Acid Rain and Air Quality in the Appalachians

Air pollution in the mountains can generally  be grouped into three categories: nitrogen  and sulfur, ozone, and haze. These  pollutants can contribute to a long list of  problems including but not limited to damage  to vegetation, acidification of streams,  and damage to human health. The next two  columns will explore the different airborne  pollutants and their effects on the local  ecosystem, beginning with sulfur and nitrogen  deposition.

As many of you who have spent hot  summer days hiking or exercising outdoors  in Macon County know, the topography  that makes the views here so breathtaking  can also both literally and figuratively take  your breath away on some days of the year.  Air quality monitors show that the Smoky  Mountains have some of the most polluted  air in the nation, especially at the higher elevations.  Why does a national park, like  Great Smoky Mountains National Park,  with nothing but trees, wildlife, and clear  streams have some of the worst air in the  eastern U.S.? Much of it has to do with topography  and where the park lies within the  southern Appalachians and the region.  Emissions from coal fired power plants to  the west and from surrounding urban and  agricultural areas (including exhaust from  cars and tractors, etc.) tend to get trapped in  valleys by the high mountain ridges. This  dirty air is out of reach from winds that  would otherwise help to flush it out. This  stagnant air, especially during the “dog  days” of summer, is the primary reason the  air quality is so poor relative to other areas  in the East.

Sulfur compounds are released primarily  from coal-fired power plants while nitrogen oxides are mostly from automobiles, utilities, and animal feed operations. According  to the National Park Service, Great  Smoky Mountains National Park has the  highest level of sulfur and nitrogen deposition  of any monitored national park. Sulfur  and nitrogen are prime contributors to acid rain, which includes precipitation in the  form of rain, snow, hail, dew, or fog- basically  anything that takes these compounds  out of the air and deposits them on the  ground, on vegetation, or in streams. These  compounds can also reach the ground without  precipitation, through a process called  “dry deposition.” Acid deposition is generally  greater at the highest elevations within  the park. This is because the tops of the  mountains receive 1) more precipitation, 2)  are often bathed in fog, which is often  lower in pH (more acidic) than rainwater,  and 3) the rugged terrain creates “eddies”  where sulfur and nitrogen oxides are deposited  in greater amounts.

Acid rain damages the forest ecosystem  in several ways. It can directly damage leaves on trees, decreasing their ability to  photosynthesize. Acid rain can also cause  the loss of critical nutrients from forest  soils, such as calcium. Both nitrogen and  sulfur compounds are plant nutrients and, in  small amounts, promote the growth of vegetation.  As levels continue to rise, however,  plant health and vegetation diversity declines.  When nitrogen and sulfur deposition  exceeds the amount that vegetation can uptake  and soils can absorb, the excess acidic  compounds begin to leach out of the soil,  taking with them positively charged nutrients  that are important for plant growth.  This upsets the charge balance of soil and  decreases the nutrients that are available to  plants, weakening trees and leaving them  more susceptible to insects, diseases and  other environmental stressors.

Coweeta researchers have been using  computer models to predict how acid deposition will impact nutrient cycles in the forest given different levels of future sulfur and nitrogen emissions. Unfortunately,  findings suggest that even with continued reductions in acid deposition it may take decades or even centuries for some areas recover from depletion of these nutrients.

Other ecosystems in the southern Appalachians  are also sensitive to high levels  of acidic deposition. When lakes and  streams become too acidic, sensitive fish species can no longer survive. The native  southern Appalachian brook trout is more  tolerant of streams with lower pH than the  non-native rainbow and brown trout. However,  brown trout cannot compete with the  other trout species for food or habitat and  are thus restricted to high elevation streams  where a barrier such as a waterfall prevents  the upstream migrations of these non-native competitors. These high elevation streams  receive greater inputs of acidic deposition relative to lower elevations, leaving the brook trout trapped between higher elevation streams that may become too acidic for  their survival and lower elevation streams  where they cannot compete with the larger  rainbow and brown trout.


This column is produced by members of the Coweeta Listening Project (CLP), a branch of the Coweeta Long Term Ecological Research Program. Views expressed here are not representative of the USDA Forest Service or the Coweeta Hydrologic Lab. Please share questions, comments, or suggestions for future topics at or Coweeta Listening Project, UGA, 210 Field St., Room 204, Athens, Georgia 30602.


Original Citation: The Coweeta Listening Project. Franklin Press. Column on "Science, Public Policy, Community." Page B4. Sep 16, 2011.