Outdoor air pollution fundamentals
 Air pollutants
 Air pollution standards and measurement
 Air pollution problems
 Air pollution in Hong Kong
 Air pollution meteorology
1. Definition of air pollutants
 Air pollution may be defined as the presence in the
atmosphere of substance(s) added directly or indirectly in
such amounts as to affect living and non-living things
adversely.
 What is classified as a pollutant therefore depends upon
recognition of which substances cause adverse effects. It
is an ever-changing definition.
 Centuries ago only soot or odoured gases may have been
considered air pollutant. Now we recognize that pollutants
can cause more subtle effects than producing unpleasants
smells. Even CO2 is now considered a pollutant.
Air Pollution ?
 The key points of air pollution: i) substance; ii) man-made
or naturally produced; iii) with adverse effects to living and
non-living things; and iv) high concentrations or large
amount.
 Questions arise: How do we define the above vague
points? How is adversity defined? At what concentration
should a pollutant be considered high? What are the
substance that should be considered as pollutants?
2. Common air pollutants
 Particulate matter:
– total suspended particulate (TSP) (typical size <
100 mm)
– respirable suspended particulate (RSP) (typical size
< 10 mm) (PM10, particulate matter of size < 10 mm)
 Gaseous pollutants:
– Primary gaseous pollutants: SOx, NOx, CO, volatile
organic compound (VOC), Pb;
– Secondary gaseous pollutants: peroxyacetylnitrate
(PAN), ozone (O3)
 Photochemical pollutants:
– VOC, O3, PAN, CFC, greenhouse gases (CO2,
H2O)
2.1 Air pollutants sources and properties
 Natural pollutant sources
 Volcano eruption: emitting smoke, particulate
matter, SO2, H2S, CH4…
 Fires: emitting smoke, unburnt hydrocarbons,
CO, CO2, NOx...
 Dust or sand storms dispersing dust
 Oceans are emitting corrosive salt aerosols
 Lightning produces NOx and O3
 Normal human respiration produces CO2
 Artificial or anthropogenic sources
 Stationary sources: combustion, fuel usage,
waste incineration, industrial processes…
 Mobile sources: all emissions and exhausts from
transportation
2.2 Sources of air pollutants in Hong Kong
Particulate matter
Pow er generation
36.7%
Aircraft
Marine
0.5%
Fuel combustion
4.3%
3.7%
Vehicle
54.8%
Sulphur dioxide
Pow er generation
86.4%
Fuel combustion
Vehicle
Marine
6.6%
1.7%
5.0%
Aircraft
0.3%
Nitrogen oxides
Vehicle
35.8%
Pow er generation
42.9%
Aircraft
3.6%
Marine
13.1%
Fuel combustion
4.6%
Carbon monoxide
Vehicle
89.3%
Fuel combustion
3.2%
Marine
Aircraft
2.1%
2.5% Pow er generation
2.9%
2.2 Particulate matter
 typical size varies: gravel
2000µ, sand 20 ~ 1000µ,
hair, 50µ, RSP with
health effects 0.1 ~ 10µ
 Sizes that cause
significant air pollution
problems are 0.01 ~ 50µ,
as larger particles tend to
settle
 Sources of PM: Natural:
wind, sandstorm, forest
fires, volcano eruption;
Anthropogenic: industry,
automobiles
 Size of particles
 Large particles (2.5 ~ 250µ): produced in mechanical, crushing,
crashing, milling or grinding processes. Most mechanical
processes cannot produce particles of size smaller than 10µ
 Fine particles (0.1 ~ 10µ): produced in combustion, evaporation,
condensation, settling, e.g. tobacco smoke contains particles of
condensed hydrocarbons at 0.01 ~ 1µ. As usual the finer the size,
the more volatile the material.
 Agglomeration of fine particles: Fine particles tend to stick
together when they get close together due to electrostatic and Van
Der Waal’s forces.
 Aerosols: Particles small enough to remain suspended in the
atmosphere for a long time are referred to as aerosols
 Effects of particulate matter pollutions
 Visibility: Particles are able to scatter lights with wavelengths
close to the particle size. Because of this particulate matter
pollutions usually yield hazy days and visible smog. Since
visible lights have wavelength between 0.4 ~ 0.8µ, hazy
days are caused by secondary particles.
 Health: Inhalable lung-damaging dust ranges from 0.5 ~ 5µ;
asthma, respiratory syndromes, bronchitis, decreased lung
functions.
 Climate: Fine particles can be called condensation nuclei in
meteorology. When wet air reaches saturation condition, the
existence of fine particles makes it easier for water vapour to
condense and form tiny droplets, forming fog and mist. It
also leads to formation of clouds.
2.3 Gaseous pollutants
 Both N and S are essential to our bodies.
However N and S oxides are strong irritants that
cause health damage at high concentrations.
They also undergo atmospheric reactions to form
PM10 in urban areas.
 N and S oxides react with water and O2 to form
nitric and sulphuric acid, which are principal
contributors to acid rain.
 Both N and S have many sources, the main of
them being combustion or chemical plants.
 S oxides are formed from the sulphur
contaminants in fuels or incomplete combustion in
sulphur ores. N oxides come mainly from
atmospheric nitrogen due to lightning.
 Effects of NOx pollutions
 smog problems: respiratory
problems, visibility issues
 acid rain
 nutrient overload in water:
decreasing water quality
 toxic atmosphere
 global warming
 Since 1970, all air pollutants
have shown a decreasing
trend except NOx, which has
increased around 10%.
Anthropgenic sources of NOx and SOx
 Effects of SOx pollutions
 PM formation
 acid rain
 respiratory problems
2.4 VOC
 VOCs are those organic compounds whose room
temperature vapour pressures are greater than about
0.0007 atm. It usually contains carbon bonded with H, N
or S and can vaporise at significant rates
 VOCs are contributors to the problem of photochemical
oxidants (smogs, ozone): NOx + O2 + VOC O3 + smog
sunlight
 Some VOCs are infrared absorbers and thus contribute to
greenhouse effects. Other are known to be toxic or
carcinogenic.
 Most VOCs are emitted from smaller sources like
automobiles, paints, solvent usage, nail polish and vanish,
correcting fluids. Plants, due to stringent laws, produce
comparatively less VOC as emissions
2.5 Ozone
 O3 layer absorbs ~3% of UV radiation. It controls the
amount of incoming and outgoing solar radiation to and
from the Earth surface, thus maintaining a normal evolution
of global climate.
• Ozone occurs in two layers of the atmosphere.
• The layer surrounding the earth's surface is the
troposphere.
• Here, ground-level or "bad" ozone is an air pollutant
• that damages human health, vegetation, and many
common materials. It is a key ingredient of urban smog.
• The stratospheric or "good" ozone layer extends upward
from about 10 to 30 miles and protects life on earth from
the sun's harmful ultraviolet rays (UV-b).
Any chlorofluorocarbon (CFC) molecule in the upper
atmosphere tends to become stripped of a chlorine atom
photochemically, by bombardment with radiation. That
loose atom combines readily with any nearby ozone
molecule, to form normal oxygen and chloro-monoxide:
Cl + O3  ClO + O2
The monoxide then combines with any available atom of
oxygen in the atmosphere to form normal oxygen and
chlorine:
ClO + O  Cl + O2
So the outcome is a chlorine atom, as at the beginning,
once again available to destroy more ozone. The
chlorine thus acts as a catalyst. This catalytic process
occurs most effectively on a foreign surface, for
instance on an ice crystal. Since the lower
stratosphere is cold over Antarctica in winter, the little
water vapour there is forced to condense in polar
stratospheric clouds. There is good evidence that
every winter the catalytic process of CFCs virtually
totally consumes the ozone between 15~25 km over
Antarctica. Any CFC has a lifetime in the troposphere
of ~100 years, so it can remain available to destroy
ozone for a long time. Even if CFC production were
stopped today, what is in the atmosphere already
would damage the ozone layer for decades.
Nitric oxide, NO, is a catalyst in the same way. First it
reacts with ozone to form oxygen and nitrogen oxide
NO2, and then that reacts with loose atoms of oxygen to
create O2 and NO once more:
NO + O3  NO2 + O2
NO2 + O  NO + O2
The reactions occur most readily on the surface of ice
particles found in the winter stratosphere above
Antarctica, where temperatures are ~ -80 °C.
One common misconception:
O3 is poisonous. Its only function is to block radiation.
O3 is also a pollutant itself, it is the chief reason for smog
 In 1985, a team led by British
scientists J. Farman (1985)
shocked the scientific community
by reporting massive annual
decreases of stratospheric ozone
over Antarctica in the polar
spring.
 In fact such phenomenon has
been detected as early as 1977
aboard Nimbus-7 satellite, only
that the observations were being
discarded as wrong data.
 Same thing actually happen in Arctic.
However, since temperature is slightly
higher in Arctic, ozone depletion is less.
 The effect of a compound on
stratospheric ozone depletion is assessed
by the unit ozone depletion potential
(ODP). ODP is usually defined as the
total steady-state ozone destruction,
vertically integrated over the
stratosphere, that results per unit mass
of species i emitted per year relative to
that of CFC-11.
O3,i
ODPi 
O 3,CFC11
compound
CFC-11 (CFCl3)
CFC-12 (CF2Cl2)
CFC-113 (CFCl2CF2Cl)
CFC-115 (CF2ClCF3)
HCFC-22 (CF2HCl)
HFC-225ca (CF3CF2CHCL2)
ODP
1.00
0.82
0.92
0.40
0.04
0.02
 In September 1987, Montréal Protocol on Substances
that Deplete the Ozone Layer, each signer agreed to
reduce the production of CFC-11 ~ CFC-114 and
halons. Consumption of these FCFs was frozen at 1986
level from 1989 and is to be reduced to 80% and then
50% of these values at 1994 and 1999.
 For undeveloped countries, production must not exceed
110% of the 1986 levels.
2.6 Greenhouse effect
 The atmosphere absorbs a high fraction of long-wave
radiation of the earth. Greenhouse effect
 Water in vapour form is the principal agent for this
effect. CO2 is the next in importance only. CO2 is not
as important because its concentration is much lower
and its main infrared absorption is localized in a narrow
band near 1.5 x 104 nm.
 However the amount of water in the atmosphere is
fairly constant, whereas the amount of CO2 is
increasing quickly due to rapid growth in industrial
activities for the past century.
3. Local air pollution problems
 Air pollution problems appear in 3 main effects:
 Effects on health
Smoke particles that enter and deposited on the alveoli
can cause tuberculosis. Other particles might adsorb
gas which causes more intensive irritation. Gases
and particles might also penetrate into the
bloodstream, e.g. suspended lead from vehicle
exhausts can causes significant nervous problems.
 Ecological effects
Plants and animals are also susceptible to air pollution
effects. Fluorine emitted by factories can damage
plants nearby and significantly lowers the value and
vitality of the crop nearby
 Effects on materials
Many acidic gases, like fluorine, chlorine, SOx, NOx can
attack metals and concrete and etch glass
3.1 Some severe air pollutions
 London smog (1952)
– Peak daily concentrations nearly 4000 µgm-3 of SOx and 6000
µgm-3 of smoke
 Los Angeles photochemical smog (1940s)
– Huge amount of O3 found by photochemical reactions between
NOx from automobile emissions, peroxyacetyl nitrate (PAN) and
solar radiation
 Acid rain (1968)
– Buildings damage and ecological changes in Scandinavia
 Bhopal chemical plant accident (1984)
– 3300 people died and more than 200000 suffered from respiratory
and eye diseases when 40 tonnes of methyl-isocyanate (MIC)
were accidentally released
 Chernobyl radiation accident (1986)
– More than 250 curies of radioactive isotopes were
released in a nuclear power plant explosion. Entire
Northern and Eastern Europe were affected. 30
casualties, countless radiation sickness
 Antarctica ozone hole (since 1983)
– Ozone depletion at Antarctica due to CFC
 Forest fire in Indonesia (1997)
– More than 30000 people suffered from respiratory
problems, visibility often less than 30 m, it was
reported that breathing the air was the same as
smoking 100 cigarettes per day.
4. The atmosphere
Average composition of
dry clean air by volume
 N2
78.084%
 O2
20.946%
 Ar
0.934%
 CO2
0.036%
 Ne
0.002%
 He
0.001%
 ...
Typical concentrations of
various gases in clean air (ppm)









H2O
CO2
CH4
CO
O3
NH3
NO2
SO2
H2S
0 ~ 70000
360
1.5
0.1
0.02
0.01
0.001
0.0002
0.0002
 Concentrations are usually expressed ‘by volume’
because there is a direct relationship between the
volume, partial pressure and the number of
molecules present. (Dalton’s law)
 In ambient air at 1 atm, there are ~2.693 x 1019
molecules / cm3 (Loschmidt number)
 Two concentration units are commonly used in
reporting atmospheric species abundance
C
concentrat ion of species i in ppm  i 106
C
where Ci and C are moles/volu me of species i and air
mi
mass concentrat ion mi in mgm , Ci 
10-6
Mi
-3
where M i is the molecular weight of species i
5. Air quality standards
 The legislative basis for air pollution abatement in the USA
is the 1963 Clean Air Act and its amendments. The Act
and its amendments provide for the establishment of two
kinds of national ambient air quality standards.
 Primary ambient air quality standards: those measures to
protect public health with an adequate margin of safety
 Secondary ambient air quality standards: specify a level of
pollutant concentrations requisite to the public welfare from
any known or anticipated adverse effects associated with
the presence of such air pollutants in the air. These effects
include damage to crops and vegetation, wildlife, visibility,
climate and economy.
 Air quality standards are based solely on the effects of air
pollution, not by scientific or economical standards.
 Three kinds of studies have been conducted: animal
testing, short-term exposures to human volunteers, and
epidemiological studies.
 National air quality standards (1997): standards not to be
exceeded once a year
Pollutant Averaging time Primary (µgm-3) Secondary (µgm-3)
SOx
annual
80
daily
365
PM10
annual
50
50
daily
150
150
PM2.5
annual
15
15
daily
65
65
CO
8 hours
10
10
1 hour
40
VOC
3 hours
160
160
NOx
annual
100
100
Pb
3 months
1.5
1.5
O3
8 hours
80
80
 Air quality objectives of Hong Kong (1987): daily
threshold not more than once a year, hourly threshold
not more than 3 times a year
Concentration in Microgrammes per Cubic Metre (i)
Averaging Time
Pollutant
1hr
8hrs
24hrs
3mths
1yr
Health Effects of Pollutant at Elevated
Ambient Levels
350
80
Respiratory illness; reduced lung function;
morbidity and mortality rates increase at higher
levels.
Total Suspended Particulates
260
80
Respirable fraction has effects on health.
Respirable Suspended
Particulates (v)
180
55
Respiratory illness; reduced lung function; cancer
risk for certain particles; morbidity and mortality
rates increase at higher levels.
150
80
Respiratory irritation; increased susceptibility to
respiratory infection; lung development
impairment.
Sulphur Dioxide
800
Nitrogen Dioxide
300
Carbon Monoxide
30 000
Photochemical Oxidants (as
ozone) (vi)
Lead
Impairment of co-ordination; deleterious to
pregnant women and those with heart and
circulatory conditions.
10 000
Eye irritation; cough; reduced athletic
performance; possible chromosome damage.
240
1.5
Affects cell and body processes; likely neuropsychological effects, particularly in children;
likely effects on rates of incidence of heart
attacks, strokes and hypertension.
5.1 Air pollution index
 The Air Pollution Index (API) of USA is the compact
expression of the air quality levels referring to five major
pollutants: PM10, SO2, CO, O3, and NO2, based on the
National Ambient Air Quality Standards (NAAQS).
 The Hong Kong API is based upon the Hong Kong Air
Quality Objectives (HKAQO). Our API is slightly more
sophisticated and is comprised of 9 sub-indices: TSP
(daily average), RSP/PM10 (daily average), SO2 (daily
average), SO2 (hourly average), NO2 (daily average),
NO2 (hourly average), CO (8-hour average), CO (hourly
average) and O3 (hourly average).
 Calculations of API:
 1) Calculate sub-index Ii of the ith pollutant
 Xi observed concentration for the ith pollutant
 Xi,j concentration for the ith pollutant and the jth breakpoint
 Xi,j+1concentration for the ith pollutant and the (j+1) breakpoint
 Ii,j API value for the ith pollutant and the jth breakpoint
 Ii,j+1 API value for the ith pollutant and the (j+1)th breakpoint
API
values
0
50
100
200
300
400
500
TSP
Daily
0
80
260
375
625
875
1000
RSP
Daily
0
55
180
350
420
500
600
SO2
Daily
0
80
350
800
1600
2100
2620
SO2
Hourly
0
400
800
1600
2400
3200
4000
NO2
Daily
0
80
150
280
565
570
940
NO2
Hourly
0
150
300
1130
2260
3000
3750
CO
8-hour
0
5
10
17
34
46
57.5
CO
Hourly
0
15
30
60
90
120
150
O3
Hourly
0
120
240
400
800
1000
1200
 The maximum of all sub-indices are then selected as
the API
API  Max[ I1 , I 2 ,..., I 9 ]
Air Quality Status
Air Pollution
Level
API
Health Implications [1]
Air quality significantly
worse than both short-term
and long-term AQOs.
Severe
201 to 500
People with existing heart or respiratory illnesses
will experience significant aggravation of their
symptoms and there will be also widespread
symptoms in the healthy population. These
include eye irritation, wheezing, coughing, phlegm
and sore throat.
Air quality worse than both
short-term and long-term
AQOs.
Very High
101 to 200
People with existing heart or respiratory illnesses
will notice mild aggravation of their health
conditions. Generally healthy individuals may also
notice some discomfort.
Air quality within the shortterm AQOs but worse than
the long-term.
High
51 to 100
Very few people, if any, may notice immediate
health effects. Long-term effects may, however,
be observed if you are exposed to such levels for
a long time.
Air quality within all AQOs.
Medium
26 to 50
None expected for the general population.
Air quality well within all
AQOs.
Low
0 to 25
None expected.
5.2 Measurement of air pollution
 The air quality monitoring network in Hong Kong of the
Environmental Protection Department (EPD) comprises
fourteen fixed monitoring stations as of July 1999 to meet the
following objectives:– To understand air pollution problems in order that cost-effective
policies and solutions can be developed;
– To assess how far standards and targets are being achieved or
violated;
– To assist the assessment of public's exposure to air pollution; and
– To provide public information on current and forecast air quality.
 Other stations have been used in the past and there are
more planned for the future. In addition to the EPD's stations,
other independent monitoring units are operated, for
example, those being operated by the power companies in
order to assess the air quality impact of their power stations.
A typical sampling urban sampling station
High volume RSP and TSP samplers
Solar radiation detector
Acid rain collector
Wind anemometer
tapered element oscillating
microbalance - continuous RSP
Gaseous pollutants analyzer
Mobile air sampler
6. Air Pollution in Hong Kong
 Chiefly vehicular emission related - TSP, RSP &
photochemical pollution
 Large industrial sources are relatively under control
 Hong Kong is extremely mountainous. This makes
trapping of air pollutants easy. Modelling and prediction of
air pollutants dispersion also becomes difficult.
 Pollution events usually happen in hot summer or winter.
OZONE
(Hourly Maximum)
700
1996
600
1998
1999
1999
400
1999
1997
1999
300
1997
3
Air Quality Objective: 240 m g/m
1999 1996 1996
200
1998 1999
1996
1999
1999
100
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Be
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0
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Concentration ( m g/m3)
500
7. Atmospheric stability
subadiabatic
z0 + dz
B
A
z0
superadiabatic
C
z0 - dz
adiabatic
T0 - dT
T0
T0 + dT
7.1 Lapse rates
 A lapse rate is the change of temperature with unit rise
of elevation.
 Under adiabatic condition, the lapse rate near the
Earth surface is ~-9.8 K km-1. If the numerical value of
the lapse is greater in magnitude than this value,
superadiabatic lapse; lower, subadiabatic lapse.
 For calculation purposes, meteorologists and
engineers have defined a standard atmospheric lapse
of about by ~-6.5 K km-1 by the approximate average of
all observations. It simply indicates the the adiabatic
assumption is not too appropriate.
What does not the atmosphere always have an
adiabatic lapse rate as its actual profile?
It is because other processes such as winds and solar
heating leading to the dynamic temperature
behaviour is seldom adiabatic. Other processes
exert an equal influence on the prevailing
temperature profile than does the adiabatic rising and
falling of air parcels.

One common fallacy: Cold air is more dense, and
therefore heavier than warm air. Therefore, when the
air aloft is colder than the air near the ground, the
atmosphere is unstable: the cold air will sink and the
warm air will rise.
7.2 Temperature-elevation curve
 The temperature-elevation curve is the principal
determination of atmospheric stability.
 Suppose an air parcel is at point A on the figure. If
it is displaced suddenly upward or downward, fast
enough to be considered reversible and adiabatic.
It would then move to B or C.
 If the air parcel is displaced slowly, then it could
transfer heat to or from the air around it. The
process is then unadiabatic.
 If the surrounding air has adiabatic lapse rate,
wherever the air parcel moves, it will reach the same
temperature and pressure as the surrounding air.
Because of this, further movement is prohibited. This
condition is neutral stability: simple displacement do
not lead to restoration to original position, nor do they
make the parcel move further.
 When the sun comes up and heats the ground
surface and the air above it, there will be a layer of
warmed air near the ground, where the lapse rate is
practically adiabatic.
 If the surrounding air has a subadiabatic lapse rate,
then if it is moved upwards, it will follow the adiabatic
curve, making itself colder and denser than the
surrounding. Gravity will then drive it back down. If it
is moved downwards, it becomes warmer and less
dense, and gravity will push it upwards. For
whatever disturbance, the parcel will be restored to
its original position. The atmosphere is stable.
 When the ground cools at evening, temperature
increases with height near the ground. This is called
inversion. Because of the difference is temperature
gradient, the situation inside the inversion is
extremely stable.
 If the surrounding air has a superadiabatic lapse rate,
then if it is moved upwards, it will follow the adiabatic
curve, making itself warmer and less dense than the
surrounding. Gravity will then drive it further upward.
If it is moved upwards, it becomes colder and denser
than the surrounding, and gravity will push it further
downwards. Whatever disturbs it from its original
position, the parcel will continue to move away from
its original position. The atmosphere is unstable and
vertical motion is spontaneous if any minute trigger
exists.
 By afternoon, enough heat has been transferred from
the warmed ground to the surrounding. The heated
adiabatic air, extends to large height (~2 km) where it
meets the stable air above. The air there has a
standard lapse rate and thus superadiabatic.
7.3 Examples of instability
 Thermals : A thermal is a column of warm-air
bubbles, expanding as they rise due to
atmospheric instability. Thermals commonly occur
on warm, calm afternoons above hot grounds.
The air lifts off, entrains more air and bubbles and
form a column of ~1 km vertical height. The lift is
great enough to support soaring
eagles without
rising hot air column
flapping their wings.
slowly falling
warm air
Hot ground
low
pressure
warm air column
near ground
 Tornadoes : A tornado is the
result of extreme instability,
consisting of a violent vortical air
flow and a tapering funnel of
twisting cloud. Although the
formation of tornadoes is similar
to thermals, it depend on three
conditions: 1) a sufficient amount
of convective energy, 2) stable
PBL to prevent thunderstorms to
release instability and 3) strong
increase of wind speed with
respect to height.
7.4 Mixing heights
 Due to the thermals, there will be
vigorous vertical mixing from the
clouds with irregular
ground at ~2 km, and then negligible
vertical mixing above that. The rising tops but flat bottoms
air columns provide good vertical
clouds tops near
mixing and induce large-scale three-mixing height
dimensional turbulence, which
mixing
induce horizontal mixing. Pollutants
height
released at ground-level will be
mixed uniformly up to that mixing
height, but not above it. Mixing
heights set a limit to air pollutants
dispersions.
8. Effect of atmospheric stability on
air pollutant dispersion
natural adiabatic lapse
 Looping : The plume is formed in
unstable condition. The flow
pattern follows closely that of a
flow past bluff body forming a
turbulent wake behind it.
actual temperature lapse
 Coning : A typical buoyant plume
which grows by mixing. It is
formed in neutral atmospheric
condition. The plume is usually
bifurcated and rises by its own
buoyancy.
 Fanning : The plume is seen to reach
its equilibrium level quickly. The
plume starts off bifurcated by meets
strong inversion and travels stably. It
is formed in stable atmospheric
condition.
 Fumigating : The plume is formed by
being trapped by a strong stable
inversion above and a well mixed
unstable air below it. It occurs usually
in the morning when sunshine causes
the layer beneath the inversion to be
mixed by convection.
 Lofting : If the inversion is
weak, the plume may
penetrate it and then
continue to spread upwards
but not downwards due to
the inversion.
 Thermalling : It is formed
when the thermals are well
developed, and the plume is
carried away by convection
and is broken into lumps.
 Flagging : When the efflux velocity of
the plume is not large enough to
carry the effluent, some pollutants
are trapped near the stack hole. This
is then brought down by gravity.
 Bifurcation : The buoyancy of a hot
plume generates a double vortex
circulation in the plume with an
updraught. This is due to internal
convective motion of the plume