Ozone and Air Quality in the Southern Appalachians

(Editor’s note: The last column in this series began a discussion on air pollution  in the mountains.)  

The National Park Service reports that  the Great Smoky Mountains National Park  has some of the worst air quality of any of  the parks that they monitor. Several different  types of pollutants contribute to this  poor air quality, including acidic deposition  of sulfur and nitrogen, ozone, and haze  causing particles; this column will focus specifically on ozone. 

Not to be confused with the naturally occurring ozone layer which filters the  sun’s ultraviolet rays in the stratosphere,  ground level ozone forms when nitrogen oxides combined with volatile organic compounds are exposed to the sun on warm  days. Nitrogen oxides (NOx), are primarily  derived from the burning of fossil fuels while volatile organic compounds (VOCs)  are gases that trees release, though VOCs  can also be found in paints, gasoline, and other chemicals. The sun’s energy is the  catalyst that causes a reaction between NOx  and VOCs to form ozone, or O3. Because the southern Appalachian Mountains have both ample vegetation (primary source of  VOCs) and inputs of NOxf rom both local and regional sources, ozone levels can exceed levels that threaten human health.  Ozone can cause coughing, chest pains,  sinus inflammation, and damage to lung tissue.  Concentrations tend to be higher in  high elevation areas, where they can reach  twice the level of nearby cities such as Atlanta  and Knoxville. This is due to both the transport of ozone and increased formation due to increased solar inputs. 

Ozone also can impact vegetation by entering plants through their stomata.  Stomata are the tiny pores found on leaves  through which plants take in carbon dioxide and release oxygen and water vapor. You can recognize ozone damage on plants by the presence of stippling, or discoloration, on the upper leaf surface between the veins.  Interestingly, there is significant variation in sensitivity to ozone both between and within species, though fast growing species tend to be more heavily affected than their slower growing counterparts. Astudy in Great Smoky Mountains National Park documented that over 90% of black cherry trees and milkweed showed signs of ozone damage. 

Once inside the plant, ozone can cause  damage in a number of ways. First, it damages cell walls and chloroplasts, impairing  the process of photosynthesis (or the plant’s ability to make food for itself). With its source of food depleted, the plant has less energy to devote to growth. Second, there is evidence that exposure to high levels of ozone damages a plant’s ability to regulate its stomata, leading to increased water use  and stress. In past columns we talked about  how trees play important roles in regulating  the water cycle in forests by taking up water though their roots and releasing excess water as vapor through a process called transpiration. Researchers have found that following exposure to ozone  stomata are less sensitive to environmental cues telling them to open and close, meaning  that stomata open wider and stay open longer than they should, causing an affected tree to lose too much water. This problem is compounded by the fact that vegetation is exposed to the highest concentrations of ozone on hot sunny days, when they also need the most water. In fact, researchers have pointed out that water stress can impair plant growth more than the loss of energy from reduced levels of photosynthesis. 

It is also possible that the increased rates of transpiration following periods of high ozone exposure can negatively impact streams. Researchers in the southern Appalachian region have found that increases in transpiration rates of ozone-effected vegetation were significant enough to impact water quantity, water quality and stream ecology. Increased transpiration rates meant decreased soil moisture content, which led to a reduction in stream baseflow. Reductions in streamflow can lead to increased concentrations of pollutants and nutrients, increased water temperatures, and decreased  concentrations of dissolved oxygen.  These factors all decrease overall water quality which is important to the survival  and diversity of aquatic wildlife. 


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 cwtlistn@uga.edu 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. Oct 7, 2011.