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What is Air Pollution?
•By OECD (Organization for Economic and
Cooperation Development):
• The presence of harmful constituents of natural
or synthetic materials in the air in excess
quantity as a consequence of natural or human
activities.
•By AMA (American Medical Association):
• It is an excessive amount of foreign matters in
the air which adversely affect the well being or
cause damage to environment.
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Harmful constituents:
• Any air pollution materials or combination of
materials - such as physical, chemical or
biological substances emitted into the
atmosphere (or ambient air) and in indoors
• Ambient air – that portion of the atmosphere
external to buildings to which public has access
•But which materials or substances must be
considered harmful?
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Pollutant vs. Contaminant
•Contaminant:
• The contamination means the state of being
contaminated.
• Any substance/material added to the atmosphere
causing changes in its biogeochemical composition.
• It is a potential pollutant.
• e. g. any foreign substance
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•Pollutant:
• The undesirable state of the environment being
contaminated is pollution.
• The substance that cause air pollution is called
pollutant
• It must be a contaminant responsible for causing
adverse effects on the atmosphere.
• Any contaminant is a potential pollutant.
• e. g. CO2, SO2, NOx, SPM, etc.
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Pollutant classification
• Based on the formation
• They are classified as primary and secondary
pollutants.
• Primary pollutant:
• The pollutants which are directly emitted into the
atmosphere from sources as a direct consequence
of human activity
• They retain original form (native/unmodified)
• e. g. SO2, NOX (mostly NO), CO, SPM, Pb or
VOC.
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•Secondary pollutant:
• They are formed as a consequence of chemical
reactions taking place in the atmosphere
• e. g. the primary pollutants undergo chemical
reactions to form other species.
• They change their basic form after released from
the sources due to:
• Oxidation, decay, reaction with or other form of
pollutants or primary pollutants.
• e. g. SO3, H2SO4 (Acid rain), PAN (Peroxy-Acetyl
–Nitrate, HC+O2+NO2+hν), O3 (Ozone formation),
and NO2
•They essentially enter as the primary pollutants
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Forms of pollutants:
•The pollutants are found in two forms:
• Gaseous
• Particulate matter (fine dust)
• Both of these forms have different chemical
compositions
• They can be classified as primary and secondary
gaseous and particulate pollutants
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Classification by state of matter
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Primary gaseous compounds
• It is the air-like substance (combustible or
noncombustible) comprised of chemical
compounds, discharged into the atmosphere,
e. g.
• Sulfur compounds (SO2, H2S)
• Nitrogen compounds (NO, NH3)
• Carbon compounds (CO, HC)
• Halogen compounds (Flouride, Chloride,
Bromide, etc)
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•A small discrete mass of solid or liquid that
remains dispersed in gas/liquid emissions.
•Aerosol:
• The gaseous suspension or dispersion of fine solid
and liquid particles.
• e.g. spray of fine particles.
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Primary particulate compounds
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•Particulate is classified as per the size (i.e.
aerodynamic diameter of particles).
•The size of a particle is an important factor.
•It characterizes the particles in the atmosphere.
•They can be classified as
• Coarse particles (> 2.5 m), and
• Coarse particles are less important as they are removed
fast from the atmosphere due to gravitational fallout,
also less harmful.
• Fine particles (<= 2.5 m) – also known as
respirable particulate matter.
• Fine particles are more toxic and have more adverse
effects on human health and visibility.
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Sizes of Particulate matter
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Particles in different size modes
•PM10 (Inhalable fraction) - particles less than
equal to 10 m in size, e.g. inhalable particles
•PM2.5 (Respirable fraction) - particles less than
equal to 2.5 m in size, e.g. respirable particles
•PM10-PM2.5 (Coarse fraction) - coarse particles
•PM0.1 (Ultrafine fraction) - particles less than
equal to 0.1 m or 100 nm in size - ultrafine
particles or nanoparticles. 38
Types of primary particulate
matters
•Air ions:
• are formed from solar and cosmic radiation,
radioactive material, industrial combustion process
•Carbonaceous particles:
• made of soot, unburnt carbon
•Particles from automotive emissions:
• lead in the form of oxides and sulfides
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•Particles containing heavy metals:
• iron, arsenic, chromium, vanadium, titanium etc.,
•Particle containing light metals:
• Like sodium, aluminum, magnesium.
•Large particles:
• airborne from human activity, dust and sand,
suspended dust
•Biological particles:
• fungi, bacteria, pollens grains
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Secondary gaseous pollutants
• Atmospheric chemical reactions are
responsible for the transformation of primary
pollutants into stable end products.
• The chemical reactions are mostly photochemical
•The secondary pollutants are
• NO2 is formed from NO, and
• O3 is formed from O2 via photochemical reaction
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Secondary particulate
compounds
•Primary gases can transform into secondary
particulate compounds.
•They may form in gaseous phase or aqueous
phase with OH radical
• SO2 to SO4
2- (sulfate) (i.e. SO2+OH+O2 = SO4
2-)
• NO2 to NO3
- (nitrate)
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•Oxidation of primary gases such as sulfur and
nitrogen oxides into sulfuric acid (liquid) and
nitric acid (gas) in the absence of NH3.
•In the presence of ammonia – ammonium salts
such as ammonium sulfate, ammonium nitrate.
•Secondary particles are more harmful to health
as are very fine in size.
•They increase the acidity of the atmosphere.
•Secondary particles are strong light scatterers.
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Sources of air pollution
•Natural:
• It is defined as the pollution not caused by any
human activity
•Anthropogenic:
• It is defined as the pollution caused by human
activity
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Natural sources
•Volcanic eruption:
• CO, Sulfur, Chlorine, Soot particles (unburnt
carbons), HC, H2S, SO2, CH4
• e.g. Icelandic volcanic cloud spread all over the
Europe – leading to the cancellation of flights
•Forest fire:
• It is caused due to lightening and responsible for
unburnt HC, SO2, NOx, particulate, and smoke,
uncontrolled, CO, CO2 and ash.
• Controlled burning of forests – cause germination
of trees which renew the forest but adds to pollution 61
•Dust storms:
• A large area of land with no or little vegetation
causes dust storms
• They entrain a large amount of particulate matter
• e.g. airborne particulate of size range 10 - 100 µ
• e.g. deserts – Sahara, Arid land in northern India,
i.e. in Rajasthan
• The environmental problems caused - are
particularly the visibility reduction
• Which may affect air travel
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•Oceans:
• Oceans produce salt particles, also called aerosols
– corrosive to metals and paints
• Specially in coastal areas (like Mumbai, etc.)
•Plants and trees or vegetation:
• Vegetation are extensive sources of natural
pollutants.
• VOCs - in the presence of sunlight and humidity
produce non-methane-hydrocarbon (NMHC)
• Isoprene (carcinogenic pollutant)
• Blue haze over forested area, from the atmospheric
reaction of VOCs (VOCs react with NOx, SO2 on warmer days)
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•Pollen grain:
• from flowers, fine particles from weed, tree, grass
• it has peculiar irritating properties
• cause allergic reactions in humans
• Typical size range from 10 to 50
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Anthropogenic sources
• Industrial processes:
• Particulate, SO2, fluoride, H2S, CO2 and NO
• Thermal power plants:
• Use fossil fuels such as coal, oil, natural gas
• They release particles, HC, SO2, NOx, fly ash
• Domestic:
• Cooking, heating, painting and other indoor
sources such as photocopier, printer (O3, VOCs,
PM), and tobacco smoke
• Fumes from paints, hair spray, aerosol sprays, and
other solvents.
• In developing countries – burning of traditional
biomass like wood, crop waste, dung.
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•Transportation: specially in urban areas:
• 72% ambient air pollution is due to vehicles
(automobiles) alone
• 60% - 70% of CO, which is inert
• 20% - 25% of HC and
• 30% respirable particulate matter, fine particles
•Agricultural activities:
• For better yields biocides, herbicides, insecticides,
pesticides are commonly used
• During harvesting time from mill operations, cotton
gins – particles are released
• Organophosphate pesticide, chlorinated pesticide in
form of aerosol particles, and hazardous substances
become airborne
• Similarly, from livestock (animals) – CH4, NH3
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•Fugitive source:
• is another important source of air pollution – such
as evaporative emissions, leakages from storage
tanks, or equipment
• e.g. mostly volatile in nature (VOCs)
•Open burning of plastic:
• is practiced in several parts of the world which
releases mostly chlorine and chlorides in various
forms
• e.g. solid waste disposal
•Population increase:
• use natural resources, pressure on fossil fuel use 74
•Nuclear power plants:
• releases radioactive substances
• e.g. radionuclides such as radon, strontium,
krypton, etc.
•Deforestation:
• disturbs the balance of O2 and CO2
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Key Pollutants
• CO:
• major source is incomplete combustion of fuel (gasoline or
petrol driven cars)
• Colorless, odorless but poisonous gas
• NOx – (NO + NO2):
• produced from an incomplete combustion process at high
temperature, 90% NOx is in form of NO2
• another source - decomposition of organics in soil (soil
bacteria)
• Produced from thunderstorms of electric discharge
• Black carbon:
• Is a light absorbing material and yields a large positive
radiative forcing – participate in climate change 80
•VOCs:
• Entire set of vapor phase atmospheric organics
excluding CO, CO2
• Are the non-methane group of organic compounds
NMHC (non-methane hydrocarbon)
• Benzene, toluene, xylene are carcinogens – cause
leukemia through a prolonged exposure
• 1,3-butadiene is another dangerous compound
• incomplete combustion in presence of sunlight
produce NMHC (alkanes, alkenes, alkynes groups)
• Outdoor – traffic, industry
• Indoor – paints, furniture, varnish
•Methane:
• Efficient greenhouse gas – contributes to climate
change
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•Tropospheric O3 (ground level ozone):
• ground level ozone - due to VOC and NOx in the
presence of sunlight (sources are vehicles –
human activity)
• e.g. urban areas having more ozone pollution due
to increase in traffic
•Stratospheric O3:
• natural action of O2 and O gives (O2 + O) O3
(natural source) – it filters the UV rays so helpful to
humans and environment
• But this layer is destroyed by CFC (Chloro-fluro-
carbon) type of materials released from sprays,
deo, and refrigerators.
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•NH3 (ammonia):
• emitted from agricultural processes
• microbial decomposition of uric acid and urea,
(source - landfill site), smell of urine
• e.g. farming, livestock
• pungent odor
• it is widely used but caustic and hazardous
•SO2:
• emissions of black smoke, coal burning is the
major source
• causes acid rain problem
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•Acid rain: also known as acid deposition
• Produced when primary pollutants like SO2 and
NOx react with moisture in the atmosphere to form
sulphuric acid (H2SO4) and nitric acid (HNO3) and
return to the earth with rains (also called as wet
deposition)
•Radionuclides: – radioactive substances
• Natural emissions (Radon, Rn)
• Atmospheric nuclear tests (Strontium, Sr)
• Accidental releases in nuclear power plants
(Iodine, I; Cesium, Cs)
• Processes in the nuclear industry (Krypton, Kr)
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•Green House Gas (GHG):
• Produced from various sources such as
• H2O, CO2, N2O (soil, sewage, animal manure,
burning of fossil fuel), CH4 (anaerobic
decomposition), etc.
•Persistent Organic Pollutants (POP):
• Long-lived organic compounds – resistant to
environmental degradation so persist longer –
capable of long range transport
• Pesticide and dioxin
• Pesticides are used in industrial processes such as
polyvinyl chloride, pharma industries, etc.
• e.g. aldrin, DDT, endrin, hexachlorobenzene, PCBs,
furans, and carcinogenic PAHs
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Homework: week 1
•Find out about the role of gaseous radicals
in atmosphere and discuss with the help of
examples.
•Study the series of chemical reactions that
occur in the formation of ground level
(tropospheric) ozone.
•Study the secondary sources of fine
particles found in atmosphere. 91
