Assistant Professor, National Law University, Jodhpur
Composition: Nitrogen and Oxygen
"Minor" Gases (% by volume)
Carbon Dioxide 0.036 - Greenhouse
Methane 0.00014 - Greenhouse Gas
Ozone (variable) - Absorbs UV; eye
and respiratory irritant; damages
Water vapor (variable) - Greenhouse
Gas; absorbs long-wave radiation
Particulates (variable) - Absorbs
long-wave or reflects short-wave
radiation; condensation nuclei.
Atmosphere temperature and Chemistry is controlled by Trace Gases.
Substances in the Lithosphere tend to become more reduced over time.
Thus biomass (CH2O) is slowly transformed through multiple steps to
substance containing no oxygen atoms and then to compounds with large
Carbon to Hydrogen ratio and then finally pure Carbon.
Gravity prevents the gas molecules to escape from planet.
Point Source (Stationary) – Industry
Non Point Source (Mobile) – Transportation Vehicles
The trace atmospheric constituents that are present in unexpectedly
high concentration or in concentrations which cause some
detrimental effect are called air pollutants.
Air Pollution occurs when substances are released into air by human
activities in such concentrations as are sufficient to cause
detrimental effect on human health, vegetation, animals, and
property or interferes with the biosphere impacting the mankind.
contamination of the air by noxious gases and minute particles of
solid and liquid matter (particulates) in concentrations that endanger
Air pollution only occurs outdoors
1. Man Mad
(1) Industrial / Manufacturing.
(2) Mining / Metallurgical
(3) Transport etc.
2. Natural activities
(2) Volcanoes or anthropogenic activities
3. Combustion of gasoline and other hydrocarbon fuels in cars, trucks, and
Burning of fossil fuels (oil, coal, petro chemicals)
Everyday radioactive fallouts
Dust from fertilizers
These occur when:
the rate of emission or formation of pollutants is greater than (>)the
rate of dispersion.
Pollutants are dispersed through chemical destruction or removal by
winds or vertical transport via physical processes.
London smog/Bhopal disaster are examples of this type of sever air
URBAN AIR POLLUTION
Massive growth in size of cities, exponential growth in human
population, and rapidly growing number of automobiles especially
1. PRIMARY POLLUTANTS 2. SECONDARY POLLUTANTS
These are directly released
/injected in the atmosphere.
Ground Level Ozone
Carbon Dioxide, CFCs
Volatile Organic Compounds
These pollutants are formed in
the atmosphere through
Ground Level Ozone
1. Primary Aerosol
These are directly released in
2. Secondary Aerosols
These are formed in air via
gas to particle conversion
Directly released due to:
Dispersion of dust through wind/human activities
Ocean is big source of marine salt (NaCl)
1. Inorganic Aerosols
These are inorganic compounds, Soluble: Salts like sodium
chloride, Ammonium sulfate, and Insoluble: Minerals etc.
2. Organic Aerosols
Made up of organic compounds, Carbonaceous particles, Polycyclic
Organic Compounds and Aromatic Hydrocarbons
Creation of heat islands due to waste heat
Lower average wind speeds
Increase in fog frequency
Changes in vertical structure of the lower boundary layer
Formation of meso-scale circulation systems
It affects urban temperature , moisture content and formation of
It is unique to urban environment due to dense network of sources.
Power Plant Plumes
Excess heat stored in buildings/other structures
Lack of Plant Cover / Rapid Deforestation
Due to incomplete combustion of fuel. Correct air to fuel ratio is
If the mixture is rich(more fuel), there is insufficient oxygen to
completely oxidize hydrocarbons to carbon dioxide. It causes
oxygen deficiency by combining with hemoglobin to form carboxy-
hemoglobin, which is formed 250 times more than the oxy-
Released by Exhaust Emissions, Evaporative Losses from Fuel
These major sources of Toxic and Carcinogenic, Polycyclic
hydrocarbons and Volatile Organic Compounds
Due to partial oxidation of hydrocarbons into aldehydes.
Smoke / Soot
In diesel vehicles the emission of soot/smoke is most visible.
It is formed due to incomplete oxidation of carbon.
The soot particles are polymeric substances having carbon chains
Due to free valence the soot particles have sticking property.
High temperature of the combustion engine leads to the formation
Petrol Engines release large amounts of CO
Diesel engines release large amounts of NOx, Smoke, Particulate,
Proper Maintenance of the Engine
Petrol Tank must be full to control evaporative losses.
Catalytic converter / Fuel Injection system must be used.
Blending fuels with bio-fuels
The Earth’s Atmosphere is Basically Oxidizing in Nature.
The Major Oxidant is Oxygen(21%) and Mostly Oxygen- Based Compounds
and Oxy – Radicals
Other Oxidants are Cl2
CO is removed by Atmospheric Oxygen
Reactions are very slow at ambient conditions
All reactions in atmospheric chemistry are photochemistry based as sun
provide major source of energy.
All reactions are cyclic in nature that is they repeat indefinitely.
The hydroxyl radical (OH) dominates the daytime oxidation chemistry in the
troposphere, controlling the atmospheric lifetime of the majority of trace
species that are emitted natural or via man's activities.
OH is primarily produced by the photolysis of ozone to form O(1D) followed
by reaction with H2O. It is the primary daytime oxidising species responsible
for the removal of CO, CH4 (and higher hydrocarbons), H2, NO2, H2S,
(CH3)2S, NH3, the hydrochlorofluorocarbons (HCFCs) and
The concentration of OH defines the oxidising capacity of the atmosphere
and hence the ability to control levels of species that contribute to global
warming, acid rain or photochemical smog. Intermediate peroxy radicals, of
which HO2 is the simplest, are generated during the oxidation of trace gases
and a fast photochemical cycle links these radicals with OH.
1. Is the gas water soluble or fully oxidised (If Yes) Gas Eventually returns
to earth surface.
1. Does the Gas photo-decompose in sunlight (If Yes) Free Radicals are
1. Does Gas molecule have multiple bonds to with OH* can add to (If Yes)
Free Radicals are produced.
1. Does gas molecules have an H that OH* can abstract in exothermic
reaction (If Yes) Free Radical is produced
First step in photochemical reaction:
XY + h XY* absorption
XY* X + Y decomposition
XY* + other reactants (can be toxic) products formed can be toxic
E.g. NO2 +h NO + O (generate ozone)
Homolysis (Breakdown) of covalent bonds lead to the production
of free radicals:
Radicals normally take part in so-called chain reactions and the
following steps are involved:
Most molecules in the atmosphere contain pairs of valence
A few stable molecules have an odd number, eg. NO has 11
Electron dot structure: The most important free radical in
atmospheric chemistry is OH, N=O
O* + M O + M + kinetic energy
Concentrations of OH are difficult to measure due to high reactivity
OH + CO H + CO2 70%
OH + CH4 CH3 + H2O 30%
Above reactions in unpolluted atmosphere. They React further to form
H + O2 + M HOO + M (Important oxidants)
If other gases are available, the hydroxyl radical can undergo two
additional types of reactions:
OH + HCHO HC=O + H2O
Addition across a double bond:
OH + H2C=CH2 H2C-CH2OH
• A great variety of organic species is present in the environment
• Biogenic or anthropogenic origin
• Many reactions initiated by hydroxyl or other radicals
Photo-dissociation of NO2
NO2 + hυ (λ<430nm) O(3P) + NO
O(3P) + O2(+M) O3(+M) where M is a third body.
O(3P) is ground state oxygen atom
O3 + NO O2 + NO2
The Tropospheric concentration of OZONE is about 30-60 ppb in Delhi .
It dissolves in H2O, the value of Henry’s law constant is 1×10-2 mol L-1 atm-1.
Henry’s Law states that the amount of gas dissolved in solution varies
directly with the partial pressure of that gas over the solution. Stated
another way, the higher the pressure exerted on the solution, the more
gas the solution will hold.
Henry’s Law is well demonstrated by the example of the gases held
under pressure in carbonated soda.
Henry's law can be put into mathematical terms (at constant
temperature) as p= kHC where p is the partial pressure of the solute in
the gas above the solution, c is the concentration of the solute and kH is
a constant with the dimensions of pressure divided by concentration.
The constant, known as the Henry's law constant, depends on the
solute, the solvent and the temperature.
The most important atmospheric oxidant is OH radical((6±3)×106 molecules
cm-3) and it is a key reactant as it destroys all pollutants
O3 + hυ (290-306) O(1D) + O2
O(1D) + H2O 2OH (O(1D) is excited state atom)
This accounts for most of sulfate production in troposphere via oxidation of
OH* does not add to any multiple bond in any fully oxidized species such as
CO2, SO3, and N2O5 since such a process are endothermic and therefore
very slow to occur at atmospheric temperature.
HO + CO H + CO2
H + O2 (+M) HO2(+M)
2HO2 H2O2 + O2
In atmosphere, H2O2 has been found to be in the range.
In atmosphere H atom life time is very short and is very reactive.
No transportation as wind is not there.
Value of its Henry’s law constant is 7×104 mol L-1atm-1 which is very high as
compared to KH of O3.
The reactions of Hydrocarbons( CH4, etc.) with OH radical produce organic
OH + CH4 H2O + CH3
CH3 + O2 + M CH3O2 + M
Reaction of CH3O2 with HO2 forms:
CH3O2 + HO2 CH3OOH + O2
Thus, many peroxides are formed. They are all strong oxidants.
Studies on the atmospheric determinations of H2O2 and other peroxides in
our country have not been done.
The importance of its chemistry has been realized over the past three
NO3 plays an important role in both Troposphere and Stratosphere.
Formation Reaction: NO2 + O3 NO3 + O2
Its concentration is highest during night and lowest during the day. Its night
time concentration is ~ 109 molecule cm-3. It is easily photolyzed.
OH radical in day and NO3 radical in night.
NO3 + hυ NO2 + O
NO3 + hυ NO + O2
It is most important oxidant during night time more than 100 times more than
OH ions but its reactivity is low.
- is one of the strongest oxidants, and stronger than SO5
It is formed via oxidation of SO2 dissolved in water or generated in aqueous
phase, where it exists as HSO3
2-, generally referred to as Sulfur(IV):
Mn+ + HSO3
2- M(n-1)+ +SO3
- + O2 SO5
- + SO3
- + SO4
Estimated SO4- = 1-3x10-14 m/l very low concentration and hence it cant be
In dust – free and unpolluted atmosphere pH of rain water and other
aqueous systems is determined by following reactions:
CO2(g) + H2O ↔ CO2
.H2O; KH = 4.5x10-2 mol L-1 atm-1
.H2O ↔H+ + HCO3
-; K1 = 3.8х10-7 mol L-1 (carbonic acid)
These Eqs. show, [H+] = ( K1KHpCO2)½ , With = pCO2 =380 ppm = 3.8 х 10-4
atm, one gets, pH = ~ 5.6
So Reference or Background pH for Rain Water = 5.6, If the pH of rain
water is less than 5.6, it is called acid rain
GAS PHASE AQUEOUS PHASE
OH Radical (Most Important)
1. Hydrogen Peroxide
3. Oxygen Catalyzed by
- Metal ions (Fe, Mn, Cu)
- Particulate Matter
( Dust, Fly Ash, etc.)
Oxidation of SO2
SO2 + OH(+M) HOSO2 (+M)
HOSO2 +O2 HOSO2.O2
.O2 SO3 + HO2
SO3 + H2O H2SO4
HO2 + NO OH + NO2
OH is regenerated / OH is never destroyed
OXIDATION OF NO/NO2
NO+ OH HNO2
HNO2 OH + NO
NO + HO2 NO2 + OH
NO + O3 NO2 + O2
NO2 + O3 NO3 + O2
OH + NO2 HNO3
NO3 + NO2 N2O5
In Atmospheric Waters, Dissolved SO2 Generally Present as HSO3
SO2 + H2O SO2.H2O
SO2.H2O H+ + HSO3
- H+ + SO3
In Regions of Low pH( 4.5 - 6) Rain Water, it is present as Bisulfite (HSO3
In Regions of High pH( 6.5 - 8.5) Rain Water, it is present as Sulfite (SO3
Dissolved SO2 is Collectively Referred to as Sulfur(IV)
In Absence of Transition metal Ions, Oxidation of SO2 in Aqueous
Phase is Slow.
The Trace Transition metal Ions, Present as Impurity in All Forms of
Atmospheric Waters Strongly Catalyze the Autoxidation.
From Atmospheric Point of View, Important Metal Ions in Order of
Decreasing Importance are:
1. Ammonia/ Ammonium Ions
Present as Gaseous Ammonia and Ammonium Aerosols: (NH4)2SO4
2. Volatile Organic Compounds: A large number of volatile organic
compounds (VOCs), hydrocarbons, alcohols, carboxylic acids, aldehydes,
terpenes, phenols, polycyclic aromatic, hydrocarbons(PAHs)etc. are found
in trace amounts in atmosphere.
The VOCs such as ethanol, benzene, phenols are known to scavenge
- radical and inhibit the sulfur(IV) autoxidation as in Eq.:
- + organics SO4
2- + non-chain products
2. Suspended Particulate Matter: Dust, Fly Ash, Rock and Mineral Particles,
Carbonaceous Particles, Metal Oxides