Sources of Air Pollutants
The number of different types of pollution sources in modern society
is almost endless. Hence in this chapter we will look at only the most
significant sources of air pollutants. For convenience pollutant sources
can be grouped into mobile or transportation sources, and stationary
sources. Approximately 50 - 70% of all air pollution arises from transportation-combustion
sources, 15-25% from heavy industrial stationary sources and as much
as 25% from other stationary sources.
Table 2.1 lists the major sources of pollution in the USA during the
late 1980’s. It provides estimates of the amounts emitted of the
five most significant air pollutants. Levels in Australia will vary
somewhat, but as our social demographics are similar to the United States
these are also a good estimate of the approximate break up of pollutants
sources. Note however that the total amount emitted in Australia will
be far less than those in the USA. One pleasing aspect is that there
has been a significant reduction in the amounts of CO and hydrocarbons
when compared to the previous decade, whilst the levels of other pollutants
has been steady or shown only a slight increase.
2.1 – Estimated emission of 5 primary pollutants in the USA 1987
There are many different types of transportation sources, not all of
which are significant sources of pollution. Unfortunately the most important
transportation sources at present are major polluters.
Motor vehicles are a very significant source of pollution. They produce
many different pollutants including carbon monoxide, carbon dioxide,
hydrocarbons, oxides of nitrogen, lead particulates and even small amounts
of oxides of sulfur. Motor vehicle exhaust accounts for 40% of all hydrocarbon
air pollutants and 90% of all nitrogen dioxide. Additionally the pollutants
from motor vehicles can react in the atmosphere to form different and
even more reactive (and dangerous) pollutants such as photochemical
smog. Many trucks run on diesel fuel. Whilst this is more economical
it is also a source of very dangerous hydrocarbons and is a major source
of the extremely carcinogenic polycyclic aromatic hydrocarbons (PAH’s).
Lead is rapidly decreasing in significance as a pollutant from motor
vehicles as more and more use unleaded fuels. IN the USA the introduction
of unleaded fuel in 1978 produced a reduction in atmospheric lead over
the next decade of 94%. A similar reduction is expected in Australia.
Additionally vehicles running on unleaded fuels emit lower levels of
oxides of nitrogen and sulfur as the catalytic converters in the exhaust
systems help to reduce these.
Aircraft and trains are less significant sources of pollution compared
with road transport vehicles.
Aircraft run on kerosene, which is burnt fairly efficiently, but as
they fly very high in the atmosphere the pollutants – most of
which are hydrocarbons – are spread and diluted in the upper atmosphere.
Many trains now run on electricity, and hence contribute very little
to air pollution. The discharge associated with high voltage electrical
cables used to power them can however, give off ozone, which is very
toxic and a powerful oxidising agent. Despite this electric trains are
probably the most environmentally friendly forms of mass transport systems
available in modern society.
Stationary (Industrial) Sources
Again these are many and varied in nature. Some of the more important
furnaces - and their combustion of carbonaceous fuels
ovens and dryers
process systems which produce volatile chemicals, gases, etc.
Solvent evaporation can come from solvent-based paints, leaking pipe
joints, maintenance work involving the dismantling of pumps or breaking
of pipelines, spills, unloading /loading procedures, contaminated ground.
Solvent vapour is an important part of photochemical pollution (see
The emission of waste gases. fumes, vapours and smokes to the atmosphere
are usually by the use of a smoke stack or chimney. A backyard incinerator,
or even a whole city or industrialised area could also be thought of
as a stack emission to the atmosphere. Although such emissions are usually
the waste products of combustion, many waste solvent vapours are vented
to the atmosphere in this manner.
The stack emission becomes a plume in the atmosphere. The plume is an
area of concentrated waste emissions that slowly become diluted with
the other atmospheric gases.
How this dilution happens will depend on a number of factors:
Nature of the waste emission
Toxic emissions will need to be very dilute if they are allowed to be
vented to the atmosphere.
Volume of the waste
Is the emission constant or only at certain times in the process. For
example, pottery kilns will only vent waste products when the kilns
are operating. Coal-fired electrical generating plants will vent waste
products all the time.
Many cities in the world are located in delta areas surrounded by hills
or mountains. Melbourne, Sydney, Los Angeles and Mexico City are examples
of cities located in delta areas. During periods of low wind and cooler
temperatures, these cities suffer from photochemical smog.
The direction of prevailing winds is important with stack emissions.
The area the wind takes the emissions may be affected by photochemical
smog, acid rain or fallout of pollutants.
Queenstown, in the south-east of Tasmania is a famous example of the
effect of stack emissions. The smelter emissions created a "fan" of
bare hills in the direction of the prevailing wind. This fan continues
for some miles beyond Queenstown.
Emissions from aluminium smelters also are linked with fluorine damage
to plants and animals in a "fan" made by the prevailing wind.
The Existing Atmosphere
In very polluted cities, the introduction of more stack emissions may
not be desirable.
Large coal-fired electrical plants are built in country areas, where
the waste gases can be vented to clean air. If these plants were built
in urban areas, the photochemical smog will be much worse. If photochemical
smog is already a problem, the introduction of more waste gases must
be done carefully, or where possible avoided entirely. Any new waste
gases may react strongly with the photochemical smog. An example would
be sulfur dioxide.
The effects of plumes are considered local within 500 metres of the
stack, and regional beyond this.
The mixing or dispersion of the waste gases and products into the atmosphere
is called plume behaviour. In stable air, and where the vertical movement
of the plume is slow, a fanning plume is produced. This wide, shallow,
spreading plume is very common after calm clear nights.
A temperature inversion limits the rise of the plume into the upper
atmosphere. The following diagram shows normal air movements, and a
temperature inversion. A layer of warm air limits the rise of the plume
into the upper atmosphere, and creates a higher concentration of polluted
air at lower levels. This plume exists for several hours.
2.1 - A Fanning Plume
windy conditions the plume can swirl up and down. This is a looping
plume, and is common in the afternoon. Moderate and strong winds are
formed on sunny days creating unstable conditions. This plume exists
for several hours.
2. 2 – A Looping Plume2
moderate winds and overcast days, the plume may become a coning plume.
This plume is wider than it is deep, and is elliptical in shape. This
plume exists for several hours.
2. 3 – A Coning Plume2
fumigating plume is short-lived (fraction of an hour), but reaches the
earth's surface. Fumigating plumes occur when the conditions move from
stable to more unsettled.
A fanning plume might have developed overnight under stable conditions.
As the day heats up, unstable air is produced. This unstable air affects
the fanning plume, causing the plume to move vertically up and down.
These plumes can cause localised pollution. Fumigating plumes become
looping or coning plumes as the air conditions stabilise.
2. 4 – A Fanning Plume2
the plume is above the inversion layer, it becomes a lofting plume.
Normal wind direction and speed will disperse the plume into the atmosphere
without effect from ground warming or cooling.
2. 5 – A Lofting Plume2
number of factors are used to establish the amount of stack emission
allowed, and its concentration to the atmosphere. These include:
smoke stack (chimney) height,
chemical reactivity, and
existing air pollution problems
Increasing the height of smoke stacks results in the emission of pollutants
higher up in the atmosphere. In theory this means that the pollutants
will be more diluted by the atmosphere if and when they return to the
ground – hence the effect on those closer to the stack is decreased
compared to if the emission occurred at a lower height, or at ground
level. Whilst this is true it also means that the pollutants are spread
over a much greater area with taller stacks, and more individuals may
be affected. In general however, higher stacks allow emission of higher
levels of pollutants.
Some types of topography harbour pollutants better than others. In general
areas in low valleys surrounded by mountains, with little wind are not
conducive to natural removal of pollutants. Hence lower pollution levels
can be tolerated in these areas. By contrast open flat areas with high
levels of prevailing wind allow rapid dispersal of pollutants.
High temperature emissions will rise to a higher level into the atmosphere.
This may mean that the pollutants will go up into the atmosphere, away
from the local area under most conditions, whilst low temperature emissions
may fall rapidly, and blanket the area surrounding the stack.
High rates of emission, or emission of highly toxic or highly reactive
species require lower limits to be set on the emission source.
Fugitive Emissions and Other Sources
Fugitive emissions are those which escape from a process rather than
being discharged. These often have serious consequences because their
levels are not monitored and they are untreated when entering the atmosphere.
There are many sources of fugitive emissions including:
industrial sources (particulate fluorides from aluminium smelters)
small business (e.g. dry cleaning)
agriculture (e.g. dust from ploughing)
natural sources (e.g. volcanoes, forest fires)
Often fugitive emissions are the result of poor maintenance of plant
and equipment and can be eliminated by standard operating procedures
that involve timed maintenance and quality control checks, but some
are almost impossible to control (such as those from natural sources).
An example of the former is the particulate and gaseous fluoride emissions
from aluminium smelting. These are emitted unintentionally when the
casting areas of aluminium potlines are opened or incorrectly sealed.
Standard operating procedures that ensure that the casts are sealed
greatly reduces the problem.
Types of Air Pollutants
Essentially there are four types of air pollutants;
particulate pollutants and
Only the first three of these are addressed in this module. Noise is
such a large area that it is addressed in its own module, but you should
be aware that the great majority of noise pollution occurs through the
Primary and Secondary Air Pollutants
Not all of the pollutants found in the atmosphere are the direct result
of emissions. Many of the substances found in the atmospheres that are
regarded as pollutants arise from chemical reactions in the atmosphere
with other substances or light. Chemical reactions that require light
in order to proceed are referred to as photochemical reactions.
Pollutant substances that are directly emitted into the atmosphere are
known as primary pollutants.
Those substances that are not directly emitted into the atmosphere,
but rather are formed by chemical reactions in the atmosphere are referred
to as secondary pollutants.
The term particulate refers to very small solid or liquid particles.
Individual particles may vary in size, geometry, chemical composition
and physical properties. They may be of natural origin (such as pollen
or sea spray) or man made (dust, fume and soot). They provide a reactive
surface for gases and vapours in the formation of secondary pollutants.
Particles also diffuse light reducing visibility. Atmospheric particles
come from stack emissions, dusty processes, unsealed roads, construction
work and many other sources.
Particulate matter may be classified under the following headings:
Dusts - large solid particles (>100um) carried into the air.
Fume - solid particles (frequently metallic oxides) formed by condensation
of vapours from a chemical reaction process or physical separation process.
These particles are quite small, typically between 0.03 - 0.3um in diameter.
Mist - liquid particles formed by condensation of vapours or chemical
reaction. For example,
SO3 + H2O H2SO4.
Typically they are 0.5 - 3.0um in diameter.
2.6 – the size ranges of common atmospheric particles1
- solid particles formed as a result of incomplete combustion of carbonaceous
materials. Typical diameter is between 0.5 - 1.0um.
Spray - a liquid particle formed by the atomisation of a parent liquid.
Particulate matter makes up the most visible form of air pollution.
Pollutant particles in the 0.001 to 10 um range are commonly suspended
as aerosols near sources of pollution in urban atmospheres such as industrial
plants, highways and power plants.
Atmospheric particles range in size from 0.005 - 500mm. The smallest
of these are clusters of molecules whilst the largest are easily visible
with the naked eye. Note that sizes given are not the physical size,
but rather the aerodynamic equivalent diameter – which relates
the particle to the behaviour of an equivalent spherical particle.
Particles less than 1mm in diameter behave much like gases (remain suspended,
may coalesce, move in fluid streams), whilst larger particles are more
like solids (affected by gravity, don’t stay suspended long, don’t
coalesce). There is a marked variation of particle composition with
size. The smaller particles generally derive from chemical reactions
and are frequently acidic, whereas the larger particles (10mm or greater)
are usually generated mechanically by bulk materials and have a tendency
to be basic. It is the former which, irrespective of chemical composition,
are the most dangerous to health, since they are not readily filtered
out in the nose and throat and penetrate into the lungs.
In urban areas there is an approximately even distribution between fine
and coarse particles, but this is weather dependent. Under calm conditions
there are more fine particles than coarse, as the coarser particles
tend to settle if there is no turbulence. Fine particulate matter also
tends to be transported and spread over much greater distances as it
has a much longer residence time in the atmosphere. An extensive analysis
of suspended particles collected from a range of urban areas in the
United States showed a total concentration of 105mg/m3, and included
the following components: sulfate (10.6), organic material (6.8), nitrate
(2.6), iron (1.58), lead (0.79), zinc (0.67), arsenic (0.02) and cadmium
Particle Behaviour in the Atmosphere
Particles in the atmosphere undergo many changes, both physical and
chemical. They may grow in size, absorb or desorb gases from their surfaces,
change their electrical charge, collide or adhere with other particles,
or absorb water. These may change the particle size and affect its atmospheric
One of the major factors in particulate behaviour is the uptake and
release of water. This process not only dramatically changes the specific
gravity of the particle, but also them to form sulfate and nitrate aerosols.
This in turn may dramatically change their pH, chemical reactivity,
toxicity and even its surface charge. Particles with a size of greater
than 3mm are generally negatively charged, whilst those less than 0.01mm
are almost always positively charged. These electrical charges have
a substantial effect on the coagulation and rates of deposition of the
Total Suspended Particulates (PM10, PM2.5)
The size distribution of particles suspended in the atmosphere shows
that most particles are concentrated into three main size groups. Larger
particles are most often around 10mm in size, whilst the smaller particles
occur in size groups centered around 0.2 and 0.02mm. Only particles
of 10mm can penetrate into the human lung, so it is common practice
to analyse air for only this fraction to estimate its potential danger
to human health. This is called PM10 sampling. Particles 2.5mm in size
can penetrate deep into the lung tissue and are especially dangerous.
For this reason a new standard has been developed which allows testing
of this very fine particulate matter. It is referred to as PM2.5 sampling.
Both PM10 and PM2.5 are discussed in the following chapter in more detail.
particulate matter occurs in a vast array of compounds. When collected,
they can be fractionated into various chemical groups. Of these, the
polycyclic aromatic hydrocarbons (PAH) have received most attention.
They tend to be found adsorbed on soot and dust particles, and are formed
from smaller hydrocarbons at high temperatures. Effluent from a coal
furnace may contain over 1 mg/m3 of PAH compounds, and cigarette smoke
almost 0.1 mg/m3. As a result, urban atmospheres have shown PAH levels
approaching 20 ug/m3.
PAH compounds can be synthesised from saturated hydrocarbons as small
as methane and ethane. Mixtures of ethane and ethene heated at above
500oC have yielded over ten PAH compounds. C-H and C-C bonds are broken
in the small hydrocarbons, forming radicals. These undergo dehydrogenation
and combine to form stable aromatic rings in a process called pyrosynthesis.
Currently, lead is the most serious atmospheric heavy-metal pollutant.
Its primary source is exhaust from vehicles using leaded petrol. Lead
in the form of the organometallic tetraethyl lead Pb(C2H5)4 - was included
in petrol to promote more even combustion. However, in the process,
it undergoes the following reaction:
Pb(C2H5)4 + O2 + halogenated organic compounds
CO2 + H2O + PbCl2 + PbBrCl + PbBr2
The lead halides produced are sufficiently volatile to exit through
the exhaust system and condense in the atmosphere to form particles.
By 1975, annual lead emissions in the Sydney region were estimated to
have reached about 1100 tonnes. As a result, Australian governments
have enforced a gradual decrease in the levels of lead added to petrol
and increased the sale of unleaded petrol. A bonus from this is that
the absence of lead in petrol makes possible further improvements in
the exhaust emissions of CO and NOx by the use of catalytic converters,
which are poisoned rapidly by lead emissions.
The American experience has shown that reducing the lead levels in petrol
has a rapid and considerable effect on atmospheric lead, and also on
its rain-induced deposition. The rate at which lead levels have decreased
indicates that, unlike other pollutants such as chlorofluorocarbons,
lead has a very short atmospheric lifetime.
Australia-wide legislation has meant that all cars built after 1985
must operate on lead-free petrol. Before the introduction of lead reduction
program in New South Wales, atmospheric lead levels in Sydney, Newcastle
and Wollongong were in excess of the recommended maximum (1.5 mg/m3
- 90-day average) at all times! In the last three months of 1987, lead
levels in the central business district exceeded the standard on one
day, but were below it on the 90-day average. However, in Port Kembla,
numerous high daily readings (peaking at 9.4 mg/m3) produced a 90-day
average of 2.0 mg/m3, well in excess of the standard. This was due not
to vehicle exhausts, but base metal smelting operations. It is estimated
that after the year 2000, vehicle emissions will play a negligible role
in atmospheric lead.
include substances that are gases at normal temperature and pressure
as well as vapours of substances that are liquid or solid at normal
temperature and pressure. The gaseous pollutants of greatest importance
include carbon monoxide, hydrocarbons, hydrogen sulfide, nitrogen oxides,
ozone and other oxidants, and sulfur oxides. Carbon dioxide could be
added to this list as it has potential to dramatically alter our climate.
Pollutant concentrations are measured in micrograms per cubic meter
(ug/m3) or parts per million (ppm).
1 ppm = 1 volume of gaseous pollutant
106 volumes of (pollutant + air)
At 25°C and 101.3 kPa the relationship between ppm and ug/m3 is;
ug/m3 = ppm x molecular weight x 103
Carbon Oxides (CO and CO2)
Significant quantities of the carbon oxides, carbon monoxide (CO), and
carbon dioxide (CO2), are produced by natural and anthropogenic sources.
Because of its health implications, CO is considered to be a major atmospheric
pollutant. Carbon dioxide is relatively non-toxic, but it’s significant
potential for causing global climatic change makes direct and indirect
emissions to the atmosphere a serious pollution problem.
monoxide is a colourless, odourless and tasteless gas. The Earth's atmosphere
has an average burden of around 530 million tonnes (about 0.00001%),
with an average residence time of 36 to 100 days. Much of the CO in
the atmosphere occurs naturally as it is emitted from volcanic eruptions,
photolysis of methane and terpenes, decomposition of chlorophyll, forest
fires and microbial action in oceans.
Anthropogenic sources include transportation, solid waste disposal,
agricultural burning, steel production, etc. It is also emitted directly
into the atmosphere through the inefficient combustion of fossil fuels.
It is removed by reactions in the atmosphere which change it to CO2
and by absorption by plants and soil micro-organisms. In combustion,
carbon is oxidised to CO2 in a two step process.
2C + O2 2CO
2CO + O2 2CO2
Annual global emissions are estimated to be 3 x 109 to 6.4 x 1011 tons
per year for natural and 2.75 x 108 tons per year for man made sources.
Carbon monoxide is emitted if insufficient oxygen is present for the
second step to proceed.
Background levels of CO tend to vary greatly depending on location.
Average global levels are about 0.2ppm. Peak concentrations tend to
occur during autumn months when large volumes are generated by the decomposition
of chlorophyll in leaves.
In urban areas a diurnal concentration pattern occurs (see figure 2.7).
This is associated with the daily transport cycles where most vehicles
are used during the morning and evening.
2.7 – CO levels in a typical urban area over a 24hr period1
the internal combustion engine contributes much of the man-generated
CO (the EPA estimates 90% in the Sydney region), maximum levels of this
gas tend to occur in congested urban areas at times when the maximum
number of people are exposed, such as during rush hours. At such times,
CO levels in the atmosphere may become as high as 50-100ppm. Since the
introduction of the Clean Air Act in NSW in 1972, and the control of
vehicular emissions, the levels of CO in the central city district have
dropped from an average of 25ppm to around 10ppm. The accepted standard
is 9ppm over an eight-hour period. Sydney exceeds this on an average
of 50 days per year.
CO is removed from the air mostly by conversion to CO2. This may occur
through aerial oxidation or through the action of soil microorganisms.
It is estimated that soil microorganisms remove more than 5 x 108 tonnes
per year in the USA alone. This is greatly in excess of the man made
emission rate. Hence we should not see great increases in the ambient
levels of CO. The reason for very high concentrations occurring in urban
areas is that high emission rates are combined with a lack of soil.
Carbon dioxide is produced when organic matter is combusted, weathered,
or biologically decomposed. It is removed from the atmosphere by plants
in photosynthesis and released by biological reactions.
Over hundreds of millions of years CO2 has been withdrawn from the atmosphere
and stored in coal, oil and natural gas. The intensive use of these
fuels in the past century, however, has resulted in significant CO2
emissions and an increase of atmospheric concentrations. Base values
of CO2 have reportedly increased about 25% since 1850. Since 1958, CO2
values measured at Mauna Loa Observatory in Hawaii have increased from
310 to more than 350ppm.
Significant seasonal variations are also observed to occur in CO2 levels,
which reach a maximum in the northern hemispheric spring and a minimum
in autumn. This seasonal variability appears to be associated with growing
season photosynthetic needs and metabolic releases of CO2 in excess
of plant uptake at the end of the growing season.
Not all CO2 emitted to the atmosphere from anthropogenic sources contributes
to increased atmospheric levels. Because of its solubility in water,
the oceans are a major sink for CO2, absorbing 50% of all man made emissions.
The world's forests, particularly tropical forests, also serve as a
As a thermal absorber (read greenhouse gas), CO2 prevents some infrared
emissions from the Earth being radiated back to space. It is though
that in time this may greatly change the planet’s global heat
balance – leading to global warming – the so-called Greenhouse
A variety of sulfur compounds are released to the atmosphere from both
natural and anthropogenic sources. The most important of these are the
sulfur oxides (SOx) and hydrogen sulfide (H2S). Although significant
SOx emissions may occur from volcanic eruptions and other natural sources,
man made emissions are responsible for much of the atmospheric emissions.
These are produced by roasting metal sulfide ores and by combustion
of fossil fuels containing appreciable inorganic sulfides and organic
sulfur. Of the four known sulfur oxides, only SO2 is found at appreciable
levels in the atmosphere. Sulfur trioxide (SO3) is emitted directly
into the atmosphere in ore smelting and fossil fuel combustion and is
produced by the oxidation of SO2. Because it has a high affinity for
water, it is rapidly converted to sulfuric acid.
The formation of SO2, SO3, and sulfuric acid in the atmosphere is summarised
in the following equations.
S + O2 SO2
2 SO2 + O2 2SO3
SO3 + H2O H2SO4
Sulfur dioxide may be directly absorbed by water bodies such as the
oceans to form sulfurous acid. This is one of the sources of acid rain,
which has dramatically affected the environment in Europe and North
SO2 is an acidic colourless gas which may remain in the atmosphere for
periods up to several weeks. It can be detected by taste and odour in
the concentration range of 0.38 - 1.15ppm. Above 3 ppm, it has a pungent,
It is estimated that 65 million tonnes of SO2 per year enter the atmosphere
as a result of man's activities, primarily from the combustion of fossil
fuels. Of these, coal is by far the greatest contributor. In the United
States, it is estimated that almost 60% of SO2 emission are the result
of coal-fired power stations.
Background levels of SO2 are very low, about 1ppb. In urban areas maximum
hourly concentrations vary from less than 0.1 to more than 0.5ppm. Maximum
hourly concentrations in the range of 1.5 to 2.3ppm have been reported
near large metal smelters. Since the implementation of significant SO2
control measures in the early 1970s, many urban areas in the United
States report markedly reduced ambient SO2 levels (<0.10 ppm).
SO2 is removed from the atmosphere by both dry and wet deposition processes.
It is believed that plants are responsible for most SO2 removal that
occurs by dry deposition. SO2 can also dissolve in water to form a dilute
solution of sulfurous acid (H2SO3). This water can be in clouds, in
rain droplets, or at the surface.
A major sink process for SO2 is its gas-phase oxidation to H2SO4 and
subsequent aerosol formation by nucleation or condensation. Sulfuric
acid will react with ammonia (NH3) to form salts including ammonium
hydrogen sulfate (NH4HSO4), ammonium sulfate [(NH4)2SO4] or mixed salts
with ammonium nitrate (NH4NO3).
About 30% of atmospheric SO2 is converted to sulfate aerosol. Sulfate
aerosols are removed from the atmosphere by dry and wet deposition processes.
In dry deposition, aerosol particles settle out or impact on surfaces.
In wet deposition, sulfate aerosol is removed from the atmosphere by
forming rain droplets (in cloud) or being captured by falling rain droplets
(below cloud). These removal processes are called rainout and washout.
H2S is a very toxic gas with a characteristic rotten egg odour. This
odour can be detected at concentrations as low as 0.5ppb. Although H2S
is quite toxic, H2S levels in the atmosphere appear to be too low to
pose a threat to human health. The principal concerns associated with
H2S are its smell and its effects causing deterioration of lead-based
Background levels of H2S are approximately 0.05ppb. Natural sources,
which include anaerobic decomposition of organic matter, natural hot
springs and volcanoes, produce approximately 100 x 106 tonnes per year
world wide.. Anthropogenic sources, which include oil and gas extraction,
petroleum refining, paper mills, rayon manufacture, and coke ovens,
account for global emissions of 3 x 106 tonnes per year.
The major sink process for H2S is its atmospheric conversion to SO2.
This SO2 is then removed from the atmosphere in the gas phase or as
an aerosol by wet or dry deposition processes.
There are five major gaseous forms of nitrogen in the atmosphere. These
include molecular nitrogen (N2), ammonia (NH3), nitrous oxide (N2O),
nitric oxide (NO), and nitrogen dioxide (NO2). N2 is the major gas in
the atmosphere. N2O is present in unpolluted air as a result of microbial
action, whilst NO and NO2 are significant air pollutants. NH3 is not
considered a significant man made pollutant, but enormous quantities
are generated through natural emissions.
Nitrous oxide is a colourless, slightly sweet, nontoxic gas. It is a
natural constituent of the atmosphere at an average concentration of
0.30ppm. It is widely used as an anaesthetic in medicine and dentistry.
It is called laughing gas because it induces a kind of hysteria. It
is a product of natural processes in the soil, produced by anaerobic
bacteria. It can photolytically dissociate in the stratosphere to produce
Nitric oxide is a colourless, odourless, tasteless, relatively nontoxic
gas. It is produced naturally by anaerobic biological processes in soil
and water, by combustion processes and by photochemical destruction
of nitrogen compounds in the stratosphere. On a global basis, natural
emissions of NO are estimated to be approximately 5 x 108 tones per
Major anthropogenic sources include automobile exhaust and stationary
sources, such as fossil fuel-fired electric generating stations, industrial
boilers, incinerators, and home space heaters.
Nitric oxide is a product of high-temperature combustion.
N2 + O2 2NO
As this reaction is endothermic, the equilibrium moves to the right
at high temperatures. At lower temperatures, it shifts completely to
the left. If the cooling rate is rapid, the equilibrium is not maintained
and high NO emissions result. High combustion temperatures and rapid
cooling promote high NO emissions.
In 1970 combined worldwide emissions of NO and NO2 were estimated to
be about 5.3 x 107 tonnes per year - about 10% of that estimated to
have been produced by natural sources.
Nitrogen dioxide is a coloured gas, which is light yellowish orange
at low concentrations and brown high concentrations. It has a pungent,
irritating odour , and is extremely corrosive especially in wet environments.
It is also toxic, as it can cause anoxia.
Some of the nitrogen dioxide in the air is produced by the direct oxidation
2NO + O2 2NO2
At low atmospheric NO levels, this occurs slow, accounting for less
than 25% of all NO conversion. Photochemical reactions involving O3,
peroxy radical (RO2) and reactive hydrogen species such as OH·,
HO2, H2O2, are the primary means by which NO is converted to NO2 in
the atmosphere. Some of the more important reactions are shown below.
NO + O3 NO2 + O2
RO2 + NO NO2 + RO
HO2 + NO NO2 + OH
Background concentrations of NO and NO2 are approximately 0.5 and 1ppb
respectively. In urban areas, 1 hour average concentrations of NO may
reach 1-2ppm, with maximum NO2 levels of approximately 0.5ppm. The decay
rate of NO is rapid as polluted air moves away from urban to rural areas,
with concentrations dropping to near background levels.
Atmospheric levels of NO are related to the transportation/work cycle.
Peak concentrations are observed in the early morning hours, with a
second smaller peak late in the day (See Figure 2.8). Peak morning NO
concentrations are followed several hours later by peak levels of NO2
produced by the chemical and photochemical oxidation of NO.
Atmospheric levels of NO and NO2 also show seasonal trends. Emissions
of NO are greater during winter months when there is an increased use
of heating fuels. Since the conversion of NO to NO2 is related to solar
intensity, higher NO2 levels occur on warm sunny days.
Nitrogen oxides in motor vehicles exhausts have been controlled by legislation
as with CO. In this case, the catalytic converter included in the exhaust
system encourages the reduction of NOx to N2. These catalysts include
rhodium and CuO. Australian Design Rules limit the emission of NOx from
vehicle exhausts to 1.9g/km, and authorities expect that this will continue
to maintain the levels in Sydney below the recommended standard of 0.16ppm
(over a 1 hour average).
2.8 – Levels of NO, NO2, and ozone on a typical smoggy day in
a large city – Taken from Los Angeles1
The most significant sink for NO is its conversion by both direct oxidation
and photochemical processes to NO2. A major sink process for NO2 is
its conversion to nitric acid as is shown below.
OH· + NO2 + M HNO3 + M
where M is an energy-absorbing species (generally O2 or N2). NO2 is
also converted to nitric acid by night-time chemical reactions involving
NO2 + O3 NO3· + O2
NO2+ NO3· N2O5
N2O5 + H2O 2HNO3
NO3· is the nitrate free radical. It is the key factor in nighttime
chemistry. The reaction product of NO2 and NO3 is dinitrogen pentoxide
(N2O5). This reacts with water rapidly to produce HNO3.
A portion of the HNO3 in the atmosphere will react with ammonia (NH3)
or other alkaline species to form salts such as NH4NO3.
Nitrate aerosol is generally removed by the dry and wet deposition processes
in much the same way as sulfate aerosol.
This is considered a relatively unimportant man made pollutant. Approximately
4 x 106 tons are emitted per year on a worldwide basis. This may be
compared to natural emissions, with a worldwide annual emission rate
estimated to be 1.2 x 109 tonnes per year. Most of this is comes from
biological decomposition. Background concentrations vary from 1 to 20ppb.
The average atmospheric residence time is approximately 7 days.
Ammonia has a significant effect on atmospheric sink processes of strong
acids. Reactions with sulfuric and nitric acid in the atmosphere produce
ammonium salts. Ammonium sulfate is the principal sulfate species collected
on particulate sampling devices. Ammonia itself may also be oxidised
in the atmosphere in a series of chemical reactions to produce nitrates.
These are produced in the atmosphere by the reaction of nitrogen oxides
and hydrocarbons. Examples are the peroxyacyl nitrates (PAN’s)
and peroxybutylnitrates (PBN’s). These are discussed in more detail
in the section on photochemical smog.
These are just simple organic materials in the atmosphere. The definition
is used more broadly here than in organic chemistry where it refers
simply to substances made from carbon and hydrogen. In the atmosphere
simple hydrocarbons react with substances containing oxygen, nitrogen,
sulfur, chlorine bromine and even some metals to form hydrocarbon derivatives.
Atmospheric hydrocarbons exist in gas, liquid and solid phases. Of these
the gases and volatile liquids are the most significant pollutants.
Solid hydrocarbons are generally of higher molecular weight and exist
as condensed particles in atmospheric aerosols.
Methane (CH4) is the most common hydrocarbon in the atmosphere and it
is formed from many natural sources such as termites, cows and general
decomposition of organic matter. It and the other alkanes found in the
atmosphere are fairly unreactive.
The atmospheric hydrocarbons that are the most significant in terms
of chemical reactivity are the alkenes. Many highly reactive alkene
hydrocarbons are formed naturally by plants (such as terpenes from citrus
plants and eucalyptus haze). The greatest source of these non-methane
hydrocarbons are motor vehicles. Significant quantities are also emitted
from petroleum processing. Alkenes are the major air pollutant responsible
for photochemical smog and other gross oxidants in the atmosphere. Once
in the atmosphere they combine with O2 to form many different oxygenated
hydrocarbons including alkanones, alkanals, alkanoic acids, alkanols
and ethers. Some dicarboxylic acids are also formed by photochemical
reactions, and are present in the atmosphere as aerosols. An example
of this is propanedioic acid (old name malonic acid).
Some oxygenated hydrocarbons are emitted into the atmosphere directly.
For example methanal (old name formaldehyde), acrolein and some other
simple alkanals are major by-products of combustion processes and are
present in significant amounts in motor vehicle exhausts – especially
those running on diesel fuel.
Aromatic hydrocarbons are not very reactive, but are important in urban
atmospheric chemistry as they undergo reactions with other very reactive
chemical oxidants to form toxic substances. Some toxic atmospheric aromatic
hydrocarbons are produced directly by combustion such as benzo[a}pyrene
and other polyaromatic hydrocarbons (PAH’s).
Benzo [a] pyrene
Hydrocarbons are emitted from a variety of natural and man made sources.
They are important pollutants because of their role in atmospheric photochemistry.
Both biological and geological processes release hydrocarbon compounds
naturally. Sources include plant and animal metabolism, vaporisation
of volatile oils from plant surfaces, biological decomposition, and
emission of volatiles from fossil fuel deposits. U.S. emissions of natural
hydrocarbons including methane are estimated to be more than 7 x 107
tons/year, with some estimates of biogenic non-methane hydrocarbons
(especially natural alkenes such as isoprene and pinene) ranging from
3.3 to 6.6 x 107 tons/year. Man made emissions of hydrocarbons in the
United States have been estimated to be about 3 x 10' tons/year, with
worldwide emissions estimated at 9 x 107 tons/year. Hence it can be
seen that one country provides 33% of the worlds man made hydrocarbon
Globally, anthropogenic emissions represent only about 5% of total hydrocarbons
emissions. In the United States, man made emissions may contribute 23-40%
of the total atmospheric load. The impact of man made emissions is particularly
significant in urban areas.
Hydrocarbons are released to the atmosphere from a variety of human
activities. Sources include transportation, petroleum refining, oil
and gas production and distribution, chemical manufacturing, industrial
and commercial organic solvent use, and food processing.
Significant portions of hydrocarbons released to the atmosphere are
from mobile sources, with light-duty motor vehicles accounting for approximately
75%. Hydrocarbon emissions from motor vehicles result from both evaporative
losses and incomplete combustion. Controls on individual vehicles have
reduced hydrocarbon emissions in exhaust gases by about 85% since their
introduction in Australia in 1985.
In the past most efforts to control hydrocarbons have focused on motor
vehicles. Stationary facilities have received less attention from regulatory
authorities, although their importance is now being recognised.
Identification of hydrocarbons species in urban atmospheres involves
the use of GC/MS techniques. Studies of atmospheric samples and emission
sources, identify hundreds of hydrocarbons and their derivatives as
being present in polluted urban air.
Identification of hydrocarbons in urban atmospheres is complicated,
as many individual compounds are present in only sub-trace amounts.
As a result, the likely presence of many compounds is inferred from
smog chamber studies and from analyses of motor vehicle exhaust. Over
400 hydrocarbons and oxy-hydrocarbon derivatives have been found in
vehicle exhaust gases. Many oxygenated hydrocarbons are found with methanal
being the most significant.
Polluted ambient atmospheres contain a variety of alkane (e.g. propane,
butane), alkene (e.g. ethene, propene), polyunsaturated (e.g. ethyne,
propadiene), aromatic (e.g. benzene), and polycyclic aromatic hydrocarbons
Individual hydrocarbon concentrations are not routinely monitored by
control agencies. Quantitative data is based on the measurement of total
non-methane hydrocarbons. This is usually averaged over the 6-9 a.m.
period. The problem with this is that the 6-9 a.m. measurement period
reflects motor vehicle emissions and gives little indications of other
Total non-methane hydrocarbon measurements show a diurnal pattern in
most cities with two peaks, one from 6-9 a.m. and another broader peak
in the late afternoon. These peaks reflect motor vehicle traffic and
local meteorological dispersion characteristics.
Total non-methane hydrocarbon concentrations in urban areas range from
1 - 10ppm.
There is a very large number of hydrocarbons, oxy-hydrocarbons, and
other derivatives in polluted atmospheres. Information on sink mechanisms
for specific compounds is relatively limited. The most important sink
processes are photochemical conversion of hydrocarbons to CO2 and H2O
or to soluble or condensable products such as dicarboxylic acids - a
major component of photochemical aerosol. These aerosols are removed
from the atmosphere by both dry and wet deposition processes.
When air pollution regulatory control first commenced, CH4 was considered
to be an unimportant hydrocarbon in polluted atmospheres. Measurements
of total hydrocarbons subtracted the concentration of CH4. Hence the
ambient air quality standard for hydrocarbons is a non-methane hydrocarbons
Relative to alkenes, aromatics, and even other alkanes, CH4 is relatively
unreactive and therefore of little significance in urban photochemical
reactions that produce elevated O3 levels. The significance of CH4 is
that relatively unreactive hydrocarbons play an important role in O3
formation as polluted air masses travel long distances downwind of urban
sources. Methane has also been recognised as one of the trace gases
that may have a significant effect on global climate through the greenhouse
Methane is by far the most abundant hydrocarbon in the atmosphere, with
a 1980 concentration of 1.65ppm. It has been increasing at a rate of
1.2-1.9% per year. The rate itself is also increasing. Increases of
CH4 over the last 300 years, as determined from measurements of air
bubbles trapped in Antarctic ice, can be seen in Figure 2.9. Note the
significant inflection of the curve in this century.
The increase of CH4 is due to both increased emissions and to CO-caused
depletion of OH·, which is the most important sink for CH4 in the
2.9 – Historical global levels of methane in the atmosphere1
and Photochemical Smog
is a normal component of the Earth’s atmosphere, but most of it
is found in the middle stratosphere where it plays an important role
in the controlling the amount of UV light reaching the planet’s
surface (see the Ozone Hole). This is one case where depletion of the
substance results in air pollution – as the loss of ozone is causing
deterioration in quality of life.
Ozone is not listed as a major primary air pollutant in the lower atmosphere
either, but due to its high toxicity and its involvement in the production
of other pollutants it is a very important source of atmospheric pollution.
Over 90% of what is broadly referred to as photochemical smog is ozone.
Sources are electrical discharge both natural (such as lightning) and
man made (such as electric trains), and upper atmospheric chemical reactions
such as the reaction of molecular oxygen with oxygen atoms.
O2 + O + M O3 + M
In this reaction M is any third substance (usually O2 or N2) that removes
the energy of the reaction and stabilises O. In the lower atmosphere
(troposphere) the only significant source of atomic oxygen is the photolysis
NO2 + hn NO + O*
The reaction of O* with O2 produces O3, which reacts immediately with
NO to regenerate NO2.
NO + O3 NO2 + O2
All these reactions proceed rapidly with approximate concentration of
20ppb under solar noon conditions in mid latitudes at atmospheric NO2/NO
concentration ratios equal to 1. Hence concentrations of ozone remain
low unless imbalances in the levels of NO2 or other alternate chemical
reactants are available.
term refers to an atmosphere laden with secondary pollutants that form
in the presence of sunlight as a result of chemical reactions in the
atmosphere. Photochemical smog arises in urban areas, where there is
a heavy build-up of vehicle exhausts. It is greatly exacerbated by weather
Under normal conditions, the primary air pollutants are dispersed over
a large region or to the upper atmosphere. A good prevailing wind is
important for cities and large urban areas to help reduce smog. At certain
times of the year, when the wind is very still, the primary pollutants
build up over cities. Autumn tends to be worse for photochemical smog
than other times of the year.
2.10 – Normal dispersion pattern of pollutants from a large urban
autumn, the days are sunny and warm, with cool nights. Under still conditions,
a warm inversion layer forms under a layer of higher cooler air. Large
urban areas store heat, which provides the warmth for the inversion
layer. The inversion layer limits air mixing and dispersal trapping
primary pollutants at lower altitudes over urban areas.
2.11 – Conditions which favour smog formation in a large urban
primary pollutants such as NOx, and hydrocarbons are trapped in the
lower atmosphere and subjected to UV radiation from the sun –
photochemical smog forms.
These ingredients produce the pollutants that characterise photochemical
smog. These products are termed gross photochemical oxidants, and are
defined by their ability to oxidise iodide ion to elemental iodine.
They include ozone (O3), hydrogen peroxide (H2O2), organic peroxides
(ROOR'), organic hydroperoxides (ROOH) and by far the most serious to
health, peroxyacyl nitrates (RCO3NO2), known as PAN's. The latter are
formed by the irradiation of mixtures of alkanals, ozone and nitrogen
Reactions Occurring in the Formation of Photochemical Smog
The key chemical reactants in the formation of photochemical smog are
NO2 and hydrocarbons. The reactions undergone by these substances in
the atmosphere are many and varied. Many of the reaction mechanisms
are not well understood. This lack of knowledge has caused several control
schemes to fail, but several reactions have major roles in elevating
the levels of O3.
In the lower atmosphere O3 concentrations are often much higher than
those that occur from NO2 photolysis alone. This is because there are
chemical reactions that convert NO to NO2 without consuming O3. In polluted
and even weakly polluted atmospheres, these changes in O3 chemistry
can be attributed to peroxy radicals (RO2) and other species produced
by the oxidation of hydrocarbons as shown in the reactions below. M
is any species that will stabilise the molecule (generally N2).
RO2 + NO NO2 + RO
NO2 + hn NO + O*
O* + O2 + M O3 + M
Net: RO2 + O2 + hn RO + O3
This process is presented diagrammatically in Figure 2.12.
2.12 – Important reactions in the formation of photochemical smog1
rate of O3 formation is closely related to the concentration of RO2.
Peroxy radicals are produced when hydroxy radicals OH· and HOx
react with hydrocarbons. Hydroxy radicals are produced by reactions
involving the photolysis of O3, carbonyl compounds (mostly alkanals),
and nitrous acid.
In polluted atmospheres, O3 concentrations are directly related to the
intensity of sunlight, NO2/NO ratios, the hydrocarbon type and concentrations,
and other pollutants, such as alkanals and CO, which react photochemically
to produce RO2. The increase in NO2/NO ratios caused by atmospheric
reactions involving RO2 results in significant increases in lower atmosphere
A quick summary of the reactions involved in smog formation can be compressed
into four stages. This also explains the time variations in levels of
hydrocarbons, ozone, NO2 and NO (see Figure 2.13).
1. Primary photochemical reaction producing oxygen atoms:
NO2 + hn NO + O*
2. Reactions involving oxygen species (M is an energy-absorbing third
O* + O2 + M O3 + M
NO + O3 NO2 + O2
Because the latter reaction is rapid, the concentration of O3 remains
low until that of NO falls to a low value. Automotive emissions of NO
tend to keep O3 concentrations low along freeways.
3.Production of organic free radicals from hydrocarbons, RH:
O + RH R· + other products
O3 + RH R· + and/or other products
(R· is a free radical that may or may not contain oxygen.)
4. Chain propagation, branching, and termination by a variety of reactions
such as the following:
NO + ROO· NO2 + and/or other products
NO2 + R· products (e.g. PAN)
Some of the many other reactions which are known to occur in photochemical
smog formation are listed below.
O + hydrocarbons HO·
HO· + O2 HO3·
HO3· + H alkanals, alkanones
HO3· + NO HO2· + NO2
HO3· + O2 O3 + HO2·
HOx· + NO2 PAN's
While all hydrocarbons can become involved in the formation of smog,
there are considerable differences in their reactivities. The simplest,
methane, is very slow to react, having an approximate atmospheric lifetime
of more than 10 days. In general, branched alkenes and alkyl aromatic
compounds are the most reactive. It is interesting to note that experiments
to date have shown certain naturally-occurring alkenes (such as d-limonene
from citrus fruits) are the most reactive compounds.
Given the complex series of reactions involved and the changing levels
of vehicle emissions during a day, it is perhaps not surprising that
the concentrations of the major components vary considerably over a
24-hour period. A typical pattern of variations is shown in the figure
2.13 – Variations in smog pollutants with time of day
As the morning rush hour begins, NO begins to rise rapidly, followed
by NO2. As the latter reacts with sunlight, ozone and other oxidants
are produced. The hydrocarbon level increases in the morning, and then
decreases as the compounds are oxidised to form PAN's and other species.
As an air mass moves toward an urban center, it picks up NO, and hydrocarbons.
Within a time scale of an hour, OH· begins to degrade hydrocarbons,
producing RO2. As the air mass moves over the urban center, O3 precursors
peak and then decline with increasing downwind distance. Ozone concentrations
increase and are sustained over a period of 1-5 hours as the more reactive
alkene and aromatic hydrocarbons are depleted by photochemical reactions.
After a 5-10 hours travel time downwind, moderately reactive hydrocarbons
increasingly play a more important role in net O3 production. Ozone
levels in the air mass subsequently decrease due to dilution, conversion
of NO2 to HNO3, and surface adsorption. Under nighttime conditions,
O3 production ceases.
Protected by the inversion layer, O3 may persist aloft with a half-life
of as much as 80 hours. This allows O3 to be transported over long distances
giving rise to higher concentrations at remote sites, which occur away
from the normal noon maximum.
At sunrise, the inversion breaks up, bringing O3 and other products
isolated aloft during the nighttime hours to the ground, where they
mix with the pollutants held in by the inversion layer, and begin the
cycle all over again.
In unpolluted atmospheres O3 concentrations near the ground are in the
range of 10-20ppb (0.01-0.02ppm) during the warm months of the year.
O3 concentrations over landmasses with large motor vehicle numbers are
often elevated well above this even at remote sites.
In urban areas, peak 1-hour summertime O3 concentrations are usually
higher than those reported for remote non-urban sites. Concentrations
near or above the 0.3 standard of 0.12ppm have been reported for numerous
cities throughout the United States during the 1980’s, despite
two decades of efforts to control O3 precursors.
It is in the precursor- and sunlight-rich Los Angeles basin that O3
concentrations reach their highest levels. One-hour concentrations in
the range of 0.20-0.40ppm are not uncommon during the summer months.
The warm, sunny summer weather of the central coast of New South Wales
has meant that the Sydney Basin has been subject to severe photochemical
smog production, particularly before the advent of motor vehicle emission
controls. The National Health and Medical Research Council (NHMRC) has
set an ozone standard of 0.12ppm (1-hour average), which should not
be exceeded on more than one day per year. In the mid- 1970's, the atmosphere
in Sydney reached levels of ozone above this standard on an average
of more than 20 days per year, reaching maximum levels of 0.3ppm. Increased
legislative controls on the exhaust of NOx and H have resulted in a
decrease of 80% in the levels of hydrocarbons, and 45% in emissions
of nitrogen oxides. As a consequence, days where ozone levels are in
excess of the NHMRC standard decreased to about four within ten years,
and are expected to continue falling.
Ozone can be removed from the atmosphere in a number of ways. These
include reactions with surfaces including plants, soil, and a variety
of man made materials such as rubber. Most O3 produced in the atmosphere
is removed by chemical processes, typically involving NOx. One of the
principal scavengers of O3 is NO. Nighttime reactions with NO2 destroy
refers to halogenated hydrocarbon compounds which have been (and still
are) used as refrigerant gases and propellants in aerosol cans.
Many halogenated hydrocarbons released into the atmosphere as a result
of human activities. These include volatile or semi-volatile man made
substances that contain one or more halogens such as chlorine, fluorine,
or bromine, in addition to hydrogen and carbon. Halogenated hydrocarbons
are unique because of their environmental persistence. They include:
the semi-volatile chlorinated hydrocarbons such as DDT, Chlordane, Dieldrin,
and Aldrin (widely used as pesticides)
polychlorinated biphenyls (PCB’s) used as solvents and transformer
polybrominated biphenyls used as fire retardants
dichloromethane, trichloroethene, perchloroethene, tetrachloroethene,
and tetrachloromethane used as solvents, and
chlorofluorocarbons (CFC’s) used as refrigerants, degreasing agents,
foaming agents, and aerosol propellants.
Chlorofluorocarbons pose a serious atmospheric threat because of their
great stability that leads to damage the O3 layer, and their ability
to absorb infrared energy and behave as greenhouse gases. Chlorofluorocarbons
represent a variety of chemical species. The three most commonly used
(making them the most common atmospheric contaminants) are trichlorofluoromethane
(CFC13), dichlorodifluoromethane (CF2C12), and trichlorotrifluoroethane
(C2C13F3). These are generally described as CFC-11, CFC-12, and CFC-113.
Because there is no sink in the lower atmosphere, CFC concentrations
in the atmosphere increase with time. For CFC-11 and CFC-12, atmospheric
lifetimes are 75 and 111 years, respectively.
smelters are a major source of both gaseous and particulate fluorides,
as are brick and glass works, some smelters, steel plants and coal fired
power stations. Fluoride is very much a localised problem, of little
significance outside the Hunter region in Australia.
Minor Gaseous Pollutants
These include hydrogen sulfide, odours and noise. They emanate from
many different sources. A major source of H2S is swamps. It is extremely
dangerous if inhaled in large quantities, but this is unlikely unless
you live in a swamp or sewage treatment works.
Though unpleasant odours may cause symptoms in some individuals, the
problem of odour or odour air pollution is usually viewed as being one
of annoyance. Odour, from a regulatory point of view, is seen as a welfare
not a health issue.
Scientists define odour as when a substance is detected by the human
olfactory system. The olfactory function in humans consists of two different
organs in the nose. The olfactory epithelium (found in the highest part
of the nose) consists of millions of yellow-pigmented bipolar receptor
cells that connect directly to the olfactory bulbs of the brain. The
free endings of trigeminal nerves distributed throughout the nasal cavity
serve as another olfactory organ. Trigeminal nerves respond to odouriferous
substances that cause irritation, tickling, or burning. The chemical
senses that correspond to these two olfactory organs are not easily
separated, with many odouriferous substances or odourants stimulating
both systems. They stimulate different parts of the brain and, therefore,
may have different effects. The major function of the trigeminal nerve
system is to stimulate reflex actions such as sneezing or the interruption
of breathing when the body is being exposed to potentially harmful odour-producing
perception of odour is a physio-psychological response to the inhalation
of an odouriferous chemical substance. Odours cannot, therefore, be
measured by chemical means. However, some of the sensory attributes
of odours can be measured by exposing individuals under controlled conditions.
Elements of odour subject to measurement are:
character (quality), and
hedonic tone (pleasantness, unpleasantness)1.
The limit of detection is called the odour threshold. It may be characterized
in one of two ways. The threshold may be that concentration of an odouriferous
substance where there is a detectable difference from the background.
Alternatively, the threshold may be defined as the first concentration
at which an observer can positively identify the quality of the odour.
The former would be best described as a detection threshold, the latter
as a recognition threshold.
Though the olfactory sense in humans is not as acute as it is in many
animals, it still has the ability to detect many substances at very
low concentrations. Odour thresholds for a variety of chemicals are
given in Table 2.2. Note that humans can detect H2S at approximately
Odours differ in their character or quality. This odour parameter allows
us to distinguish odours of different substances by prior odour associations.
The characters of a variety of selected chemicals are summarised in
Table 2.2. For example, dimethylamine is described as fishy, phenol
as medicinal, 1,4-dihydroxybenzene (paracresol) as tar-like.
2.2 – Odour thresholds and characteristics of selected chemical
to the character/quality aspects of an odour is hedonic tone or the
degree of pleasantness or unpleasantness. The character of an odour
may, in many cases, be the primary determinant of whether it is pleasant
or unpleasant. The olfactory sense is highly subjective, with some odours
being pleasant to some and unpleasant to others. The difficulty of defining
some odour problems lies in part with the lack of objectivity.
The olfactory response to an odourant decreases as the odourant concentration
decreases. This decrease is nonlinear. For a substance such as pentyl
butanoate, the perceived odour intensity decreases by 50% for a tenfold
reduction in concentration. This relationship is common for many other
The perceived intensity of an odour rapidly decreases after initial
exposure. Generally within a few minutes, the odour may not be perceived
at all. This phenomenon is called adaptation. Within minutes after the
exposure ceases, sensitivity to the odour is restored. A person may
also become habituated to an unpleasant odour – this generally
occurs over a long time.
Unpleasant odours may affect our sense of wellbeing. Responses to a
variety of malodours can include nausea, vomiting, headaches, coughing,
sneezing, induction of shallow breathing, disturbed sleep, appetite
disturbance, sensory irritation, annoyance, and depression. Effects
may be physiological, psychological, or both1.
odours emanating from a nearby source result in more complaints to regulatory
agencies than any other form of air pollution. Likely sources of bad
odours are soap-making facilities, petrochemical plants, refineries,
pulp and paper mills, fish-processing plants, diesel exhaust, sewage
treatment plants, and agricultural operations including feedlots, poultry
houses, and abattoirs. Bad odours associated with such sources include
a variety of amines, sulfur gases (such as H2S, methyl and ethyl mercaptan,
and carbon disulfide), phenol, ammonia, alkanals, alkanoic acids, etc.