FORMATION OF ACID RAIN

• The process that leads to acid rain begins with the burning of fossil fuels.

• Burning, or combustion, is a chemical reaction in which oxygen from the air combines with carbon, nitrogen, sulfur, and other elements in the substance being burned.

• The new compounds formed are gases called oxides. When sulfur and nitrogen are present in the fuel, their reaction with oxygen yields sulfur dioxide and various nitrogen oxide compounds.

• In the United States, 70 percent of sulfur dioxide pollution comes from power plants, especially those that burn coal. In Canada, industrial activities, including oil refining and metal smelting, account for 61 percent of sulfur dioxide pollution.

• Nitrogen oxides enter the atmosphere from many sources, with motor vehicles emitting the largest share—43 percent in the United States and 60 percent in Canada.

• Once in the atmosphere, sulfur dioxide and nitrogen oxides undergo complex reactions with water vapor and other chemicals to yield sulfuric acid, nitric acid, and other pollutants called nitrates and sulfates.

• The acid compounds are carried by air currents and the wind, sometimes over long distances.

• When clouds or fog form in acid-laden air, they too are acidic, and so is the rain or snow that falls from them.

• Acid pollutants also occur as dry particles and as gases, which may reach the ground without the help of water.

• When these “dry” acids are washed from ground surfaces by rain, they add to the acids in the rain itself to produce a still more corrosive solution.

• The combination of acid rain and dry acids is known as acid deposition.

Major Air Pollutants

Sources of major air pollutants include individual actions, such as driving a car, and industrial activities, such as manufacturing products or generating electricity. Note: 1 cubic meter (1m³) is equal to 35.3 cu ft; 1 milligram (1 mg) is equal to 0.00004 oz; 1 microgram (1µg) is equal to 0.00000004 oz.

Pollutant

Major Sources

Notes

Carbon monoxide (CO)	Motor-vehicle exhaust; some industrial processes	Health standard: 10 mg/m3 (9 ppm) over 8 hr; 40 mg/m3 over 1 hr (35 ppm)
Sulfur dioxide (SO2)	Heat and power generation facilities that use oil or coal containing sulfur; sulfuric acid plants	Health standard: 80 µg/m3 (0.03 ppm) over a year; 365 µg/m3 over 24 hr (0.14 ppm)
Particulate matter	Motor-vehicle exhaust; industrial processes; refuse incineration; heat and power generation; reaction of pollution gases in the atmosphere	Health standard: 50 µg/m3 over a year; 150 µg/m3 over 24 hr; composed of carbon, nitrates, sulfates, and many metals including lead, copper, iron, and zinc
Lead (Pb)	Motor-vehicle exhaust; lead smelters; battery plants	Health standard: 1.5 µg/m3 over 3 months
Nitrogen dioxide (NO2)	Motor-vehicle exhaust; heat and power generation; nitric acid; explosives; fertilizer plants	Health standard: 100 µg/m3 (0.05 ppm) over a year; reacts with hydrocarbons and sunlight to form photochemical oxidants
Ozone (O3)	Formed in the atmosphere by reaction of nitrogen oxides, hydrocarbons, and sunlight	Health standard: 235 µg/m3 (0.12 ppm) over 1 hr