Acid deposition: The process by which acidic pollutants from sources like sulfur dioxide and nitrogen oxides are deposited onto Earth's surface through rain, snow, or fog.
Fossil fuels: Fuels formed from the remains of ancient plants and animals over millions of years. They include coal, oil, and natural gas.
Particulate matter: Tiny solid or liquid particles suspended in the air. Sulfur dioxide can react with other pollutants to form particulate matter, which can be harmful when inhaled.