Acid rain results when the combustion of fossil fuels releases sulfur dioxide and nitrogen oxides to the atmosphere. These contaminants are deposited to the Earth’s surface as precipitation, gases or particles and are called acid rain or acidic deposition. Acid rain was "discovered" in North America at Hubbard Brook through measurements of precipitation chemistry that were started in 1963. The measurements of precipitation chemistry at Hubbard Brook are the longest continuous measurements in North America. Subsequent research showed that acid rain was a major environmental issue for eastern North America and many parts of Europe and Asia. Research at Hubbard Brook and elsewhere have demonstrated the ecological effects of acid rain on high elevation forests and surface waters. These effects include: the acidification of soil associated with depletion of readily available calcium and magnesium and the mobilization of aluminum; effects of soil acidification on sensitive tree species such as red spruce and sugar maple; the acidification of surface waters associated with decreases in pH and acid neutralizing capacity and increases in concentrations of aluminum; and decreases in the species richness of aquatic ecosystems due to surface water acidification.
Emissions of sulfur dioxide peaked in the U.S. in the early 1970s, while emissions of nitrogen oxides have remained elevated until the early 2000s. Decreases in acid forming emissions have largely been associated with controls on emissions from electric utilities. These decreases in emissions have resulted in decreases in wet deposition of sulfate and nitrate at Hubbard Brook and elsewhere in eastern North America. Decreases in acidic deposition have started to reverse ecosystem effects of acidification, including decreases in concentrations of sulfate and nitrate, and resulting increases in pH and acid neutralizing capacity and decreases in dissolved aluminum in streamwater. Research pioneered at Hubbard Brook shows that recovery from acid rain will be slow due to the long-term depletion of available calcium and magnesium from soil that occurred under conditions of elevated acidic deposition.
Process-level studies and long-term measurements and experiments on acid rain effects at Hubbard Brook and elsewhere have been key in the development and testing of watershed biogeochemical models, like PnET-BGC. These models are used a research tools to improve understanding of the hydrologic and biogeochemical processes in forest ecosystems and used a predictive tools to inform natural resource managers on the extent and rate of ecosystem response to emission control strategies. An important initiative called "critical loads" is underway that allows scientists to interact with policy makers in the discussion of ecosystem effects of air quality management. Critical loads are the input of an air pollutant below which ecosystem effects are not observed given current knowledge. Models and empirical observations are used to set critical loads of air pollutants.