What is “acid rain” and what causes it?
—Jeff Ohmberger, Lincoln, NE
“Acid rain,” also known as acid precipitation and acid deposition, is a broad term used to describe the nitric and sulfuric acids that fall to Earth during rain, snow or fog. These chemicals form in our atmosphere to begin with when pollutants released into the air through the burning of fossil fuels blend with other substances, including water vapor. When it storms, these substances return to the Earth’s surface where they get into rivers, streams and groundwater, literally making these waters more acidic.
This acidity, in turn, damages trees and other plant life and makes it difficult for wildlife—especially aquatic life—to thrive and reproduce. The consequences of acid rain can also be seen in the cracks and discoloration on some building surfaces and on the smoothed and faded facial features on outdoor statues.
Emissions of sulfur dioxide (SO2) and nitrogen oxide (NOx) are primarily to blame. While volcanoes and other natural sources produce these chemicals, too, as much as 95 percent of the SO2 and NOx emitted in North America comes from industrial sources and the tailpipes of cars and trucks.
According to the U.S. Environmental Protection Agency’s (EPA) Acid Rain Program, the highest concentrations of acid rain in the U.S. are in western Pennsylvania and West Virginia. Prevailing winds move large masses of pollutants there from the smokestacks of the many coal-fired and other kinds of power plants dotting the banks of the Ohio River.
Meanwhile, the National Atmospheric Deposition Program (NADP), which monitors the chemistry of precipitation at some 200 locations around the United States, reports high levels of acid rain throughout the entire Northeast, extending from Indiana all the way to the Atlantic coast. NADP’s website features interactive maps detailing acid rain concentrations nationwide.
During the 1980s, public outcry over acid rain resulted in Congress amending the Clean Air Act to impose limits on industrial emissions of SO2 and NOx. While the regulations have helped, many environmentalists think more needs to be done in order to protect plants, wildlife and water throughout the northeastern U.S. and southeastern Canada. “A growing body of evidence shows that without significant additional cuts in acid rain-forming emissions, many of the problems associated with acid rain will persist for many decades,” says Ellen Baum, ecosystem expert at the Boston-based nonprofit organization, Clean Air Task Force.
Since the burning of fossil fuels accounts for most of the troublesome SO2 and NOx emissions, individuals can make a difference by reducing their energy consumption at home and by driving fewer miles in their internal combustion vehicles. Businesses can take similar steps by increasing energy efficiency at the workplace and encouraging employees to carpool or take public transit.
CONTACTS: U.S. EPA Acid Rain Program, www.epa.gov/airmarkets/arp/index.html ; National Atmospheric Deposition Program, (217) 244.5459, nadp.sws.uiuc.edu ; Clean Air Task Force, (617) 624-0234, www.catf.us .