Acid rain is a product of sulfur dioxide (SO2) and nitrogen oxides (NOx) mixing with water in the atmosphere, then falling to the ground as rain or snow. In some cases, acidic air pollutants also can settle to the earth on their own, before combining with moisture - a process known as "dry deposition."

SO2 emissions are released from power plants that burn oil or coal to generate electricity and from industrial combustion of fossil fuels. NOx emissions come from the same sources, as well as from motor vehicles with gasoline- and diesel-powered engines.

These pollutants can be carried long distances on prevailing winds aloft. A substantial portion of the acid deposition in Massachusetts is attributable to SO2 and NOx emissions from out-of-state sources.

Environmental Effects of Acid Rain

When acid air pollutants fall to the ground in rain, in snow or on their own, they lower the pH levels of lakes, rivers, and soils and damage forests, leading to a range of environmental problems. Specifically, acid deposition:

  • Can make lakes and streams so acidic that survival becomes difficult if not impossible for many species of fish and invertebrates.
  • Makes plants, their leaves and their root systems more likely to dry out or accumulate dangerous toxic metals.
  • Dissolves and washes away calcium and other minerals from the soil, thereby robbing ecosystems of nutrients essential for plant growth.
  • Causes slow but steady damage to building features and materials, including limestone sculptures and paint.

Learn more about the effects of acid rain from the U.S. Environmental Protection Agency (EPA).

What Government is Doing to Address Acid Rain

In 1990, the U.S. Congress created the Acid Rain Program within the federal Clean Air Act to reduce the adverse effects of acid rain through annual reductions in SO2 and NOx from power plants that burn fossil fuels.  Massachusetts also has established limits on SO2 and NOx for power plants, and works closely with EPA to ensure the goals of the Acid Rain Program are met.

As a member of the Conference of New England Governors and Eastern Canadian Premiers, Massachusetts committed in 1998 to a 50 percent reduction in regional SO2 emissions by 2010 and a regional NOx emissions reduction of between 20 percent and 30 percent by 2007 (which was achieved). To achieve these goals, Massachusetts has required further reductions in SO2 and NOx (and in some cases mercury and carbon dioxide) from power plants and other large sources.

To track the effectiveness of these nationwide and regional efforts, Massachusetts hosts two precipitation monitoring sites for the National Atmospheric Deposition Program (NADP). Located in Ware and Truro, these monitoring stations measure the acidity of rain and snow.

Further, the Department of Environmental Protection (MassDEP) provides funding for the Acid Rain Monitoring Project led by the Water Resources Research Center at the University of Massachusetts Amherst.

Since 1983, this project has relied on citizen volunteers to collect samples from lakes and streams across the state. The map below shows sampling sites. See a larger, printer-friendly version of the map pdf format of Map of Massachusetts Acid Rain Monitoring Sites
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Acid Rain Monitoring Project Sample Sites


Long-Term Acid Rain Trends

Nationally, according to EPA's Acid Rain Progress Report, SO2 emissions have dropped by 43 percent and NOx emissions by 51 percent from the electric power sector since 1990.

These and other emission reductions have led to a significant decrease in acid deposition. However, monitoring data collected by the Massachusetts Acid Rain Monitoring Project indicate that surface water bodies have been slow to recover, showing only slight to no improvement in acidic level, suggesting that long-term monitoring of surface waters will be needed to assess how quickly their ecosystems are recovering with SO2 and NOx emissions from power plants, factories and motor vehicles now significantly reduced.

The Committee on the Environment of the Conference of New England Governors and Eastern Canadian Premiers mapped the sensitivity of regional forests to acid deposition. This approach uses an ecological assessment of the "nutrient budgets" of these wooded areas and can be used to determine which forests are most susceptible to acid precipitation.

New England Forest Areas Most Sensitive to Acid Deposition

New England Forest Areas Most Sensitive to Acid Deposition
Critical load mapping was performed to estimate the impacts of sulfur and nitrogen compound deposition on forest ecosystem health across the region. As soil becomes more acidic, it loses nutrients that are critical to plant growth. The map shows how extensive this nutrient loss has been in southern New England. The problem grows progressively worse as the color shading of forested areas changes from green to yellow, orange and red.