Diploma of Environmental Monitoring & Technology
Study module 2
Types and sources of pollutants
MSS025009A
Air pollution monitoring
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Study module 2 – Types & sources of pollutants
APM
TYPES OF AIR POLLUTANTS
2
Primary and Secondary Air Pollutants
Particulate Pollutants
Particle size & behaviour
2
2
3
GASEOUS POLLUTANTS
5
Carbon Oxides (CO and CO2)
Sulfur Compounds
Nitrogen Compounds
Hydrocarbons
5
7
8
11
OZONE AND PHOTOCHEMICAL SMOG
14
Photochemical Smog
Minor Gaseous Pollutants
15
20
ASSESSMENT & SUBMISSION
21
Knowledge questions
Submission
Problems
Resources & references
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Types of Air Pollutants
Essentially there are four types of air pollutants;
◗ particulate pollutants
◗ gaseous pollutants
◗ odour
◗ noise
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 atmosphere.
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 sometimes referred
to as primary pollutants. These 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.
Particulate 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 manmade (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
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Liquid particles formed by condensation of vapours or chemical reaction. For example,
SO3 + H2O ⇌ H2SO4
Typically they are 0.5 - 3.0um in diameter.
Figure 2.1 – the size ranges of common atmospheric particles
Smoke
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.
Particle size & behaviour
Atmospheric particles range in size from 0.005 - 500µm. 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 (EAD) – which
relates the particle to the behaviour of an equivalent spherical particle.
Particles less than 1µm in diameter behave much like gases (remain suspended, may
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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 (10µm 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.
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 lifetime.
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
always positively
charged. These electrical charges have a substantial effect on the coagulation and rates of
deposition of the particles.
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 10µm in
size, whilst the smaller particles occur in size groups centred around 0.2 and 0.02µm. Only
particles of <10µm 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 <2.5µm 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 PM 2.5 sampling. Both PM10 and
PM2.5 are discussed in the following chapter in more detail.
Organic Particulates
Organic particulate matter occurs in a vast array of compounds. When collected, they can
be fractionated into various chemical groups. Of these, the polycyclic aromatic
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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 µg/m3.
Gaseous Pollutants
These 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/m 3) or parts per
million (ppm).
1𝑝𝑝𝑚 =
1 𝑣𝑜𝑙𝑢𝑚𝑒 𝑜𝑓 𝑔𝑎𝑠𝑒𝑜𝑢𝑠 𝑝𝑜𝑙𝑙𝑢𝑡𝑎𝑛𝑡
106 𝑣𝑜𝑙𝑢𝑚𝑒𝑠 𝑜𝑓 (𝑝𝑜𝑙𝑙𝑢𝑡𝑎𝑛𝑡 + 𝑎𝑖𝑟)
At 25°C and 101.3 kPa the relationship between ppm and µg/m3 is;
𝜇𝑔
𝑝𝑝𝑚 × 𝑚𝑜𝑙𝑒𝑐𝑢𝑙𝑎𝑟 𝑤𝑒𝑖𝑔ℎ𝑡 × 1000
=
3
𝑚
24.5
Carbon Oxides (CO and CO2)
Significant quantities of the carbon oxides, carbon monoxide (CO), and carbon dioxide (CO 2),
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
its significant potential for causing global climatic change makes direct and indirect
emissions to the atmosphere a serious pollution problem.
Carbon Monoxide
Carbon 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.
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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.
Because 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 50100ppm.
CO is removed from the air mostly by conversion to CO2. This may occur through aerial
oxidation or through the action of soil microorganisms.
Carbon Dioxide
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 380ppm.
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 CO 2,
absorbing 50% of all man-made emissions. The world's forests, particularly tropical forests,
also serve as a sink.
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
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planet’s global heat balance – leading to global warming – the so-called Greenhouse Effect.
Sulfur Compounds
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.
Sulfur Oxides
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 America.
Sulfur Dioxide
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, irritating odour.
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 SO 2 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. Since the implementation of
significant SO2 control measures in the early 1970s, many urban areas in the United States
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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 (NH 3) 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.
Hydrogen Sulfide
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 paints.
Background levels of HS 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 worldwide. 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.
Nitrogen Compounds
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
Nitrous oxide is a colourless, slightly sweet, nontoxic gas. It is a natural constituent of the
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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 NO.
Nitric Oxide
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 tonnes per year.
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
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 of NO.
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
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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.
Figure 2.2 – Levels of NO, NO2, and ozone on a typical smoggy day in a large city
Atmospheric levels of NO and NO2 also show seasonal trends because emissions of NO are
typically 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).
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
In this reaction, M is an energy-absorbing species (generally O2 or N2). NO2 is also converted
to nitric acid by night-time chemical reactions involving O3.
NO2 + O3 ⇌ NO3• + O2
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NO2+ NO3• ⇌ N2O5
N2O5 + H2O ⇌ 2HNO3
NO3• is the nitrate free radical. It is the key factor in night-time chemistry. The reaction
product of NO2 and NO3 is di-nitrogen 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.
Ammonia
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.
Organic 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.
Hydrocarbons
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.
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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 nonmethane 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.
Some oxygenated hydrocarbons are emitted into the atmosphere directly. For example
methanal (old name formaldehyde), acrolein and some other simple alkanals are major byproducts 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[]pyrene and other polyaromatic hydrocarbons (PAH’s).
Figure 2.3 Benzo [α] 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 10 7 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 10 7 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 emissions.
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
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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 (e.g., benzo--pyrene).
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
sources.
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 nonmethane 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
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component of photochemical aerosol. These aerosols are removed from the atmosphere by
both dry and wet deposition processes.
Methane
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 standard.
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 effect.
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 2.4. 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 atmosphere
Figure 2.4 – Historical global levels of methane in the atmosphere
Ozone and Photochemical Smog
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Ozone 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 of NO2.
NO2 + h⇌ 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 NO 2 or other
alternate chemical reactants are available.
Photochemical Smog
This 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 conditions.
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.
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Figure 2.5 – Normal dispersion pattern of pollutants from a large urban area2
In 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.
Figure 2.6 – Conditions which favour smog formation in a large urban area2
When 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 dioxide.
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
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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 + h ⇌ NO + O*
O* + O2 + M ⇌ O3 + M
Net reaction: RO2 + O2 + h ⇌ RO + O3
The 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 O3 levels.
Figure 2.7 – Important reactions in the formation of photochemical smog1
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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.
1. Primary photochemical reaction producing oxygen atoms:
NO2 + h ⇌ NO + O*
2. Reactions involving oxygen species (M is an energy-absorbing third body):
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,
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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.
Figure 2.8 – Variations in smog pollutants with time of day
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 2.8.
As the morning rush hour begins, NO begins to rise rapidly, followed by NO 2. 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
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nighttime hours to the ground, where they mix with the pollutants held in by the inversion
layer, and begin the cycle all over again.
Ozone Concentrations
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.
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. Night-time reactions with NO2 destroy O3.
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.
Aluminium 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.
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Assessment & submission
Knowledge questions
This section provides formative assessment of the theory. Answer all questions by typing
the answer in the boxes provided. Speak to your teacher if you are having technical
problems with this document.
◗ Type brief answers to each of the questions posed below.
◗ All answers should come from the theory found in this document only unless the
question specifies other.
◗ Marks shown next to the question should act as a guide as to the relative length or
complexity of your answer.
1. Carbon monoxide is an important air pollutant. Describe the factors that influence its
seasonal and diurnal variation in urban and non-urban areas. 4mk
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2. Explain how disruption to the carbon cycle is leading to the increase in average global
temperatures during the latter half of the twentieth century. 4mk
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3. For any four major pollutant gases give the main source, typical ambient levels and likely
sinks. 16mk
Pollutant gas
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levels
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Type answer.
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4. Explain why hydrocarbons are normally assess without the contribution of methane.
2mk
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5. Describe the typical atmospheric conditions that lead to the formation of photochemical
smog. 3mk
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6. Describe the important elements that contribute to the formation of photochemical
smog. 7 mk
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Submission
Answers
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◗ Write answers in the text-fields provided
Submission
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Resources & references
Unless specified otherwise, the references and resources listed below are for interest only.
You are not expected to purchase, download or otherwise read these resources unless the
question you are answering specifically requests that you do so. Most of the resources here
are either available from your campus library, or directly from the teacher.
References
Australian-Standards. (Various dates). AS/NZS 3580 Methods for sampling and analysis of
ambient air (entire series). Canberra: Standards Australia.
Bates, G. (2010). Environmental Law in Australia. Australia: LexisNexis-Butterworths.
Baukal Jr, C. (2004). Industrial Combustion Pollution and Control. New York, USA: Marcel
Dekker.
Burden, F. E. (2002). Environmental Monitoring Handbook. McGraw-Hill Professional.
Colls, J. (2002). Air Pollution. England: Talyor & Francis.
Manahan, S. (2000). Environmental Chemistry. Boca Raton: Lewis Publishers.
Manly, B. (2009). Statistics for environmental science and management. Boca Raton: Taylor
& Francis Group.
Schuenemeyer, J. E. (2011). Statistics for Earth and Environmental Scientists. New Jersey:
John Wiley & Sons.
Seinfeld, J. P. (2006). Atmospheric Chemistry and Physics: From Air Pollution to Climate
Change, 2nd Ed. Hoboken, USA: John Wiley & Sons.
Vallero, D. (2008). Fundamentals of Air Pollution, 4th Ed. Burlington, USA: Academic Press.
vanLoon, G. W. (2011). Environmental Chemistry: a global perspective. New York: Oxford
University Press.
Workplace Health and Safety Act 2011. (n.d.).
Workplace Health and Safety Regulation 2011. (n.d.).
Resources
1. http://www.environment.gov.au/resource/national-standards-criteria-air-pollutants-1-australia
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