Photochemical smog develops when primary pollutants like NOx and VOCs from fossil fuel combustion interact with sunlight. This produces secondary pollutants like ozone and PAN. For photochemical smog to form, sunlight, NOx, VOCs, and temperatures over 18°C are needed. Reactions between these pollutants produce the toxic components of smog. Acid rain forms from SO2 and NOx emissions reacting with water and oxygen in the atmosphere to produce sulfuric and nitric acid. This acid rain damages ecosystems and infrastructure. Topography and temperature inversions can trap smog near the ground.

1. Dr.Henry Antoine Des Voeux
Prashant Mehta
Assistant Professor
National Law University, Jodhpur

2. SMOG
• Majority of this pollution was created from the burning of coal for energy
generation, space heating, cooking, and transportation. Under the right
conditions, the smoke and sulfur dioxide produced from the burning of coal can
combine with fog to create industrial smog. In high concentrations, industrial
smog can be extremely toxic to humans and other living organisms. Example:
London Smog
• The use of other fossil fuels, nuclear power, and hydroelectricity instead of coal
has greatly reduced the occurrence of industrial smog. However, the burning of
fossil fuels like gasoline can create another atmospheric pollution problem known
as photochemical smog. Photochemical smog is a condition that develops
when primary pollutants (oxides of nitrogen and volatile organic compounds
created from fossil fuel combustion) interact under the influence of sunlight to
produce a mixture of hundreds of different and hazardous chemicals known
as secondary pollutants. Example: LA Smog

3. PHOTOCHEMICAL SMOG
To begin the chemical process of photochemical smog development the following
conditions must occur:
– Sunlight.
– The production of oxides of nitrogen (NOx).
– The production of volatile organic compounds (VOCs).
– Temperatures greater than 18 degrees Celsius.
If the above criteria are met, several reactions will occur producing the toxic
chemical constituents of photochemical smog.

4. REACTIONS
• Nitrogen dioxide can be formed by one of the following reactions. Notice that the nitrogen
oxide (NO) acts to remove ozone (O3) from the atmosphere and this mechanism occurs naturally in
an unpolluted atmosphere.
O3 + NO »»» NO2 + O2
NO + RO2 »»» NO2 + other products
• Sunlight can break down nitrogen dioxide (NO2) back into nitrogen oxide (NO).
NO2 + sunlight »»» NO + O
• The atomic oxygen (O) formed in the above reaction then reacts with one of the abundant oxygen
molecules (which makes up 20.94 % of the atmosphere) producing ozone (O3).
O + O2 »»» O3
• Nitrogen dioxide (NO2) can also react with radicals produced from volatile organic compounds in a
series of reactions to form toxic products such as peroxyacetyl nitrates (PAN).
NO2 + R »»» products such as PAN
• It should be noted that ozone can be produced naturally in an unpolluted atmosphere. However, it
is consumed by nitrogen oxide as illustrated in the first reaction. The introduction of volatile
organic compounds results in an alternative pathway for the nitrogen oxide, still forming nitrogen
dioxide but not consuming the ozone, and therefore ozone concentrations can be elevated to toxic
levels.

5. TOPOGRAPHY
Topography
• Very important for formation of
photochemical smog
• Restriction of air movement,
city in valley experience more
smog problem, than plains

6. METEROLOGY
Temperature Inversion
• Increase of air temperature with
height for some distance above
ground causing the smog trapped
close to ground
• Consequences
– Air becomes still and dust and
pollutants are no longer lifted
from surface
• Serious problem in many cities

7. SMOG EFFECTS
• Smog can irritate and inflame
pulmonary membranes, causing
chest pains, coughing, and throat
irritation.
• Other illnesses such as colds and
pneumonia can also be brought on
by exposure to smog.
• People with asthma problems are
under an even greater threat. Even
minor exposure to smog may cause
these people to get asthma attacks.
• Smog slowly ruins people's lungs to
an extent as great as that of
cigarettes.

8. ACID RAIN
• Unpolluted rainwater is slightly acidic (pH 5.6) because of the carbon dioxide from
air dissolved in it.
CO2 + H2O  H2CO3
• Rainwater with a pH as low as 5.6 to 2.5 has been recorded in some parts of the
world. They are commonly known as acid rain.
• "Acid rain" is also a broad term used to describe several ways that acids fall out of
the atmosphere. A more precise term is acid deposition, which has two parts: wet
and dry.
– Wet deposition: acidic rain, fog, and snow.
– Dry deposition: acidic gases and particles

9. CAUSES OF ACID RAIN
• Sulphur dioxide (SO2) and nitrogen oxides (NOx) are the primary causes of acid
rain.
• Acid rain occurs when these gases react in the atmosphere with water, oxygen,
and other chemicals to form various acidic compounds.
• Sunlight increases the rate of most of these reactions. The result is a mild solution
of sulfuric acid and nitric acid.

10. Formation of Sulphuric Acid (H2SO4)
• Sulphur dioxide (from the burning of fossil fuels in power plants and industrial
plants) is the primary cause of acid rain.
• In the atmosphere, sulphur dioxide is slowly oxidized to sulphur trioxide which
dissolves readily in water droplets to form sulphuric (vi) acid.
2SO2+O2  2SO3
SO3 + H2O  H2SO4
• The actual pathways are more complex.
• The formation of SO3 from SO2 is influenced by the prevailing atmospheric
conditions: sunlight, temperature, humidity, and the presence of hydrocarbons,
nitrogen oxides and particulates in the atmosphere.
• Sulphuric (iv) acid is also formed when SO2 dissolves in rainwater:
SO2 + H2O  H2SO3

11. Formation of Nitric Acid (HNO3)
• Nitrogen oxides (from the burning of fossil fuels in automobiles and power plants)
also cause the formation of acid rain.
• When released to the atmosphere, nitrogen monoxide combines with
atmospheric oxygen to form nitrogen dioxide:
2NO + O2  2NO2
• In a series of complex reaction, nitrogen dioxide combines with oxygen and water
vapour to form nitric (v) acid.
4NO2 + 2H2O + O2  4HNO3

12. Environmental Problems
(i) In water of pH less than 4.5, calcium metabolism in fresh water fish will be
affected, leading to poor health and stunted growth.
As a result, diversity and population of some fresh water species will be reduced.
(ii) In soil of pH less than 4.5, absorption of cations by plants will be affected,
resulting in death of plants.
(iii) Inflow of acidic water containing poisonous metal ions from soil will kill the
fish and water plants
(iv) Acid rain corrodes metals and accelerates the deterioration of building, rock
and statue.