APES Notes 7.2 - Photochemical Smog

Photochemical Smog Overview

  • Objective: Explain causes, effects, and reduction methods for photochemical smog.

  • Skill Practice: Describe relationships between variables in a data set.

Key Precursors of Photochemical Smog

Nitrogen Dioxide (NO2)

  • Role: Primary precursor; decomposes under sunlight to form nitric oxide (NO) and free oxygen.

  • Ozone Formation:

    • The free oxygen combines with O2 to form ozone (O3).

    • Ozone is a secondary pollutant, formed via reactions involving nitrogen dioxide.

Volatile Organic Compounds (VOCs)

  • Definition: Organic chemicals that easily evaporate at room temperature.

  • Examples: Acetone (nail polish remover), gasoline, and emissions from petrochemical processes.

  • Natural Sources: Emissions from coniferous trees contribute to VOCs.

Ozone (O3)

  • Nature: Secondary air pollutant, primarily formed in the troposphere.

  • Effects:

    • Respiratory irritant to humans.

    • Damages plant stomata, inhibiting growth.

Environmental Conditions Required for Smog Formation

Sunlight

  • Importance: Breaks down nitrogen dioxide (NO2) and drives ozone production.

Temperature

  • Impact: Warmer temperatures increase reaction rates for smog formation and accelerate VOC evaporation.

Normal Ozone Formation Process

  1. Morning Traffic: Increases NO2 production.

  2. Sunlight Activation: Breaks NO2 into NO and free O.

  3. Ozone Creation: Free O combines with O2 to produce O3.

  4. Nighttime Process: At night, ozone tends to recombine with NO, reducing overall ozone levels.

Conditions Leading to Photochemical Smog

Introduction of VOCs

  • Dynamics Altered: VOCs bind with NO, prevent it from recombining with ozone, leading to ozone buildup.

  • Result: The combination of ozone and these photochemical oxidants generates photochemical smog.

Factors Increasing Photochemical Smog Formation

  • Increased Traffic: More vehicles contribute higher NO2 emissions.

  • Higher VOC Emissions: Urban areas with more industrial activities produce significant VOCs.

  • Warmer Temperatures: Cause increased evaporation of VOCs; maximize ozone production.

  • Urban Heat Island Effect: Urbanization can lead to higher temperatures contributing to smog.

Impacts of Smog

Environmental Effects

  • Photosynthesis Reduction: Smog blocks sunlight, inhibiting plant growth.

  • Respiratory Issues: Irritates respiratory tracts of humans and animals.

Human Health Effects

  • Health Conditions: Aggravates asthma, COPD, bronchitis, and other respiratory diseases.

  • Eye Irritation: Ozone can be harmful to eyes.

Economic Costs

  • Lost Productivity: Illness due to smog leads to missed work.

  • Healthcare Costs: Linked to premature deaths and chronic health issues.

  • Agricultural Impact: Smog can negatively affect crop yields due to wider dispersal beyond urban areas.

Methods to Reduce Photochemical Smog

  • Vehicle Reduction: Lower number of vehicles and travel distance to decrease NO2.

  • Gasoline Usage Minimization: Decreasing gasoline consumption reduces VOC emissions.

  • Renewable Energy: Utilize sources like solar, wind, and hydroelectric power to avoid nitrogen oxide emissions.

  • Fossil Fuel Transition: Switching from coal to natural gas results in lower NO2 emissions.