has a large number of industrial
applications. Its single largest industrial use world-wide is in the manufacture of
batteries (60-70% of total consumption of some 4 million tones) and it is also used in
paints, glazes, alloys, radiation shielding, tank lining and piping.
As tetraethyl lead, it has been used for many years as an additive in petrol; most
airborne emissions of lead in Europe therefore originate from petrol-engine motor
vehicles. With the increasing use of unleaded petrol, however, emissions and
concentrations in air have declined steadily in recent years.
Acidification of water bodies and soils, and the consequent impact on agriculture,
forestry and fisheries are the result of the re-deposition of acidifying compounds
resulting principally from the oxidation of primary SO2 and NO2
emissions from fossil fuel combustion. Deposition may be by either wet or dry processes,
and acid deposition studies often need to examine both of these acidification routes.