Air Pollution Study Notes

Chapter 6: Air Pollution

Learning Objectives

• By the end of this chapter, students will be able to:

  • Describe the composition and structure of the atmosphere.

  • Explain the importance of the ozone layer, its depletion, and specific steps taken to address it.

  • Distinguish between indoor and outdoor air pollution and how these compare among industrialized and less industrialized countries.

  • Identify natural and anthropogenic sources of air pollution.

  • Explain the effects of air pollution on human and ecosystem health.

  • Explain how the Clean Air Act legislation works and describe its outcomes.

Chapter Outline

• 6.1 Composition and Structure of the Atmosphere

• 6.2 Ozone

• 6.3 Outdoor Air Pollution

• 6.4 Indoor Air Pollution

• 6.5 Effects of Air Pollution on Human Health

• 6.6 Acid Rain

• 6.7 Air Pollution Legislation

6.1 Composition and Structure of the Atmosphere

• The atmosphere is the layer of gases surrounding Earth, held by gravity.

• It is a complex system organized into layers that support life.

• Composition of clean dry air:

  • Top four gases make up 99.998% (unpolluted air without water vapor).

  • Nitrogen (N2): 78% - dilutes oxygen, prevents rapid burning, used to make proteins by living organisms.

  • Oxygen (O2): 20.95% - essential for life and aerobic respiration.

  • Argon (Ar): 0.93% - non-reactive, used in light bulbs and to preserve important documents.

  • Carbon Dioxide (CO2): 0.03% - essential for photosynthesis, acts as a heat-trapping blanket to prevent heat escape.

  • Water Vapor: present up to ~4%, contributing to weather patterns.

  • Full table of composition includes gasses like neon, helium, krypton, sulfur dioxide, methane, hydrogen, nitrous oxide, ozone, nitrogen dioxide, iodine, carbon monoxide.

Table 6.1: Average Composition of Clean Dry Air

• Atmospheric Layers:

  • Troposphere: Closest to Earth (~0-18 km); contains convection currents which redistribute heat and moisture.

  • Temperature: Drops from ~17°C (62°F) at sea level to ~-51°C (-60°F) at the tropopause.

  • Contains 75% of the atmosphere's mass; densest layer.

  • Stratosphere: Extending from tropopause to ~50-53 km.

  • Contains nearly 1000 times more ozone than the troposphere and minimal water vapor.

  • Temperature increases with height; prevents convection mixing.

  • The top bound is known as the stratopause.

  • Mesosphere: Extends to 85 km; no ozone; temperature decreases with altitude.

  • Thermosphere: 85-600 km; absorbs UV and X-ray radiation; can reach temperatures of up to 2000°C (3600°F).

  • Exosphere: No clear boundary; pressure and density decline until indistinguishable from outer space.

6.2 Ozone

• Ozone (O3): Molecule of three oxygen atoms; unstable and readily releases one atom, making it a strong oxidizer.

• Formed in the stratosphere from UV radiation's action on O2.

  • UV splits O2 into separate O atoms; which then combine with O2 to form O3.

  • Absorbs UV radiation, warming the stratosphere and creating a dynamic balance of its creation and destruction.

• Ozone Layer:

  • Located 17-30 km above Earth; absorbs harmful UV radiation, protecting life on Earth.

  • UV Types:

  • UV-A: Least harmful, not absorbed by ozone.

  • UV-B: More harmful, partially absorbed by ozone.

  • UV-C: Most harmful, strongly absorbed and does not reach Earth.

• Disruption of balance, leading to ozone depletion results in increased UV-B radiation on Earth's surface, causing health and environmental problems.

6.2.1 The Ozone Hole

• Area of reduced ozone layer thickness, primarily over Antarctica, intensifying every spring since first reported in 1985.

• The polar vortex isolates air from the surrounding atmosphere, conditions promote ozone-depleting reactions when sunlight returns in spring.

6.2.2 Ozone Depletion

• Ozone depletion occurs when breakdown rates of ozone exceed its formation rates, primarily due to human activities.

• Chlorofluorocarbons (CFCs): Stable, man-made compounds responsible for ozone depletion upon UV exposure, releasing chlorine atoms that deplete ozone.

  • A single chlorine atom can destroy ~100,000 ozone molecules before being neutralized.

6.2.3 The Montreal Protocol

• Established in 1987 to phase out CFCs and related compounds, leading to measurable recovery of the ozone layer.

• U.S. laws such as amendments to the Clean Air Act support this effort, mandating reduction in ozone-depleting substances and stimulating alternative solutions.

6.3 Outdoor Air Pollution

• Definition: Introduction of harmful substances into the atmosphere, resulting from both natural and anthropogenic sources.

  • Examples: Dust storms, forest fires (natural) and vehicle emissions, industrial processes (anthropogenic).

• Pollutants:

  • Primary Pollutants: Released directly from sources (e.g., CO, VOCs, NOx, SO2, particulate matter).

  • Secondary Pollutants: Formed from reactions of primary pollutants (e.g., ground-level ozone).

6.3.1 Criteria Pollutants

• The Clean Air Act established air quality standards for six pollutants:

  1. Carbon Monoxide (CO): Colorless/odorless, produced from combustion; health effects include oxygen delivery reduction.

  2. Ground-level Ozone (O3): Not emitted directly; a product of the interaction between sunlight, heat, VOCs, and NOx.

  3. Nitrogen Dioxide (NO2): Contributes to smog and acid rain; formed during combustion processes.

  4. Sulfur Dioxide (SO2): Produced from fossil fuel combustion; major source of acid rain.

  5. Lead (Pb): Emissions declined due to stricter regulations; still present near industrial areas.

  6. Particulate Material (PM): Includes a mix of solids and liquids, harmful when inhaled; PM2.5 and PM10 noted for health risks.

6.3.2 Volatile Organic Compounds (VOCs)

• Organic gases emitted from sources, both natural (plants) and anthropogenic (household products); significant in ozone formation.

6.3.3 Smog

• Mixture of pollutants leading to brownish haze, categorized as industrial or photochemical smog.

• Industrial smog arises from fossil fuel combustion; photochemical smog results from reactions among primary pollutants with sunlight.

6.3.4 Toxic Pollutants

• Hazardous substances known or suspected to cause serious health effects; primarily from human sources but can also come from natural sources.

6.4 Indoor Air Pollution

• Generally worse than outdoor pollution; indoor spaces often have higher concentrations of toxic pollutants.

• Sources: combustion materials (e.g., tobacco, wood stoves), building materials, cleaning products; radon, outdoor pollutants.

• Risks vary between industrialized (cigarette smoke, radon) and less industrialized countries (soot from biomass).

6.5 Effects of Air Pollution on Human Health

• WHO identifies air pollution as a significant health risk, linking it to respiratory issues, cardiovascular diseases, and more.

• Immediate effects: irritation, headaches; long-term effects: chronic health issues, decreased lung function.

Table 6.2: Sources and Health Effects of Criteria Pollutants

6.6 Acid Rain

• Defined as precipitation with pH < 5.6, typically caused by emissions of NOx and SO2 from fossil fuels.

• Results in environmental damage including nutrient leaching from soils, release of toxic metals, and infrastructure damage.

6.7 Air Pollution Legislation

• Major pollution events in history prompted formal U.S. laws to address air quality concerns.

  • Initial Clean Air Act in 1963; stronger provisions enacted in 1970, establishing the EPA.

  • EPA's Role:

  • Enforces regulations to reduce ambient pollutant levels, manage toxic air pollutants, and phase out ozone-depleting chemicals.

  • State and Local Governments: Tailor solutions to local conditions; responsible for developing State Implementation Plans (SIPs) to meet Clean Air Act standards.

  • Outcomes of the Clean Air Act:

  • Significant reduction in key pollutants; improved air quality despite economic growth; successful management of lead pollution, among other ongoing efforts.