Introduction to Smog Formation
Smog is a type of air pollution that reduces visibility and creates a hazy atmosphere. The term "smog" comes from combining the words "smoke" and "fog". It's a major environmental problem in many cities worldwide, affecting both human health and the environment.
Key Definitions:
- Smog: A mixture of air pollutants (smoke, gases and particles) that form a yellowish or blackish fog in the atmosphere.
- Photochemical smog: Forms when sunlight reacts with nitrogen oxides and volatile organic compounds in the atmosphere.
- Industrial smog: Results from a high concentration of sulphur oxides, particulates and other pollutants, typically from burning coal.
- Primary pollutants: Harmful substances released directly into the air (e.g., nitrogen oxides, VOCs).
- Secondary pollutants: Harmful substances formed when primary pollutants react in the atmosphere (e.g., ozone, PAN).
💨 Photochemical Smog
Photochemical smog is most common in cities with sunny, warm, dry climates and lots of motor vehicles. The main ingredients are nitrogen oxides (NOx) and volatile organic compounds (VOCs) that react in sunlight. Los Angeles is famous for this type of smog.
Formation process:
- Cars, factories and power plants release NOx and VOCs
- Sunlight triggers chemical reactions
- Ground-level ozone and other irritating chemicals form
- A yellowish-brown haze develops, especially during afternoons
🌆 Industrial Smog
Industrial smog (also called "London smog" or "sulphurous smog") forms in cool, humid conditions and was common in industrial cities that burned lots of coal. It's less common today in developed countries but still occurs in developing nations.
Formation process:
- Coal burning releases sulphur dioxide (SO₂) and particulates
- In cool, humid air, SO₂ dissolves in water droplets
- Forms sulphuric acid and acidic particles
- Creates a thick, dark fog that can last for days
The Chemistry of Smog Formation
Understanding the chemical reactions that create smog helps us see why it's so difficult to control. These reactions involve both natural and human-made pollutants.
Key Chemical Reactions in Photochemical Smog
Photochemical smog forms through a series of complex reactions that start with nitrogen oxides (NOx) and volatile organic compounds (VOCs):
- Nitrogen dioxide breaks down in sunlight:
NO₂ + sunlight → NO + O
- Oxygen atoms combine with oxygen molecules to form ozone:
O + O₂ → O₃ (ozone)
- VOCs react with hydroxyl radicals and oxygen:
VOCs + OH + O₂ → various products including aldehydes and PAN
- Nitrogen oxide can react with ozone:
NO + O₃ → NO₂ + O₂
The result is a complex mixture of ground-level ozone, peroxyacetyl nitrate (PAN), aldehydes and other irritating compounds that create the characteristic yellowish haze.
Did You Know?
Ground-level ozone in smog is chemically identical to the ozone in the upper atmosphere. But while stratospheric ozone protects us from harmful UV radiation, ground-level ozone is a harmful pollutant that can damage our lungs and harm plants!
Factors Influencing Smog Formation
Several factors affect how and when smog forms. Understanding these can help predict and prevent smog events.
🌡 Weather Conditions
- Temperature inversions trap pollutants near the ground
- Sunlight drives photochemical reactions
- Lack of wind prevents pollutant dispersal
- Humidity affects industrial smog formation
🗺 Geographical Factors
- Mountains can trap air in valleys
- Urban layout affects air circulation
- Coastal areas may experience sea breezes that push smog inland
- Altitude affects oxygen levels and combustion
🚘 Human Activities
- Vehicle emissions (major source of NOx and VOCs)
- Industrial processes release various pollutants
- Power generation from fossil fuels
- Residential heating and cooking with coal or wood
Health and Environmental Impacts
Smog isn't just unpleasant to look at it poses serious risks to human health and the environment.
🏥 Health Effects
- Respiratory problems: Coughing, wheezing, shortness of breath, asthma attacks
- Eye irritation: Burning, watering and redness
- Reduced lung function: Especially in children, elderly and those with existing conditions
- Increased susceptibility to infections: Damages respiratory tract defenses
- Long-term exposure: Can lead to chronic bronchitis, emphysema and heart disease
- Premature death: WHO estimates millions of premature deaths annually from air pollution
🌱 Environmental Effects
- Plant damage: Ozone damages leaf tissue and reduces crop yields
- Reduced visibility: Affects transportation safety and tourism
- Material damage: Accelerates deterioration of buildings and monuments
- Ecosystem disruption: Can harm sensitive species and disrupt food chains
- Climate effects: Some smog components contribute to climate change
- Acid rain: Industrial smog can lead to acidification of soils and water bodies
Case Study Focus: The Great Smog of London (1952)
In December 1952, London experienced one of history's deadliest air pollution events. A period of cold weather combined with windless conditions created a perfect storm for smog formation. Coal burning for home heating released massive amounts of sulphur dioxide and smoke particles. The resulting dense smog lasted for five days and was so thick people couldn't see their feet!
The health impacts were devastating: an estimated 4,000 people died during the smog event, with another 8,000 dying in the following weeks and months. The Great Smog led directly to the Clean Air Act of 1956, one of the first major environmental laws addressing air pollution.
This historical event demonstrates how severe smog can become when the right conditions combine with heavy pollution and shows why environmental regulations are so important.
Modern Smog Hotspots
While many developed countries have reduced smog through regulations, it remains a serious problem in many parts of the world.
Global Smog Challenges
- Beijing, China: Experiences severe smog events due to coal power plants, industrial activity and increasing vehicle numbers. The government has implemented emergency measures during extreme events, including shutting down factories and restricting driving.
- New Delhi, India: Suffers from some of the world's worst air quality, especially during winter months. Contributing factors include vehicle emissions, industrial pollution, construction dust and agricultural burning practices.
- Mexico City, Mexico: Its high altitude (2,240m) and surrounding mountains create perfect conditions for photochemical smog. The city has implemented various control measures including vehicle restrictions.
- Los Angeles, USA: Despite strict regulations, still experiences photochemical smog due to its geography, climate and large number of vehicles. However, air quality has improved significantly since the 1970s.
Preventing and Controlling Smog
Tackling smog requires action at individual, local and national levels. Here are some key approaches:
💡 Technological Solutions
- Catalytic converters in vehicles reduce NOx and VOC emissions
- Scrubbers in industrial chimneys remove SO₂
- Low-VOC products like paints and solvents
- Electric vehicles eliminate direct emissions
- Renewable energy reduces fossil fuel combustion
- Particulate filters capture fine particles
📄 Policy Approaches
- Emission standards for vehicles and industry
- Air quality monitoring networks
- Public transport investment to reduce car use
- Urban planning that promotes walking and cycling
- Clean fuel requirements (low-sulphur fuels)
- Smog alerts and emergency measures during high pollution events
Summary: Key Points About Smog
- Smog is a mixture of pollutants that creates a visible haze in the atmosphere
- The two main types are photochemical smog (from vehicle emissions + sunlight) and industrial smog (from coal burning)
- Smog formation involves complex chemical reactions between primary pollutants
- Weather conditions like temperature inversions and lack of wind make smog worse
- Smog causes respiratory problems, damages plants, reduces visibility and harms ecosystems
- Prevention measures include emission controls, renewable energy and better urban planning
- While many developed countries have reduced smog, it remains a serious problem in many developing nations
Taking Action: What Can You Do?
Even as students, you can help reduce smog:
- Walk, cycle, or use public transport instead of cars when possible
- Encourage parents to maintain vehicles properly and avoid unnecessary idling
- Support tree planting initiatives (trees help filter air pollutants)
- Conserve energy at home to reduce power plant emissions
- Learn about air quality in your area and spread awareness
- Consider air pollution when making future career and lifestyle choices