3.1. Setting the Context

In response to growing concern, the United States launched the National Acid Precipitation Assessment Program (NAPAP) in the 1980s. This monumental, federally funded research effort was designed to create a definitive scientific understanding of the causes, effects, and potential controls of acid rain. Over a decade, NAPAP coordinated a massive body of research, and its findings provided the critical scientific foundation for subsequent U.S. environmental policy, including the Clean Air Act Amendments of 1990.

3.2. Scope of the NAPAP Research

The scale of the program was immense. Over its lifetime, more than $600 million was spent by U.S. federal agencies on acid rain research coordinated through NAPAP. The comprehensive scope of this effort is illustrated by the four major volumes of its 1990 state of science reports:

• Volume I: Emissions, Atmospheric Processes, and Deposition
• Volume II: Aquatic Processes and Effects
• Volume III: Terrestrial, Materials, Health, and Visibility Effects
• Volume IV: Control Technologies, Future Emissions, and Effects Valuation

3.3. Key Findings on Aquatic Ecosystems

The final assessment report, published in 1991, delivered several key conclusions regarding the impact of acid rain on surface waters and fish, which had been a primary public concern:

• Prevalence of Acidic Waters: In acid-sensitive regions of the U.S., 4% of lakes and 8% of streams were found to be chronically acidic. Florida had the highest percentage of acidic waters among all states.
• Characteristics of Acidic Lakes: The research noted that acidic lakes tended to be smaller than nonacidic ones.
• Causality: The report affirmed that acidic deposition was a direct cause of surface water acidification. However, it also acknowledged other causes, such as naturally produced organic acids and acid mine drainage.
• Documented Fish Losses: Historical records confirmed that fish populations had been lost due to acidification in specific regions, including the Adirondacks, New England, and the mid-Atlantic Highlands.

3.4. Future Areas of Concern

Beyond aquatic ecosystems, NAPAP also highlighted the potential economic impact of acid deposition on materials. The accelerated deterioration of paint, metal, and stone due to acid deposition was identified as an area warranting careful future evaluation.

3.5. Concluding Transition

The comprehensive assessment provided by NAPAP underscored the real-world effects of acid rain. To study and address these effects, however, scientists first needed a shared, precise technical vocabulary to measure and discuss the phenomenon accurately.