13.2 Air pollution sources and types

Air pollution comes from both natural and human-made sources. Volcanoes, wildfires, and dust storms release pollutants naturally. Meanwhile, vehicles, industries, and agriculture contribute to human-caused pollution. Understanding these sources is crucial for addressing air quality issues.

Air pollutants can be primary, emitted directly, or secondary, formed through chemical reactions. Major pollutants include particulate matter, ozone, and nitrogen oxides. These substances can lead to smog formation and acid rain, causing significant health and environmental impacts.

Air Pollution Sources and Types

Sources of air pollution

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• Natural sources
  • Volcanic eruptions emit sulfur dioxide, ash, and other particulates into the atmosphere
  • Wildfires release smoke, particulate matter, and volatile organic compounds (VOCs) during combustion (forest fires)
  • Dust storms in arid regions (Sahara Desert) generate large amounts of particulate matter that can be transported long distances
  • Biological decay of organic matter produces methane and other organic compounds (wetlands)
• Anthropogenic sources
  • Transportation sector, including vehicles (cars, trucks), aircraft, and ships, emits carbon monoxide, nitrogen oxides, and particulate matter
  • Industrial processes such as manufacturing and power generation (coal-fired power plants) release sulfur dioxide, nitrogen oxides, and particulate matter
  • Agricultural activities, including livestock farming (cattle) and fertilizer use, emit ammonia and methane
  • Residential and commercial sources, such as heating, cooking, and waste incineration, produce particulate matter and VOCs (wood-burning stoves)

Primary vs secondary pollutants

• Primary air pollutants are emitted directly from sources
  • Carbon monoxide from vehicle exhaust
  • Sulfur dioxide from coal-fired power plants
  • Particulate matter from construction sites (dust)
• Secondary air pollutants form in the atmosphere through chemical reactions involving primary pollutants
  • Ozone forms from nitrogen oxides and VOCs in the presence of sunlight
  • Sulfuric acid forms from sulfur dioxide and water vapor
  • Secondary particulate matter forms from reactions involving ammonia, nitrogen oxides, and sulfur dioxide (ammonium sulfate)

Characteristics of major air pollutants

• Particulate matter (PM)
  • Solid or liquid particles suspended in the air classified by size: PM10 (diameter < 10 μm) and PM2.5 (diameter < 2.5 μm)
  • Health effects include respiratory issues (asthma), cardiovascular disease, and premature death
• Ozone (O3)
  • Highly reactive gas formed in the troposphere as a component of photochemical smog
  • Health effects include respiratory irritation, reduced lung function, and aggravation of asthma (wheezing)
• Nitrogen oxides (NOx)
  • Includes nitric oxide (NO) and nitrogen dioxide (NO2), contributes to the formation of ozone and acid rain
  • Health effects include respiratory issues, increased susceptibility to infections (pneumonia), and aggravation of asthma

Formation of smog and acid rain

• Photochemical smog
  1. NOx and VOCs react in the presence of sunlight in urban areas with high concentrations of these pollutants (Los Angeles)
  2. Reactions produce ozone and other oxidants
  3. Stagnant atmospheric conditions and temperature inversions trap pollutants near the ground
• Acid rain
  1. SO2 and NOx are oxidized in the atmosphere to form sulfuric acid (H2SO4) and nitric acid (HNO3)
  2. Acids dissolve in water droplets and fall as acid rain, snow, or fog
  • Environmental effects include acidification of lakes and streams (fish kills), damage to forests and crops (leaf damage), and corrosion of buildings and infrastructure (limestone erosion)