Section 15 vid 3

Photochemical Smog Formation

• Definition: Photochemical smog is a mixture of primary and secondary pollutants, primarily featuring photochemical ozone as its main component.

• Components: Contains over 100 chemicals, including volatile organic compounds (VOCs) and nitrogen oxides (NOx).

• Formation Process:

  • VOCs and NOx react in the presence of UV light and oxygen, leading to the formation of tropospheric ozone.

  • This process is a photochemical reaction, making tropospheric ozone a secondary pollutant.

• Health Effects: Resulting smog can cause irritated eyes and throats, particularly on sunny days in urban areas.

Urban Areas Affected

• Major cities known for dense photochemical smog include:

  • Argentina: Buenos Aires

  • Brazil: Sao Paulo

  • Mexico: Mexico City

  • China: Beijing, Shanghai

  • Australia: Sydney

  • USA: Salt Lake City, Denver, Los Angeles

Historical Events

• Great Smog of London (1952): Resulted in 4,000 deaths over five days following earlier smog events in 1880 and 1911.

• Other significant smog incidents:

  • Industrial smog disasters killed approximately 2,500 people in 1956, '57, and 1962.

  • The Clean Air Act of 1956 in the UK improved air quality regulations.

  • The Killer Smog of Donora (1948): 20 deaths and 7,000 illnesses in a Pennsylvania town due to emissions from industrial plants.

  • NYC Smog (1963): Injured and sickened thousands.

Types of Smog

• Photochemical Smog vs. Industrial Smog:

  • Photochemical Smog: Results from chemical reactions involving primary pollutants and sunlight.

  • Industrial Smog: Formed from sulfur dioxide and soot particulates due to the burning of fossil fuels like coal and oil.

• Chemical Reactions:

  • Sulfur dioxide reacts with oxygen to form sulfur trioxide, which further reacts with water vapor, creating sulfuric acid leading to acid rain.

  • SPM (Suspended Particulate Matter) gives industrial smog its gray color.

Clean Air Regulations

• Emissions Control: Developed countries enforce rigorous emissions control policies for cleaner air in industrial plants, favoring tall smokestacks and pollution controls.

• Developing countries, e.g., Ukraine, India, and China, still struggle with heavy industrial smog due to inadequate regulations.

• The Clean Air Act of 1970 in the USA set emission standards for vehicles and industries, with amendments in 1977 and 1990.

Factors Influencing Smog Formation

• Influenced by:

  • Type of fuels used

  • Quantity of industrial activities

  • Population density

  • Regional topography and climate

• Mitigating Factors:

  • Precipitation helps cleanse air by coagulating pollutants.

  • Wind can dilute and remove polluted air, but urban buildings and mountains can obstruct airflow.

Temperature Inversion

• Normal Conditions: Warm air containing pollutants mixes with cool air above, dispersing them.

• Inversion Conditions: A temperature inversion traps pollutants near the ground when a layer of warm air overlays cooler air, leading to smog accumulation.

  • Types of Inversion:

    • Subsidence Inversion: Occurs when warm air at high altitudes moves over a cooler region, trapping pollutants.

    • Radiation Inversion: Happens when ground-level air cools more rapidly than air above, creating a warm layer that keeps pollutants trapped.

• Geographical Impact: Regions with surrounding mountains or valleys, like Los Angeles and Mexico City, are prone to prolonged inversions during summer months, exacerbating smog issues.

Urban Heat Islands

• Urban infrastructure (e.g., asphalt, concrete) absorbs heat, contributing to higher temperatures in cities:

  • Leads to enhanced photochemical reactions and greater smog formation.

• Mitigation Strategies:

  • Implementing green roofs

  • Stricter vehicle emissions policies

  • Increasing green spaces and tree conservation.

Conclusion

• Key Takeaway: Prolonged thermal inversions can increase pollution residence time due to geographical features.

  • Remarkable events of photochemical smog stress the importance of understanding and addressing air pollution for public health.