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Acid Rain

Emissions from power plants, industry, and vehicles react with water in the air, which then falls to the ground as rain or snow containing nitric and sulfuric acids.

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. A substantial portion of acid deposition in Massachusetts can be attributed 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.

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What Government is Doing About Acid Rain

The U.S. Congress in 1990 created a federal Acid Rain Program to reduce the adverse effects of acid rain through annual emission reductions from power plants that burn fossil fuels. Massachusetts also has established emission limits on power plants and works closely with the U.S. Environmental Protection Agency (EPA) to address acid rain.

As a member of the Conference of New England Governors and Eastern Canadian Premiers, Massachusetts more than two decades ago committed to a 50 percent reduction in regional SO2 emissions by 2010 and a 20 percent to 30 percent reduction in regional NOx emissions by 2007 (which was achieved).

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 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.

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Long-Term Acid Rain Trends

According to EPA, electric power sector emissions of both SO2 and NOx emissions by more than half since 1990.

These and other emission reductions have led to a significant decrease in acid deposition. But monitoring data collected in Massachusetts indicates that bodies of water have been slow to recover, showing only slight improvement, so long-term monitoring is needed to assess how quickly their ecosystems bounce back.