Chapter 2
Sources, Types & Distribution of
Air Pollution
Major Sources of Air Pollution
• The number of different types of pollution
sources in modern society is almost endless.
• We look at only the most significant sources of
air pollutants.
• Mobile (50 - 70%), and stationary sources.
• 15-25% from heavy industrial stationary sources
and as much as 25% from other stationary
sources.
Major Sources of Air Pollution
Emissions (106 tonnes / year)
Source
CO
Particles
H/C’s
NOx
SOx
Total
%
Transport
40.7
1.4
6.0
8.4
0.9
57.4
44.9
Stationary fuel
combustion
7.2
1.8
2.3
10.3
16.4
38.0
29.7
Industrial
Processes
4.7
2.5
8.3
0.6
3.1
19.2
15.0
Solid waste
disposal
1.7
0.3
0.6
0.1
0
2.6
2.0
Miscellaneous
7.1
1.0
2.4
0.1
0
10.6
8.3
Total
61.4
7.0
19.6
19.5
20.4
127.8
%
48.0
5.5
15.3
15.3
15.9
100
Major Sources of Air Pollution
• The table is from USA. The total amount
emitted in Australia will be far less
• A significant reduction in the amounts of
CO and H/C’s when compared to the
previous decade
• levels of other pollutants has been steady
or shown only a slight increase
Transportation Combustion
Sources
• The most important transportation sources
at present are major polluters
• Motor vehicles CO, CO2 H/C’s, NOx and
small amounts of SOx
• Motor vehicle exhaust accounts for 40% of
all H/C air pollutants and 90% of all NO2
Transportation Combustion
Sources
• Motor vehicles pollutants react to form more
reactive (and dangerous) pollutants such as
photochemical smog.
• diesel fuel = a source of very dangerous H/C’s
(PAH’s).
• Pb has decreased in significance, and according
to the latest national SoE report, is no longer
considered a problem
• vehicles running on unleaded fuels emit lower
levels of NOx and SOx
Transportation Combustion
Sources
• Aircraft and trains are less significant sources of
pollution compared with road transport vehicles.
• Aircraft run on kerosene, burnt efficiently, but
they fly very high in the atmosphere the
pollutants – most of which are H/C’s – are
spread and diluted in the upper atmosphere.
DISCUSS NEW PROBLEM
• Trains mostly run on electricity = contribute very
little to air pollution (except CO2 & some ozone)
Stationary Combustion Sources
• 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.
Stationary Combustion Sources
• Solvent evaporation (fugitive) from;
– solvent-based materials
– leaking pipe joints
– maintenance work
– spills, unloading /loading procedures
• an important part of photochemical
pollution
Stack Emissions
• Emission of waste gases, fumes, vapours
and smokes to the atmosphere are usually
by the use of a smoke stack or chimney.
• 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.
Stack Emissions
How dilution happens depends on many factors
• Nature of the waste emission
– Toxic emissions need to be very dilute
• Volume of the waste
– Is emission constant or only at certain times in the process.
• Local topography
– Many cities located in areas surrounded by hills or mountains.
– low wind and cooler temperatures = photochemical smog.
Stack Emissions
• Prevailing climate
– direction of prevailing winds
– e.g. Queenstown, Tasmania
• The Existing Atmosphere
– In very polluted cities, more stack emissions
not desirable.
– e.g. build power stations in the country away
from NOx from cars
Plume Behaviour
• Effects of plumes are considered local
within 500 metres of the stack, and
regional beyond this.
• Mixing or dispersion of the waste gases
and products into the atmosphere = plume
behaviour.
Types of Plumes
•
•
•
•
•
Fanning plumes
Looping plumes
Coning plumes
Fumigating
Lofting
The Fanning Plume
• Fanning Plumes
– Require stable air and slow vertical movement of the
emission
– common after calm clear nights
– temperature inversion limits the rise of the plume into
the upper atmosphere
The Fanning Plume
– creates a higher conc. of polluted air at lower levels
– exists for several hours
– Commonly seen from Eraring Power station
Looping plumes
• Looping plumes
– Require windy conditions which cause the plume can
swirl up and down
– common in the afternoon.
– Moderate and strong winds are formed on sunny days
creating unstable conditions
– Exists for several hours.
Coning plumes
• Coning plumes
– Require moderate winds and overcast days
– wider than it is deep, and is elliptical in shape
– exists for several hours.
Fumigating plume
• Fumigating plume
– Is short-lived (fraction of an hour), but reaches the earth's
surface.
– occur when the conditions move from stable to unstable
– A fanning plume develops overnight under stable
conditions but as the day heats up, unstable air is
produced
Fumigating Plume
• Fumigating plume (cont)
– unstable air causes the plume to move up and down can cause localised pollution.
– become looping or coning plumes as the air conditions
stabilise.
Lofting plume
• Lofting plume
– When plume is above the inversion layer (or there is
no inversion), it becomes a lofting plume.
– Normal wind direction and speed will disperse the
plume into the atmosphere without effect from ground
warming or cooling.
Stack emissions
• factors used to establish the amount of stack
emission allowed, and its conc. to the
atmosphere include:
–
–
–
–
–
–
–
smoke stack (chimney) height,
local topography,
temperature,
emission rates,
chemical reactivity, and
existing air pollution problems
wind allow rapid dispersal of pollutants.
Fugitive Emissions
• Fugitive emissions are emissions which
escape from a process rather than being
discharged
• They often have serious consequences
because their levels are not monitored and
they are untreated when entering the
atmosphere
Fugitive Emissions
• There are many sources of fugitive emissions
including:
– industrial sources (particulate fluorides from
aluminium smelters)
– small business (e.g. dry cleaning solvents)
– agriculture (e.g. dust from ploughing)
– natural sources (e.g. volcanoes, forest fires)
Fugitive emissions
• Often the result of poor maintenance of
plant and equipment
• Can be eliminated by SOP’s that involve
timed maintenance and quality control
checks
• Some are almost impossible to control
(e.g. natural sources)
Types of air pollutants
• There are four types of air pollutants;
– particulate pollutants and
– gaseous pollutants,
– odour and
– noise.
Primary vs Secondary pollutants
• Not all of the pollutants found in the
atmosphere are the direct result of
emissions.
• Many pollutants arise from chemical
reactions in the atmosphere with other
substances or light (photochemical
reactions).
1° vs 2° pollutants
• Pollutant substances that are directly
emitted into the atmosphere = primary
pollutants.
• Substances not directly emitted into the
atmosphere, formed by chemical reactions
in the atmosphere = secondary pollutants.
Particulate Pollutants
• Very small solid or liquid particles
• Individual particles may vary in size,
geometry, chemical composition and
physical properties
• May be of natural origin (pollen or sea
spray) or man made (dust, fume and soot)
Particulate Pollutants
• Provide a reactive surface for gases and
vapours in the formation of secondary
pollutants
• Particles also diffuse light reducing
visibility
• Come from stack emissions, dusty
processes, unsealed roads, construction
work and many other sources
Particulate Pollutants
• Dusts
– large solid particles
• Fume
– solid particles (metallic oxides) formed by
condensation of vapours from a chemical
reaction process or physical separation
process
• Mist
Particulate Pollutants
– liquid particles formed by condensation of vapours or
chemical reaction.
• Smoke
SO3 + H2O
H2SO4
– solid particles formed as a result of incomplete
combustion of carbonaceous materials.
• Spray
– a liquid particle formed by the atomisation of a parent
liquid.
Particle Size
• Particles range in size from 0.005 500m.
• Smallest of these are clusters of
molecules whilst the largest are easily
visible with the naked eye.
• 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.
Particle Size
• Particles less than 1m in diameter behave like
gases (remain suspended, may coalesce, move
in fluid streams),
• Larger particles act like solids (affected by
gravity, don’t stay suspended long, don’t
coalesce).
• Smaller particles generally derive from chemical
reactions, whereas the larger particles (10m or
greater) are usually generated mechanically and
tend to be basic.
Particle Size
• Smaller particles most dangerous to health,
• In urban areas there is an approx. even
distribution between fine and coarse particles,
this is weather dependent.
• Calm conditions more fine particles than coarse,
• Fine particulate matter spread over much
greater distances
Particle behaviour in the atmosphere
• Particles can undergo many physical and
chemical changes;
– grow in size,
– absorb or desorb gases from their surfaces,
– change electrical charge,
Particle behaviour in the atmosphere
• Particles can undergo many physical and
chemical changes;
– collide or adhere with other particles,
– absorb water.
– changes the particle size and affect its
atmospheric lifetime.
Total Suspended Particles
(TSP)
• Most particles concentrated into three
main size groups
• Larger particles around 10m in size
• Smaller particles in size groups centred
around 0.2 and 0.02m.
TSP
• Only particles of <10m penetrate into the
human lung
• Analyse air for only this fraction to
estimate its potential danger to human
health = PM10 sampling.
• Particles <2.5m in size can penetrate
deep into the lung tissue and are
especially dangerous = PM2.5 sampling
Organic Particulates
• PAH most significant
• Found on soot and dust particles, and are
formed from smaller H/C’s at high
temperatures (coal furnace effluent may
contain 1mg/m3 of PAH cigarette smoke
0.1mg/m3)
• Urban atmospheres PAH levels ~20 ug/m3
but is highly variable
Lead Particulates
• Was the most serious atmospheric heavymetal pollutant, but is no longer
• primary source was exhaust from vehicles
Gaseous Pollutants
• CO, H/C’s, H2S, NOx, O3 and other
oxidants, and SOx
• 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)
Gaseous Particulates
• 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 Monoxide
• a colourless, odourless and tasteless gas.
• atmosphere has an avg. burden of around 530
million tonnes (about 0.00001%),
• avg. residence time of 36 to 100 days.
• Much of the CO in the atmosphere occurs
naturally from volcanic eruptions, photolysis of
methane and terpenes, decomposition of
chlorophyll, forest fires and microbial action in
oceans.
Carbon Monoxide
• Anthropogenic sources = transportation, solid
waste disposal, agricultural burning, steel
production, etc.
• emitted directly into the atmosphere through the
inefficient combustion of fossil fuels.
• removed by reactions in the atmosphere which
change it to CO2 and by absorption by plants
and soil micro-organisms.
Carbon Monoxide
• It is removed by reactions in the
atmosphere which change it to CO2 and
by absorption by plants and soil microorganisms.
• In combustion, carbon is oxidised to CO2
in a two step process.
2C + O2
2CO
2CO + O2
2CO2
Carbon Monoxide
• Typical conc's
– Background levels of CO tend to vary greatly
depending on location.
– avg. global levels = 0.2ppm.
– Peak conc's during autumn months when large
volumes are generated by the decomposition of
chlorophyll in leaves.
– In urban areas = diurnal conc. pattern
Carbon Monoxide
• The internal combustion engine contributes
much of the anthropogenic CO (up to 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.
Carbon Dioxide
• Since the Clean Air Act in NSW in 1972
(and subsequent acts), the levels of CO in
Sydney have dropped from an avg. of
25ppm to around 10ppm
• The accepted standard is 9ppm over an
eight-hour period
http://www.environment.nsw.gov.au/air/24hr.htm
Carbon Monoxide
• Sinks
– CO is removed from the air mostly by
conversion to CO2
– This may occur through aerial oxidation or
through the action of soil micro-organisms
– The reason for very high conc's 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
– biologically decomposed
• It is removed from the atmosphere by plants in
photosynthesis and released by biological
reactions
Carbon Dioxide
• 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 has resulted in
significant CO2 emissions and an increase of
atmospheric conc's
• Since 1958, CO2 values measured at Mauna
Loa Observatory in Hawaii have increased from
310 to more than 350ppm.
Carbon Dioxide
• Significant seasonal variations are also
observed to occur in CO2 levels
• 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.
Carbon Dioxide
• 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.
Carbon Dioxide
• As a thermal absorber (read greenhouse
gas), CO2 prevents some IR emissions
from the Earth being radiated back to
space
• 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)
• 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 Oxides
• 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.
Sulfur Oxides
• 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
• 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 conc. range of 0.38 - 1.15ppm
• Above 3 ppm, it has a pungent, irritating
odour
Sulfur Dioxide
• 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 (and oil) is by far the
greatest contributor, even in Australia
Sulfur Dioxide
• Background levels of SO2 are very low,
about 1ppb
• In urban areas maximum hourly conc's
vary from less than 0.1 to more than
0.5ppm.
Sulfur Dioxide
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.
Sulfur Dioxide
• A major sink process for SO2 is its gasphase oxidation to H2SO4 and subsequent
aerosol formation by nucleation or
condensation
• Sulfuric acid will react with ammonia (NH3)
to form a variety of salts
Sulfur Dioxide
• 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.
• The principal concerns associated with
H2S are its smell (foul) & toxicity (same as
HCN)
Hydrogen Sulfide
• Background levels of H2S are approx.
0.05ppb
• Natural sources, which include anaerobic
decomposition of organic matter, natural
hot springs and volcanoes
• Anthropogenic sources include oil and gas
extraction, petroleum refining, paper mills,
rayon manufacture, and coke ovens
Hydrogen Sulfide
• 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).
Nitrogen Compounds
• N2 the major gas in the atmosphere.
• N2O present in unpolluted air due to
microbial action
• NO and NO2 significant air pollutants
• NH3 not considered a significant man
made pollutant, but enormous quantities
generated through natural emissions.
Elemental Nitrogen (N2)
• ~78% of the air we breathe
• Relatively inert (unlike O2)
• Significant biological use by microbes
Nitrous Oxide (N2O)
• colourless, slightly sweet, non-toxic gas.
• natural part of the atmosphere avg. conc.
0.30ppm.
• used as anaesthetic in medicine and dentistry
(laughing gas)
• product of natural soil processes, produced by
anaerobic bacteria.
• photolytically dissociates in stratosphere to NO.
Nitric Oxide (NO)
• colourless, odourless, tasteless, relatively
non-toxic gas.
• produced naturally by;
– anaerobic biological processes in soil and
water,
– combustion processes and by photochemical
destruction of N compounds in stratosphere.
Nitric Oxide (NO)
• Major anthropogenic sources include;
– automobile exhaust
– fossil fuel-fired electric generating stations
– industrial boilers
– incinerators
– home space heaters
Nitric Oxide
• Nitric oxide is a product of hightemperature combustion.
N 2 + O2
2NO
Nitrogen Dioxide (NO2)
• light yellow to orange colour at low conc’s
and brown at high conc’s.
• pungent, irritating odour , and extremely
corrosive especially in wet environments
• toxic - can cause anoxia
Nitrogen Dioxide (NO2)
• Some of the NO2 in air produced by direct
oxidation of NO
2NO + O2
2NO2
Nitrogen Dioxide (NO2)
• At low atmospheric NO levels, oxidation is
slow, accounts for <25% of NO conversion
• Photochemical reactions involving O3,
peroxy radical (RO2) and reactive
hydrogen species such as OH, HO2,
H2O2, are primary means by which NO is
converted to NO2 in the atmosphere.
Nitrogen Dioxide (NO2)
• Other NO2 formation mechanisms
NO + O3
NO2 + O2
RO2 + NO
NO2 + RO
HO2 + NO
NO2 + OH
Nitrogen Dioxide
• Background conc’s of NO and NO2 are
approx. 0.5 and 1ppb respectively
• In urban areas, 1 hour avg. conc’s of NO
may reach 1-2ppm, with max NO2 levels of
approx. 0.5ppm.
• decay of NO rapid as polluted air moves
from urban to rural areas, with conc’s
dropping to near background levels.
Nitrogen Dioxide
• Atmospheric NO level related to
transport/work cycle.
• Peak conc's observed in early morning
hours, with a second smaller peak late in
the day (See Figure 2.8).
• Peak morning NO conc's followed several
hours later by peak levels of NO2
produced by the chemical and
photochemical oxidation of NO.
Nitrogen Dioxide
• Atmospheric levels of NO and NO2 also
show seasonal trends
• Emissions of NO greater during winter
when there is 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 Dioxide
• NOx in vehicle exhausts controlled by legislation
as with CO
• catalytic converter in the exhaust system
increases reduction of NOx to N2.
• Australian Design Rules limit emission of NOx
from exhausts to 1.9g/km
• to maintain the levels in Sydney below the
recommended standard of 0.16ppm (1 hour
avg.).
Figure 2.8 – Levels of NO, NO2, and ozone on a smoggy day in Los Angeles
Nitrogen oxides (NOx)
Sinks
• most significant sink for NO is conversion
by both direct oxidation and photochemical
processes to NO2
• A major sink process for NO2 is its
conversion to nitric acid
Nitrogen Oxides (NOx)
OH + NO2 + M
HNO3 + M
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
NO2+ NO3
N 2O 5 + H 2O
NO3 + O2
N 2O 5
2HNO3
Nitrogen Oxides (NOx)
• NO3 is nitrate free radical
• key factor in night-time chemistry
• reaction product of NO2 and NO3 is
dinitrogen pentoxide (N2O5) - reacts with
water rapidly to produce HNO3
Nitrogen Oxides (NOx)
• Some of the HNO3 in the atmosphere
reacts 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 (NH3)
•
•
•
•
relatively unimportant man made pollutant
Most comes from biological decomposition
Background conc's vary from 1 to 20ppb
The avg. atmospheric residence time is
approx. 7 days
Organic Nitrates
• produced in the atmosphere by reaction of
NOx and hydrocarbons
• Examples are peroxyacyl nitrates (PAN’s)
and peroxybutylnitrates (PBN’s).
• discussed in detail in photochemical smog
section
Hydrocarbons
• organic materials in the atmosphere.
• In the atmosphere simple hydrocarbons react
with substances containing
– oxygen,
– nitrogen,
– sulfur,
– chlorine
– bromine
– even some metals (Pb)
Hydrocarbons
• Atmospheric hydrocarbons exist in gas,
liquid and solid phases
• gases and volatile liquids the most
significant pollutants
• Solid hydrocarbons generally of higher
MW and exist as condensed particles in
atmospheric aerosols
Hydrocarbons
• Methane (CH4) most common
hydrocarbon in the atmosphere - formed
from many natural sources;
– termites,
– cows
– decomposition of organic matter
• It and the other alkanes found in the
atmosphere are fairly un-reactive
Hydrocarbons
• atmospheric hydrocarbons of most
significance in terms of chemical reactivity
are the alkenes
• highly reactive alkene hydrocarbons are
formed naturally by plants (e.g. terpenes
from citrus plants and eucalyptus haze)
Hydrocarbons
• greatest source of non-methane
hydrocarbons are motor vehicles and
petroleum processing
• Alkenes are the major air pollutant
responsible for photochemical smog and
other gross oxidants in the atmosphere
Hydrocarbons
• Once in the atmosphere non-methane
H/C’s combine with O2 to form many
different oxygenated H/C’s including;
– alkanones
– alkanals
– alkanoic acids
– alkanols
– ethers
Hydrocarbons
• Aromatic H/C’s not very reactive, but can
react with other very reactive chemical
oxidants to form toxic substances, such
as;
– benzo[]pyrene
– poly-aromatic hydrocarbons (PAH’s)
Benzo[]pyrene
Hydrocarbons
• H/C’s emitted from a variety of natural and
man made sources
• important pollutants because of their role
in atmospheric photochemistry
• biological and geological processes
release hydrocarbon compounds naturally
Hydrocarbons
Sources include;
– plant and animal metabolism
– vaporisation of volatile oils from plant surfaces
– biological decomposition
– emission of volatiles from fossil fuel deposits
Hydrocarbons
Sinks
• 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.
• aerosols are removed from the atmosphere by
both dry and wet deposition processes.
Methane
• was initially considered an unimportant
H/C
• Measurements of total H/C subtracted the
conc. of CH4
• Hence ambient air quality standard for
H/C’s is a non-methane hydrocarbons
standard
Methane
• recognised as one of the trace gases that
may have significant greenhouse effect on
global climate
Methane
• by far the most abundant H/C in the
atmosphere, with a 1980 conc. of
1.65ppm.
• It has been increasing at a rate of 1.21.9% per year. The rate itself is also
increasing.
Methane
Ozone & Photochemical Smog
• O3 a normal component of the atmosphere
• mostly in the middle stratosphere where it
controls UV light reaching the planet’s
surface
• here depletion of the substance results in
air pollution – loss of ozone is causing
deterioration in quality of life
Ozone
• not listed as a major primary air pollutant
in the lower atmosphere
• high toxicity and involvement in production
of other pollutants - very important
atmospheric pollutant
• Over 90% of photochemical smog is
ozone
Ozone
Sources
• Electrical discharges, e.g. lightning and
electrical devices
• Light driven upper atmospheric chemical
reactions e.g. reaction of molecular
oxygen with oxygen atoms
Ozone
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
Ozone
• All reactions proceed rapidly with approx.
conc. of 20ppb
• atmospheric NO2/NO conc. ratios can be
equal to 1
• Hence conc's of ozone remain low unless
imbalances in the levels of NO2 or other
alternate chemical reactants are available
Oh dear! The chemistry!
• We need to look closely at the chemistry
we have seen thus far.
•
Photochemical Smog
• refers to an atmosphere laden with
secondary pollutants that form in the
presence of sunlight as a result of
chemical reactions in the atmosphere
• arises in urban areas, where there is a
heavy build-up of vehicle exhausts
• greatly exacerbated by weather conditions
Photochemical Smog
• normally 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 reduce smog
• At certain times of the year, when wind is very
still, primary pollutants build up over cities.
• Autumn worst for photochemical smog
Photochemical Smog
Photochemical Smog
• In autumn, days are sunny and warm, with cool
nights
• Under still conditions, a warm inversion layer
forms under a layer of higher cooler ai
• 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
Photochemical Smog
Photochemical Smog
• primary pollutants (NOx), and H/C’s
trapped in the lower atmosphere are
subjected to UV radiation from the sun –
photochemical smog forms.
Photochemical Smog
• products called gross photochemical oxidants,
defined by their ability to oxidise I- to I2.
• 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.
Photochemical Smog
• The key chemical reactants in the
formation of photochemical smog are NOx
and hydrocarbons.
• The reactions undergone by these
substances in the atmosphere are many
and varied.
• Many of the reaction mechanisms are not
well understood.
Photochemical Smog
• In the lower atmosphere O3 conc's 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 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.
Photochemical Smog
RO2
+ NO
NO2 + h
O* + O2 + M
Net:
RO2 + O2 + h
NO2 + RO
NO + O*
O3 + M
RO + O3
Photochemical Smog
Photochemical Smog
• The rate of O3 formation is closely related
to the conc. 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.
Photochemical Smog
• In polluted atmospheres, O3 conc's are directly
related to;
–
–
–
–
the intensity of sunlight,
NO2/NO ratios,
the hydrocarbon type and conc's,
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.
Photochemical Smog
• summary of reactions in smog formation
can be compressed into 4 stages.
• explains time variations in levels of H/Cs,
ozone, NO2 and NO (see Figure 2.13).
Photochemical Smog
• 1. Primary photochemical reaction producing oxygen
atoms:
NO2 + h
NO + O*
• 2. Reactions involving oxygen species (M is an energyabsorbing third body):
O* + O2 + M
O3 + M
NO + O3
NO2 + O2
Photochemical Smog
• Because last reaction is rapid, the conc. of
O3 remains low until that of NO falls to a
low value.
• Automotive emissions of NO tend to keep
O3 conc's low along freeways.
Photochemical Smog
• 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.)
Photochemical Smog
• 4. Chain propagation, branching, and
termination by a variety of reactions such
as the following:
NO + ROO
NO2 + R
NO2 + and/or other products
products (e.g. PAN)
Photochemical Smog
• Some of the many other reactions which are known
to occur in photochemical smog formation are listed
below.
•
•
•
•
•
•
O + hydrocarbons
HO + O2
HO3 + H
HO3 + NO
HO3 + O2
HOx + NO2
HO
HO3
alkanals, alkanones
HO2 + NO2
O3 + HO2
PAN's
Photochemical Smog
• all H/C’s may form smog, but there are
considerable differences in their reactivities
• methane, very slow to react, having an approx.
atmospheric lifetime of more than 10 days
• branched alkenes and aromatic compounds the
most reactive
• naturally-occurring alkenes (d-limonene) the most
reactive compounds
Photochemical Smog
• With complex reactions and changing
vehicle emissions during a day, conc's of
the major components vary considerably
over a 24-hour period.
• typical pattern of variations shown in
fig2.13.
Photochemical Smog
• morning rush hour begins, NO rises rapidly,
followed by NO2.
• NO2 reacts with sunlight giving ozone and
other oxidants
• H/C level increases in the morning, then
decreases as compounds are oxidised to
form PAN's and other species.
Photochemical Smog
• air mass moves toward an urban center, picks
up NO, and H/C’s.
• OH begins to degrade H/C’s, producing RO2
while O3 precursors peak and then decline with
increasing downwind distance.
• Ozone conc's increase and are sustained over a
period of 1-5 hours as more reactive alkene and
aromatic H/C’s are depleted by photochemical
reactions.
Photochemical Smog
• After 5-10 hours, moderately reactive
H/C’s play a more important role in O3
production
• O3 levels decrease due to dilution,
conversion of NO2 to HNO3, and surface
adsorption
• At night no O3 produced
Photochemical Smog
• Under inversion layer, O3 may persist for
80 hrs.
• allows O3 to be transported over long
distances
• At sunrise, inversion breaks up, bringing
O3 and other products to the ground,
where they mix with the pollutants held in
by the inversion layer, and begin cycle all
over again
Photochemical Smog
• In unpolluted atmospheres O3 conc's near
ground are 10-20ppb (0.01-0.02ppm)
during the warm months
• O3 conc's over landmasses with large
motor vehicle numbers often well above
this even at remote sites
• Los Angeles basin 1 hour conc's are 0.200.40ppm
Photochemical Smog
• warm, sunny NSW central coast means
Sydney Basin has high photochemical
smog production
• (NHMRC) ozone standard of 0.12ppm (1hr
avg.) should not be exceeded on more
than one day per year.
Photochemical Smog
• Ozone removed from the atmosphere by
reactions with plants, soil, and man made
materials (rubber)
• O3 produced in the atmosphere removed by
chemical processes involving NOx
• principal scavenger of O3 is NO – Night
reactions with NO2 destroy O3
Chlorofluorocarbons (CFC’s)
What are they?
• halogenated H/C compounds used as
refrigerant gases and propellants in
aerosol cans
Chlorofluorocarbons (CFC’s)
• unique because of their environmental
persistence
• examples
– DDT, Chlordane, Dieldrin, and Aldrin (pesticides)
– polyhalogenated biphenyls (PCB’s, PBB’s) solvents
and fire retardants
– dichloromethane, trichloroethene, perchloroethene,
tetrachloroethene, and tetrachloromethane (solvents)
– CFC’s - refrigerants, degreasing agents, foaming
agents, aerosol propellants
Chlorofluorocarbons (CFC’s)
• serious atmospheric threat because of their
great stability - leads to damage the O3 layer
• Also absorb IR energy and are greenhouse
gases
Chlorofluorocarbons (CFC’s)
• most commonly used (most common
atmospheric contaminants) are;
– Trichlorofluoromethane (CFC13)
– Dichlorodifluoromethane (CF2C12),
– Trichlorotrifluoroethane (C2C13F3).
Chlorofluorocarbons (CFC’s)
• no sink in the lower atmosphere - CFC
conc's increase with time
• For CFC-11 and CFC-12, atmospheric
lifetimes are 75 and 111 years,
respectively
Chlorofluorocarbons (CFC’s)
Naming CFC’s
• The decoding system for CFC-01234a is:
– 0 = Number of double bonds (omitted if zero)
– 1 = Carbon atoms -1 (omitted if zero)
– 2 = Hydrogen atoms +1
– 3 = Fluorine atoms
– 4 = Replaced by Bromine ("B" prefix added)
Fluoride
• Aluminium smelters 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 a localised problem
Minor Gaseous Pollutants
• Hydrogen sulfide
• odour
• noise
Odour as air pollution
• odour pollution increasing importance
• from a regulatory point of view, seen as a
welfare not a health issue – this is
changing
Odour as air pollution
• odour is response to the inhalation of a
chemical substance - cannot yet be
reliably measured by chemically
• sensory attributes of odours measured by
exposing individuals under controlled
conditions
Odour as air pollution
• Elements of odour subject to
measurement are:
– detectability
– intensity
– character (quality)
– hedonic tone (pleasantness, unpleasantness)
Odour as air pollution
• limit of detection = odour threshold
characterized in 2 ways;
– detectable difference from the background
– first conc. at which an observer can positively
identify quality of odour
Odour as air pollution
• characters of a variety of selected
chemicals summarised in Table 2.2
• For example, dimethylamine is described
as fishy, phenol as medicinal, 1,4dihydroxybenzene (paracresol) as tar-like.
Chemical
ethanal (acetalydehyde)
Odour Threshold (ppm)
Odour Character
0.21
Green, sweet
propanone (acetone)
100.0
Chemical sweet, pungent
dimethylamine,
0.047
Fishy
ammonia
46.8
Pungent
benzene
4.68
Solvent
butanoic acid
0.001
Sour
dimethylsuffide
0.001
Vegetable sulfide
ethanol
10.0
Sweet
ethyl mercaptan
0.001
Earthy, sulfide
formaldehyde
1.0
hydrogen sulfide
methanol
0.00047
Hay, straw-like, pungent
Egg like sulfide
100.0
Sweet
10.0
Sweet
1,4-dihydroxybenzene
(paracresol)
0.001
Tar-like, pungent
perchloroethene
4.68
Chlorinated solvent
phenol
0.047
Medicinal
sulfur dioxide
0.47
Pungent Sulfur
toluene
2.14
Moth balls, rubbery
2-butanone
or MEK)
(methylethylketone
Odour as air pollution
• olfactory response to an odourant
decreases as the odourant conc.
decreases (nonlinear)
• responses to malodours include;
– Nausea and vomiting
– Headaches and other sensory disturbance
– Coughing respiratory ailments
– Depression
Odour as air pollution
• Odour Problems
– Bad odours generate complaints to regulatory
agencies more than any other form of air
pollution
– A new area of management that deals with
odour and noise is called modelling
Odour as air pollution
Odour Problems
• Likely sources of bad odours include;
– soap-making facilities
– petrochemical plants and refineries
– pulp and paper mills
– food-processing plants
– sewage treatment plants
– abattoirs
Odour as air pollution
• Bad odours associated with;
– amines
– sulfur gases (e.g. H2S)
– phenol, ammonia etc
– Hydrocarbons
– And many more…
Odour and the Law
• Legal/Regulated aspects of odour
• Local Council