Enduring Understanding:

• Human activities have physical, chemical, and biological consequences for the atmosphere.

Learning Objective:

• Explain the causes and effects of photochemical smog and methods to reduce it.

Essential Knowledge:

• Photochemical smog is formed when nitrogen oxides and volatile organic hydrocarbons react with heat and sunlight to produce a variety of pollutants.

• Many environmental factors affect the formation of photochemical smog.

• Nitrogen oxide is produced early in the day. Ozone concentrations peak in the afternoon and are higher in the summer because ozone is produced by chemical reactions between oxygen and sunlight.

• Volatile Organic Compounds (VOCs), such as formaldehyde and gasoline, evaporate or sublimate at room temperature. Trees are a natural source of VOCs.

• Photochemical smog often forms in urban areas because of the large number of motor vehicles there.

• Photochemical smog can be reduced through the reduction of nitrogen oxide and VOCs.

• Photochemical smog can harm human health in several ways, including causing respiratory problems and eye irritation

Volatile Organic Hydrocarbons/Compounds (VOCs)

• VOCs are called such because they can vaporize at room temperature

  • There is an increased presence in the atmosphere

• Anthropogenic VOCs include formaldehyde and gasoline

  • They are also in paint and nail polish remover, but these are in small amounts and less of a concern

• Natural sources include trees and many plant oils

• Note that these are typically things with a distinct smell

Formation

• Recall that during the combustion of fossil fuels, nitrogen oxides and volatile organic compounds/hydrocarbons (VOCs)

  • This occurs early in the day, usually as everyone is the city is driving to work

• Urban areas with many cars are the primary source of smog

  • These release the primary pollutant of nitrogen dioxide (NO2)

• VOCs are also emitted through industry and certain manufacturing processes

• The NO2 reacts with sunlight, heat, and oxygen gas (O2) to create nitrous oxide (NO) and tropospheric ozone (O3)

  • Ozone concentrations peak in the afternoon with increased sun intensity and temperatures

• In a more stable system, the sun goes down and those two products break down into their original forms again, O2 and NO2

• In the presence of VOCs, they react with the NO to create photochemical oxidants

  • This depletes the NO in the atmosphere, meaning the O3 cannot break apart at the end of the day

• The ozone and photochemical oxidants combine to create photochemical smog

• Another secondary pollutant of those processes is PANs

• Ozone levels are higher in sunny, warm climates, and in the summer

Human Health Impacts

• Photochemical smog

  • Irritates the eyes, nose, and throat

  • Can worsen existing heart and lung conditions

  • Regular long-term exposure can cause lung cancer

• Tropospheric ozone

  • Effects in humans

    • Can worsen bronchitis and emphysema

    • Can trigger asthma

    • Causes permanent damage to lung tissue

  • Additional environmental impacts

    • Can enter plants through the stomata and burn plant tissue

    • This leads to leaf damage and reduced rates of survival

Methods of Reduction

• Reducing nitrogen oxides

  • Using catalytic converters

    • Required on automobiles

    • They convert NO into O2 and N2

    • CO and hydrocarbons react to produce CO2 and H2O

    • Admittedly increase CO2, but CO is much more dangerous

  • Enforce emissions testing and standards for vehicles

• Reducing VOCs

  • Pump gas at night

  • Follow gasoline refueling instructions for efficient vapor recovery

  • Be careful not to spill fuel

  • Always tighten the gas cap securely

• Improve fuel efficiency

  • Keep engines properly tuned

  • Make sure tired are properly inflated

• Reduce the use of internal combustion

  • Make fewer trips, bike, or walk

  • Use public transportation or carpool

  • Avoid idling your engine

• Reduce health impacts

  • Minimize outdoor activities in the late afternoon in summer or on hot days

  • Wear a mask on hot days and/or if you’re at risk

Catalytic Converter

• It intakes unburned fuel, nitric oxide, nitrogen dioxide, and carbon monoxide

• They go through the converter and come out as carbon dioxide, nitrogen, oxygen, and water