The Critical Loads Mapper Tool is a EPA-USFS-NPS collaborative effort to enable decision makers, researchers, and the public to easily access information on atmospheric deposition of nitrogen and sulfur, critical loads, and their exceedances to better understand local and regional vulnerability to atmospheric pollution. This interactive mapping tool displays nitrogen and sulfur deposition levels through time for several air quality models, critical load levels for terrestrial and aquatic ecosystems in the National Critical Loads Database, and the exceedance of deposition over the critical loads as an estimate of vulnerability to air pollution.