This project evaluated the sensitivity of all Inventory and Monitoring (I&M) national parks in the United States to potential acidification effects caused by atmospheric sulfur (S) and nitrogen (N) deposition. Such effects can be caused by the addition of S, oxidized N (NOx) and/or reduced N (NHx) to natural ecosystems. Acidification can occur in both terrestrial (soil and soil water) and aquatic (fresh surface water) ecosystems. The addition of S and N from air pollution sources to national park ecosystems can alter plant, animal, and algal communities at all trophic levels and influence the mix of species that thrive in those ecosystems. This assessment includes consideration of three factors that influence acidification risk to park resources from atmospheric S and N deposition: pollutant exposure, inherent ecosystem sensitivity, and park protection mandates. National parks and networks were ranked according to each of these themes. An overall risk ranking was calculated for each park and network based on averages of the three theme rankings. Project completion was April 2011.