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Sources and Environmental Impacts of Acid Rain and Acid Deposition

Acid rain is a by-product of our industrialized society. Air pollution combines with water in the atmosphere and falls to the earth as acidic rain or snow. Discussions and reports about acid rain often use the terms acid deposition or atmospheric deposition to describe this return of airborne pollutants to earth. Pollutants can be deposited from the atmosphere in rain or snow (wet deposition) or without precipitation (dry deposition).

While many areas of New York State are not sensitive to acidity because of limestone deposits or soils which neutralize the acid, the Adirondacks, Catskills, Hudson Highlands, Rensselaer Plateau and parts of Long Island are particularly sensitive to acid deposition. The soil and bedrock in these areas are not able to counteract the acid in the rain and snow.

Students and others looking for basic information about acid rain may wish to visit EPA's Acid Rain information webpage.

Sources of Acid Deposition

The primary emissions responsible for acid deposition are sulfur dioxide (SO2) and oxides of nitrogen (NOx) from the combustion of coal, oil and natural gas. The combustion compounds are transformed into sulfuric and nitric acid and transported downwind before they are deposited.

Some of the conveniences we take for granted everyday also lead to the emissions responsible for acid deposition. The burning of fossil fuels to supply the electricity we use is a source of sulfur dioxide and nitrogen oxides. Another source is the burning of fuels to power cars, trucks, buses and airplanes. However, scientists and engineers are working on new ways to reduce harmful emissions.

Image of a smokestack at a factory

Power Plants

Modern power plants use fuel that has had the sulfur reduced before it is burned or the plants use scrubbers in the smokestacks to remove the sulfur from the emissions. NOx emissions are reduced by using specially designed low NOx combustors and selective catalytic reduction (SCR) or non-selective catalytic reduction (NSCR) in the smokestacks.

Automobiles and Other Vehicles

Image of traffic on a highway

Since the mid-1970s, two important features have been added to automobiles - catalytic converters and electronic fuel injection (EFI). Catalytic converters are located in the exhaust system to remove NOx emissions. EFI controls the formation of NOx emissions during combustion of the fuels.

And, starting in 1996, automobiles have been equipped with onboard diagnostics (OBD) that signal the driver when the various components of emission control are not operating properly. Efforts are also being made to reduce the amount of sulfur in vehicle fuels. Cars using low-sulfur fuels not only emit less SO2, but also less NOx, because catalytic converters work more efficiently with low-sulfur fuel.

Each of us can also take part in reducing acid rain. When we turn off a light, we reduce the demand for electricity from power plants. When we car pool, take public transportation, or walk, we reduce automobile emissions. Many small contributions can together make a significant reduction in emissions that lead to acid deposition.

Environmental Impacts of Acid Deposition

Acid deposition damage to trees in the Adirondacks

In the early 1970's, acid deposition was identified as a serious ecological threat to New York State's waters and forests. The primary emissions responsible for acid deposition are sulfur dioxide (SO2) and oxides of nitrogen (NOx) from the combustion of fossil fuels which are transformed and transported downwind before they are deposited. Acid deposition is of particular concern to New York State because of important and sensitive ecosystems which lie immediately downwind of the largest mid-western utilities burning fossil fuels and emitting SO2 and NOx emissions in North America.

An ecosystem is considered sensitive to acid deposition when it lacks adequate soil buffering capacity to counter the acids deposited to it. Sensitive ecosystems include the Adirondack Mountains, the Catskill Mountains and the Hudson Highlands.

Acid deposition also damages building materials by eroding the ornamental facades, statuary and other vulnerable edifices that are an important part of our heritage. In addition to being the main ingredient in acid rain formation, SO2 also leads to sulfate formation; acidic particles that can cause respiratory problems in humans.

In 1984, the "State Acid Deposition Control Act" (SADCA) required the reduction of SO2 emissions from existing sources and further NOx emission controls on new sources in New York State. SADCA also required the Department of Environmental Conservation (DEC) to set an Environmental Threshold Value (ETV) for sulfate deposition. The ETV was set at 20 kilograms per hectare. The Department established the New York State Acid Deposition Network to determine levels of actual deposition in the state for comparison to the ETV and to measure any changes in deposition that might occur as a result of the control program.

Federal Programs

The Acid Rain Program was created under Title IV of the 1990 Clean Air Act Amendments. Its overall goal is to achieve significant environmental and public health benefits through reductions in emissions of SO2 and NOx, the primary causes of acid rain. To achieve this goal at the lowest cost to the public, the program employs both traditional and innovative, market-based approaches for controlling air pollution. Specifically, the program seeks to limit, or "cap," SO2 emissions from power plants at 8.95 million tons annually starting in 2010, authorizes those plants to trade SO2 allowances, and reduces NOx emission rates. In addition, the program encourages energy efficiency and pollution prevention.

The Clean Air Interstate Rule (CAIR) and the Acid Rain Program (ARP) are both cap-and-trade programs designed to reduce emissions of SO2 and NOx from power plants. EPA's Cross-State Air Pollution Rule (CSAPR) has replaced CAIR and began implementation on January 1, 2015.

The CSAPR requires a total of 28 states in the eastern half of the U.S. to significantly improve air quality by reducing power plant emissions of SO2 and NOx that cross state lines and contribute to smog (ground-level ozone) and soot (fine particle pollution) in other states.