Air Pollution and Atmospheric Chemistry

The U.S. Clean Air Act regulates six 'criteria' pollutants: particulate matter (PM₂.₅ and PM₁₀), ground-level ozone (O₃), carbon monoxide (CO), sulfur dioxide (SO₂), nitrogen dioxide (NO₂), and lead (Pb). PM₂.₅—particles smaller than 2.5 micrometers—penetrates deep into the lungs and enters the bloodstream, causing cardiovascular disease and premature death. The World Health Organization estimates that outdoor air pollution causes 4.2 million premature deaths annually worldwide. Major sources include fossil fuel combustion (vehicles, power plants), industrial processes, and agricultural activities like burning crop residues.