Air Pollution: Sources, Primary and Secondary Pollutants - TNPSC - Environmental Scientist
Air Pollution: Sources,
Primary and Secondary Pollutants
Air
pollution is the presence of substances in the atmosphere—introduced either by
human activity or natural processes—that are harmful to humans, animals,
plants, and the environment. It has become a critical global issue, affecting
air quality, health, weather patterns, and climate. Air pollution occurs when
harmful gases, particulates, or biological molecules are introduced into
Earth’s atmosphere, causing discomfort or disease.
Sources of
Air Pollution
The
sources of air pollution are broadly classified into two categories:
1. Natural Sources
These
occur without human intervention and include:
- Volcanic eruptions – release sulfur dioxide
(SO₂), ash, and particulate matter.
- Forest fires – release carbon monoxide
(CO), carbon dioxide (CO₂), and particulates.
- Dust storms – especially in arid
regions, release large amounts of particulates.
- Pollen dispersal – contributes to allergenic
particulate matter.
- Methane emissions from wetlands – a natural
source of greenhouse gas.
2. Anthropogenic (Human-made) Sources
These are
responsible for most of the modern air pollution:
- Industrial emissions – from factories and power
plants (e.g., SO₂, NOₓ).
- Transportation – vehicles emit CO, NOₓ,
hydrocarbons, and particulates.
- Burning of fossil fuels – in homes and industries
releases CO₂, SO₂, etc.
- Agricultural activities – pesticides, ammonia (NH₃)
from fertilizers.
- Construction activities – release dust and particulate
matter.
- Waste disposal – open burning of waste
releases dioxins and other toxins.
Classification
of Air Pollutants
Air
pollutants are broadly classified into two types:
1. Primary Pollutants
2. Secondary Pollutants
1. Primary
Pollutants
Definition:
Primary pollutants are those emitted directly from a source into the atmosphere
in a harmful form. These pollutants are released from identifiable sources and
cause immediate effects.
Common Primary Pollutants and Sources:
|
Pollutant |
Description |
Major Sources |
|
Carbon
Monoxide (CO) |
Colorless,
odorless gas; interferes with oxygen transport in the blood. |
Vehicle
exhaust, burning of wood and fossil fuels. |
|
Sulfur
Dioxide (SO₂) |
Sharp-smelling
gas that contributes to acid rain. |
Coal
and oil combustion in power plants and industry. |
|
Nitrogen
Oxides (NO and NO₂) |
Involved
in smog formation and acid rain. |
Motor
vehicles, combustion of fossil fuels. |
|
Particulate
Matter (PM10, PM2.5) |
Tiny
solid and liquid particles suspended in the air. |
Construction,
vehicles, burning, industrial dust. |
|
Volatile
Organic Compounds (VOCs) |
Organic
chemicals that evaporate easily and form ozone. |
Paints,
gasoline, solvents, vehicle exhaust. |
|
Lead
(Pb) |
A toxic
metal that affects the nervous system. |
Old
leaded gasoline, smelting industries. |
|
Ammonia
(NH₃) |
A
pungent gas that contributes to secondary pollutants. |
Agricultural
fertilizer use, animal waste. |
Effects of
Primary Pollutants:
- Human Health: Respiratory problems,
heart disease, brain damage.
- Environment: Acid rain, poor
visibility, soil and water pollution.
- Climate: Some primary pollutants
are also greenhouse gases (e.g., CO₂, CH₄).
2. Secondary
Pollutants
Definition:
Secondary pollutants are not emitted directly. They form in the atmosphere
through chemical reactions involving primary pollutants and
natural atmospheric compounds like water vapor, sunlight, or oxygen.
Formation
Process:
Primary
pollutants + Atmospheric components → Secondary pollutants
Common
Secondary Pollutants and Formation:
|
Secondary Pollutant |
Formation |
Effects |
|
Ozone
(O₃) |
Formed
when nitrogen oxides (NOₓ) and VOCs react in sunlight. |
Component
of photochemical smog; causes respiratory problems. |
|
Peroxyacetyl
Nitrate (PANs) |
Reaction
of NOₓ and VOCs in the presence of sunlight. |
Eye
irritation, harmful to plants. |
|
Acid
Rain (H₂SO₄, HNO₃) |
SO₂ and
NOₓ react with water vapor to form sulfuric and nitric acids. |
Damages
soil, forests, aquatic life, buildings. |
|
Smog
(Photochemical & Industrial) |
Mixture
of O₃, PANs, particulates. |
Reduces
visibility, health hazard. |
|
Secondary
Particulate Matter |
Gases
like SO₂, NOₓ, and NH₃ form aerosols. |
Penetrates
deep into lungs; worsens heart/lung diseases. |
Photochemical Smog vs Industrial Smog
|
Type |
Components |
Formation |
Region |
|
Photochemical
Smog |
Ozone,
NOₓ, VOCs, PANs |
Sunlight
+ vehicular emissions |
Urban,
sunny, dry areas (e.g., Los Angeles) |
|
Industrial
Smog |
SO₂,
particulate matter |
Coal
burning + fog + moist air |
Industrial
cities with cold, damp climates (e.g., London) |
Health and
Environmental Effects of Air Pollutants
Human
Health:
- CO – Reduces oxygen delivery
to organs and tissues.
- NO₂ – Causes inflammation of
airways.
- PM2.5 – Penetrates lungs and
bloodstream, causing cancer, asthma.
- O₃ – Reduces lung function,
worsens bronchitis and asthma.
Plants and
Ecosystems:
- Ozone damages leaf tissues,
reducing crop yield.
- Acid rain alters soil pH and
leaches away nutrients.
- Heavy metals (like lead)
poison plants and aquatic life.
Built
Environment:
- Acid rain corrodes monuments
and buildings.
- Smog stains walls and
reduces visibility.
- Particulate matter settles
on surfaces and reduces aesthetics.
Climate
Change Link
Some air
pollutants are also greenhouse gases:
- Methane (CH₄) and Carbon Dioxide (CO₂)
trap heat and warm the planet.
- Black carbon (a component of PM) reduces
ice albedo and accelerates melting.
Prevention
and Control Measures
- Industrial Regulation: Enforce emissions standards
for factories and power plants.
- Vehicular Control: Promote public transport,
electric vehicles, and regular emission checks.
- Cleaner Fuels: Use low-sulfur fuels, CNG,
and renewables.
- Green Cover: Afforestation and green
belts absorb pollutants.
- Public Awareness: Educate about pollution
sources and reduction techniques.
- Legislation: Strengthen air quality laws
like the Air (Prevention and Control of Pollution) Act, 1981 (India).
