The National Acid Precipitation Assessment Program (NAPAP) was a cooperative federal program that was first authorized in 1981 in order to coordinate acid rain research and report those findings to the U.S. Congress. The research, monitoring, and assessment efforts of NAPAP, and other groups in the 1980s, culminated in Title IV of the 1990 Clean Air Act Amendments (CAAA), also known as the Acid Deposition Control Program. Title IX of the CAAA reauthorized NAPAP to conduct acid rain research and monitoring, and to periodically assess the costs, benefits, and effectiveness of Title IV. The NAPAP member agencies were the U.S. Environmental Protection Agency, the U.S. Department of Energy, the U.S. Department of Agriculture, the U.S. Department of Interior, the National Aeronautics and Space Administration, and the National Oceanic and Atmospheric Administration.
The NAPAP published a total of four reports: 1991 (multiple volumes), 1998, 2005, and 2011. The Program was able to describe and document strong reductions in sulfur dioxide and nitrogen oxide emissions, as well as the resulting atmospheric deposition from 1980 to 2010 as various elements of the CAAA were implemented. The NAPAP officially ended with publication of the last report in 2011. To reflect the federal NAPAP role in the NADP, the network name was changed to NADP National Trends Network (NTN)