Outdoor Air Quality: Composition and Contamination

The Earth's atmospheric environment, outside of enclosed spaces, is a complex mixture of gases, particulate matter, and aerosols. Its composition varies based on location, altitude, and meteorological conditions. Understanding its components and sources of contamination is crucial for public health and environmental protection.

Primary Components of Unconfined Atmospheric Air

• Nitrogen (N₂): The most abundant gas, comprising approximately 78% of the atmosphere by volume.
• Oxygen (O₂): Essential for respiration and combustion, making up roughly 21% of the atmosphere.
• Argon (Ar): An inert noble gas, present at about 0.93%.
• Other Gases: Trace amounts of other gases such as carbon dioxide (CO₂), neon (Ne), helium (He), methane (CH₄), krypton (Kr), hydrogen (H₂), and water vapor (H₂O).

Common Contaminants and Pollutant Categories

Various natural and anthropogenic sources introduce substances that degrade air quality, posing risks to human health and ecosystems. These contaminants can be categorized as follows:

Particulate Matter (PM)

• PM₁₀: Inhalable coarse particles, with diameters generally 10 micrometers and smaller.
• PM_2.5: Fine inhalable particles, with diameters generally 2.5 micrometers and smaller. These are especially concerning due to their ability to penetrate deep into the lungs and bloodstream.
• Sources: Combustion processes, industrial activities, construction, agriculture, and windblown dust.

Gaseous Pollutants

• Ozone (O₃): A secondary pollutant formed through photochemical reactions involving nitrogen oxides and volatile organic compounds.
• Nitrogen Oxides (NO_x): Primarily nitrogen dioxide (NO₂) and nitric oxide (NO), emitted from combustion sources, especially vehicles and power plants.
• Sulfur Dioxide (SO₂): Released mainly from burning fossil fuels containing sulfur, industrial processes, and volcanic activity.
• Carbon Monoxide (CO): A colorless, odorless gas produced by incomplete combustion of carbon-containing fuels.
• Volatile Organic Compounds (VOCs): A diverse group of organic chemicals that readily evaporate at room temperature. Sources include industrial processes, vehicle emissions, and solvents.
• Lead (Pb): A heavy metal historically associated with gasoline additives, but now primarily from industrial sources.

Other Pollutants

• Ammonia (NH₃): Primarily from agricultural sources.
• Persistent Organic Pollutants (POPs): Toxic chemicals that persist in the environment and bioaccumulate in living organisms.

Sources of Atmospheric Contamination

The origins of these contaminants can be broadly classified as:

• Anthropogenic Sources: Human activities such as industrial processes, transportation, energy production, agriculture, and waste disposal.
• Natural Sources: Volcanic eruptions, wildfires, dust storms, and biogenic emissions from vegetation.

Impacts of Degraded Outdoor Air Quality

Exposure to contaminated air can lead to a range of adverse health effects, including respiratory illnesses, cardiovascular disease, and cancer. It also contributes to environmental problems such as acid rain, smog formation, and climate change.