Acid rain comes in many forms: rain, snow, sleet, hail, and fog (wet deposition), and as acid particles, aerosols, and gases (dry deposition). Acid deposition forms when sulfur dioxide (SO2) and nitrogen oxides (NOx) combine with moisture in the atmosphere to produce sulfuric acid and nitric acid.
By the 1960s, it became clear to scientists that acid deposition was devastating natural resources across New York. The Catskill and Adirondack Mountains were particularly hard hit. Soils were becoming too acidic to maintain healthy forests with noticeable tree die-offs at higher elevations. Many lakes, mountain streams, and some rivers were unable to support healthy populations of fish.
Causes of Acid Rain
Several sources that contribute to creating acid rain include:
- Combustion of fossil fuels (coal, oil, natural gas, wood, etc.) for energy.
- SO2 emissions have been reduced in modern power plants with the introduction of low-sulfur fuel and the use of scrubbers in the smokestacks to remove the sulfur from the emissions. According to the U.S. Energy Information Administration, the US consumption of coal for electric generation has dropped by over 25 percent in the last six years.
- NOx emissions are reduced by using low NOx combustors and selective catalytic reduction (SCR) or non-selective catalytic reduction (NSCR) in the smokestacks.
- Emissions from motor vehicles, airplanes, power plants, and industries.
- 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.
- Starting in 1996, automobiles have been equipped with onboard diagnostics (OBDII). This equipment signals the driver when the emission controls are not working, causing the car to create more pollution.
- Changes continue to be to reduce the amount of sulfur in vehicle fuels. Cars using low-sulfur fuels emit less SO2 and NOx. This is because catalytic converters work more efficiently with low-sulfur fuel.
- Emissions of SO2 and NOx from the Midwest.
- The Clean Air Interstate Rule (CAIR) and the Acid Rain Program (ARP) were 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 improve air quality by reducing power plant emissions that cross state lines. This pollution can contribute to smog (ground-level ozone) and soot (fine particle pollution) in other states.
Effects of Acid Rain on the Environment
Aquatic - Freshwater macroinvertebrates, plants, and fish populations are damaged when acidic water disrupts their reproductive cycle. Aluminum leaches from the soil into the water, altering the chemistry and clogging the fish's gills. As water bodies become acidified, one species after another disappears. In addition to sensitive lakes, the Adirondack region includes thousands of miles of streams and rivers that are also sensitive to acidic deposition.
Wildlife - Acid rain lowers the biological productivity of lakes and reduces the amount of forage fish available to loons. Toxicity from mercury pollution of water bodies can lead to decreased reproductive success of loons as well.
Forests - Sulfur and nitrogen deposition have caused adverse impacts on highly-sensitive forest ecosystems, especially the high-elevation, spruce-fir forests in the eastern United States. Forests are damaged because acid precipitation drains nutrients from the soil. Excess nitrogen in the air also may adversely affect tree growth. Evidence of decreased growth and dieback has been found in the Adirondacks.
Visibility - The same pollutants that cause acid rain can degrade air quality and significantly reduce visibility, even in remote areas like the Adirondack Mountains.
Architecture - For buildings, bridges and cultural resources, acid deposition can cause damage. The sulfuric acid can have a corrosive effect on limestone and marble buildings and sculptures. Dry deposition can even be more damaging to stone than wet deposition for these structures.
Human Health - Walking in acid rain is no more dangerous to people than walking in non-acid rain. However, the pollutants that cause acid rain can be harmful to people. SO2 and NO2 can react in the atmosphere to form fine sulfate and nitrate particles. These particles can enter the lungs and cause lung disease, heart attacks, and difficulties for people with asthma. Acidified water can also cause metals to be leached from the soil into streams, lakes and reservoirs or old lead and copper pipes into home water supplies, causing serious illness.