The scientific evidence presented in this briefing demonstrates a well-understood environmental problem, a measurable policy success, and the clear need for continued vigilance. The data have not only characterized the chemical nature of acid rain but also supported assessments of its environmental consequences, which reinforce the importance of sustained action.

6.1. Summary of Environmental Effects

The U.S. National Acid Precipitation Assessment Program (NAPAP) conducted a comprehensive evaluation of the effects of acid deposition. Key conclusions from its 1991 Integrated Assessment Report include:

• Aquatic Systems: Acidic deposition has caused the acidification of some lakes and streams, leading to documented losses of fish populations in sensitive regions like the Adirondacks. Within these regions, an estimated 4% of lakes and 8% of streams are chronically acidic, with Florida exhibiting the nation’s highest percentage of acidic surface waters.
• Materials: Acid deposition accelerates the deterioration of manmade materials, including paint, metal, and stone. This effect has the potential for large economic impacts due to increased maintenance and replacement costs for buildings, monuments, and infrastructure.

6.2. Recommendations for Future Policy Support

To build on past successes and address remaining challenges, the scientific and monitoring infrastructure that enables effective policymaking must be maintained and enhanced.

1. Continue Long-Term Monitoring: The wet deposition monitoring network (NADP/NTN) is an invaluable national asset. Its continued, indefinite operation is essential to track the ongoing effects of legislation, including the impacts of Phase II of the Clean Air Act Amendments, and to provide a continuous, reliable scorecard of our nation’s environmental health.
2. Develop a Dry Deposition Network: Wet deposition is only part of the story. To fully account for the total transfer of pollutants from the atmosphere to the ecosystem—a concept known as mass balance—we must also measure dry deposition. The development and implementation of a long-term dry deposition monitoring network is a critical next step to achieve a complete understanding of the fate of airborne pollutants.