APES Photochemical Smog and Thermal Inversion Study Notes

Introduction to Smog

  • Smog is a portmanteau derived from "smoke" and "fog."
  • It is typically categorized into two main types:
    • Winter smog
    • Summer smog (also known as photochemical smog)

Enduring Understanding and Learning Objectives

  • Enduring Understanding (STB-2): Human activities significantly impact the atmosphere through physical, chemical, and biological consequences.
  • Learning Objective (STB-2.B): Students should be able to explain the causes and effects of photochemical smog, as well as methods to reduce it.
  • Learning Objective (STB-2.C): Students should be able to describe thermal inversion and its relationship with pollution.

Winter Smog

  • Composition: Consists of smoke and other various pollutants.
  • Sources: Primarily results from increased energy demands during winter months.
    • Furnaces
    • Wood-burning stoves and fireplaces
    • Power plants
    • Traffic
    • Industry
  • Exacerbation: Its severity is often worsened by temperature inversions (thermal inversions).

Thermal Inversion (Temperature Inversion)

  • Definition: A reversal of the normal temperature gradient in the troposphere.
  • Normal Tropospheric Behavior: Air in the troposphere typically becomes colder as altitude increases.
  • Inversion Phenomenon: Occurs when a layer of cold air at or near the Earth's surface becomes trapped underneath a layer of warmer air at higher altitudes. Essentially, the air at ground level is colder than the air above it.
  • Location: Often forms in valleys due to geographical features.
  • Impact on Pollution: This inversion layer acts as a lid, trapping pollutants (such as smoke, smog, and particulates) close to the ground, preventing their dispersion into the higher atmosphere.
    • Essential Knowledge (STB-2.C.2): Thermal inversion traps pollution close to the ground, particularly smog and particulates.
  • Example: Denver's "brown cloud" is a well-known example of pollution exacerbated by thermal inversions.

Photochemical Smog (Summer Smog)

  • Definition: Also known as summer smog.
  • Formation (Essential Knowledge STB-2.B.1): It is formed when nitrogen oxides (NOextx\text{NO}_ ext{x}) and volatile organic hydrocarbons (VOCs), which are primary pollutants, react with heat and sunlight to produce a variety of secondary pollutants. These secondary pollutants then constitute photochemical smog.
  • Key Reactants:
    • Nitrogen Oxides (NOextx\text{NO}_ ext{x}**):
      • Include nitric oxide (NO\text{NO}) and nitrogen dioxide (NOext2\text{NO}_ ext{2}). (Note: While sometimes Next2extO\text{N}_ ext{2} ext{O} is considered a nitrogen oxide, it is not strictly part of the NOextx\text{NO}_ ext{x} category in the context of smog formation.)
      • Sources: Produced from any combustion process, especially at high temperatures.
        • Internal combustion engines (e.g., motor vehicles).
        • Combustion of fossil fuels.
        • Household fires (from heat, cooking).
    • Volatile Organic Compounds (VOCs):
      • Characteristics: These are organic compounds that evaporate or sublimate at room temperature.
      • Examples: Formaldehyde and gasoline are common anthropogenic examples.
      • Natural Sources (Essential Knowledge STB-2.B.4): Trees are a significant natural source of VOCs.
    • Heat
    • Sunlight
    • Oxygen
    • Water (implicitly involved in some reactions, e.g., forming nitric acid)
  • Products (Secondary Pollutants): The reaction of these primary pollutants with heat and sunlight produces a complex mixture of secondary pollutants, including:
    • Ozone (Oext3O_ ext{3}) - a major component and oxidant.
    • Nitric acid
    • Peroxyacytyl nitrate (PAN)
    • Aldehydes
    • Acrolein
  • Environmental Factors Affecting Formation (Essential Knowledge STB-2.B.2):
    • Time of Day (Essential Knowledge STB-2.B.3):
      • Nitrogen oxide (NO\text{NO}) is produced early in the day, often peaking during morning commute times.
      • Ozone concentrations peak in the afternoon because its production is heavily reliant on sunlight.
    • Season (Essential Knowledge STB-2.B.3): Ozone concentrations are generally higher in the summer due to more intense sunlight and higher temperatures, which facilitate the chemical reactions. Photochemical smog occurs most often in summer because heat and sunlight levels are highest then.
    • Climate: More common in sunny, warm, and dry climates. Can occur year-round in tropical regions or during dry seasons.
    • Urban Areas (Essential Knowledge STB-2.B.5): Photochemical smog frequently forms in urban areas principally due to the high concentration of motor vehicles, which are significant sources of NOextx\text{NO}_ ext{x} and VOCs.
  • Global Prevalence: Present in all modern cities.

Cities Most Affected by Smog (as of November 20212021)

  • Worldwide:
    1. Beijing, China
    2. Ahvaz, Iran
    3. Ulaanbaatar, Mongolia
    4. Lahore, Pakistan
    5. New Delhi, India
    6. Rijadh, Saudi Arabia
    7. Cairo, Egypt
    8. Dhaka, Bangladesh
    9. Moscow, Russia
    10. Mexico City, Mexico
  • In the US:
    1. Los Angeles, CA
    2. Visalia, CA
    3. Bakersfield, CA
    4. Fresno, CA
    5. Hanford, CA
    6. Sacramento, CA
    7. Houston, TX
    8. Dallas / Fort Worth, TX
    9. Washington D.C. / Baltimore, MA and VA
    10. El Centro, CA

Health Effects of Smog

  • Varied Impacts: Due to the diverse chemical composition of smog, it causes a wide range of health effects.
  • Vulnerable Populations: The young, the elderly, and individuals with pre-existing health conditions are most adversely affected.
  • Immediate Symptoms:
    • Irritation of mucus membranes (eyes, nose, mouth, throat).
    • Can weaken the primary immune system.
  • Respiratory Problems (Essential Knowledge STB-2.B.7):
    • Shortness of breath
    • Coughing
    • Wheezing
    • Asthma
    • Bronchitis
    • Emphysema
    • Potentially leading to cancer
  • Long-Term Exposure (Linked With):
    • Alzheimer's disease
    • Birth defects and low birth weight
    • Premature death
  • Mortality Statistics: An estimated 7 million7 \text{ million} premature deaths worldwide each year are attributed to air pollution/smog (according to the WHO).

Methods to Reduce Photochemical Smog

  • Reduction of Precursors (Essential Knowledge STB-2.B.6): Photochemical smog can be specifically reduced by decreasing the emissions of nitrogen oxides (NOextx\text{NO}_ ext{x}) and VOCs.
  • General Strategies:
    • Energy Transition: Switch to renewable energy resources to reduce reliance on fossil fuel combustion.
    • Emission Control: Implement measures to reduce overall pollutant emissions from industrial and vehicular sources.
    • Energy Conservation and Efficiency: Conserve energy and improve energy efficiency in homes, transportation, and industry.
    • Product Choices: Use environmentally friendly products that do not release pollutants, particularly VOCs.
    • Waste Management: Practice reduce, reuse, and recycle principles to minimize resource consumption and pollution.
    • Regulation and Monitoring: Regulate and continuously monitor air quality to identify problem areas and enforce emission standards.
    • Local Consumption: Buy local products to reduce emissions associated with long-distance shipping and transportation.
  • Technical Solutions:
    • Smog Filtering Towers: Large, outdoor air purifiers designed to filter pollutants directly from the air. One Dutch company converts collected carbon into synthetic diamonds.
    • Catalytic Buildings: Buildings designed with materials that can chemically break down pollutants in the air, similar to a catalytic converter. These can also function in carbon sequestration.