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Chemical, Forensic, Food & Environmental Technology


Air Pollution Monitoring & Control


Chapter 2 - Sources, types & distribution of air pollutants


Major 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.

 

Table 2.1 – Estimated emission of 5 primary pollutants in the USA 1987

Transportation-Combustion Sources

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 sources include
furnaces - and their combustion of carbonaceous fuels
boilers
ovens and dryers
process systems which produce volatile chemicals, gases, etc.
Solvent Evaporation
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 photochemical smog).
Stack emissions
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.

Local topography
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.

Prevailing climate
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.
Plume behaviour
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.

Figure 2.1 - A Fanning Plume

In 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.

Figure 2. 2 – A Looping Plume2

With 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.

Figure 2. 3 – A Coning Plume2

The 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.

Figure 2. 4 – A Fanning Plume2

Where 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.

Figure 2. 5 – A Lofting Plume2

A 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,
local topography,
temperature,
emission rates,
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
gaseous pollutants,
odour and
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 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.
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 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.

Figure 2.6 – the size ranges of common atmospheric particles1

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

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 (0.002).
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 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 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 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.

Organic Particulates

Source

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 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.
Lead
Source
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.

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/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
24.5
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.

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.

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.

Typical Concentrations
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.

Figure 2.7 – CO levels in a typical urban area over a 24hr period1

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 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.

Sinks
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
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 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 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 SO2 emission are the result of coal-fired power stations.

Typical Concentrations
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).

Sinks
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.

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.

Typical Concentrations
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.

Sinks
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 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 tones 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

NOx Concentrations
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).

Figure 2.8 – Levels of NO, NO2, and ozone on a typical smoggy day in a large city – Taken from Los Angeles1

Sinks
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 O3.

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.
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. 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

Source

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 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 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-a-pyrene).
Hydrocarbon Concentrations
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 non-methane hydrocarbon concentrations in urban areas range from 1 - 10ppm.
Sinks
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.
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 2.9. 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 atmosphere

Figure 2.9 – Historical global levels of methane in the atmosphere1

Ozone and Photochemical Smog

Sources

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 + 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.

Photochemical Smog

Source

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.

Figure 2.10 – 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 2.11 – 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 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.

Figure 2.12 – Important reactions in the formation of photochemical smog1

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.

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 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, 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.

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.
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. 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 Sinks
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 O3.

Chlorofluorocarbons (CFC’s)

Source

This 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 insulators
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.

Fluoride

Source

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.
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.
Odour Pollution
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 substances1.

Odour Measurement

The 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:
detectability
intensity
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 0.5ppb.

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.

Table 2.2 – Odour thresholds and characteristics of selected chemical compounds1

Related 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 odourants.

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.

Odour Problems

Bad 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.

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