Acidic deposition, also known as acid rain, is caused by emissions of nitrogen oxides (NOx) and sulfur oxides (SOx) which primarily come from power plant emissions. When NOx and SOx react with water, oxygen, and sunlight in the atmosphere, they form acids like sulfuric and nitric acid which fall to earth as rain, snow, or dry deposition. Acidic deposition harms ecosystems by lowering the pH of soils and surface waters outside of their normal acidic range, which can damage forests and aquatic wildlife. The EPA addresses acidic deposition through its Acid Rain Program that implements emissions trading of SOx and NOx to reduce deposition and restore damaged environments.

1. Acidic Deposition (Acid Rain) Forms of Acidic Deposition A. Wet Deposition: Rain, Snow, Fog B. Dry Deposition: Acidic Gases and Particles 2. Causes A. NOx (1/4 from power plants) B. SOx (2/3 from power plants) 3. Basic Chemistry A. NOx and SOx react with water, oxygen, and sunlight to form sulfuric and nitric acids B. pH scale: neutral pH = 7, 100 times more acidic pH = 5. pure water pH = 7.0 natural rainwater pH = 5.6

2. 4. Effects A. Surface water Lakes and Streams typically have pH between 6 and 8. Natural soil and water have buffering agents to neutralize some acidity. Low pH affects aqueous chemistry: High Al, Low Ca Eggs affected Different Species tolerate acidic conditions better Changes in species populations, competition Ecosystem dynamics: species highly interdependent (Nitrogen also causes eutrophication; oxygen depletion) B. Forests In higher elevations, soils are thin (little buffering) Leaf Damage Limitation of nutrients less Ca, Mg, P - too much Al C. Materials Corrosion of metal, deterioration of stone Disintegration of fabrics, paint, paper Automotive coatings

3. 5. Strategies for Reducing Acid Rain Reduce Emissions from stacks and tailpipes Alternative Energy Sources Energy Conservation Environmental Restoration (e.g. liming lakes) 6. EPA's Acid Rain Program Title IV of 1990 amendments to Clean Air Act SOx and NOx emissions at power plants Objectives: a) Achieve environmental benefits through emission reductions b) facilitate: allowance trading options to minimize compliance costs c) promote: pollution prevention technologies energy efficient strategies

4. Reductions in SO 2 emissions (started in 1995) are facilitated through a market-based system for capping and trading—the centerpiece of EPA’s Acid Rain Program. The allowance trading system creates low-cost rules of exchange that minimize government intrusion. Reductions in NOx are based on emission limits for specific types of coal-burning boilers. The annual cost of these additional reductions will be approximately $200 million, at an average cost-effectiveness of $229 per ton of NOx removed. The 2005 Clean Air Interstate Rule provides states with a solution to the problem of power plant pollution that drifts from one state to another. CAIR covers 28 eastern states and the District of Columbia. The rule uses a cap and trade system to reduce the target pollutants—sulfur dioxide (SO 2 ) and nitrogen oxides (NOx)—by 70 percent.

5. Energy Conservation ENERGY STAR is a joint program of the U.S. Environmental Protection Agency and the U.S. Department of Energy helping us all save money and protect the environment through energy efficient products and practices.

6. Affected Sources under the Acid Rain Program Trends in Sulfur Dioxide Emissions

7. Trends in Nitrogen Oxide Emissions

11. Comparison of Ambient Sulfur Dioxide Concentrations Comparison of Ambient Nitrate Concentrations

12. Trends in Wet Sulfate Deposition 1989-1991 1995-1998 Trends in Wet Nitrate Deposition 1989-1991 1995-1998

13. Wet Sulfate Deposition 05-07 89-91 05-07 89-91 Wet Nitrate Deposition

14. Acidic Surface Waters Sensitive Forest Ecosystems