Beginning about 1860, fossil fuels became the primary energy source for the industrialization of the nation. Energy from coal and petroleum products greatly improved the quality of life for U.S. citizens, but combustion of these fuels led to large releases of air pollutants into the atmosphere. Consequently, there have been multiple impacts to the region, including a considerable reduction in how far and how well a person can observe scenic views (visibility impairment), as well as significant atmospheric and ground level deposition of acidic compounds in the region. Although improving, air pollution is still impacting the health of humans, wildlife, and vegetation, as well as water quality.
Clean Air Act of 1970
Air pollution in Western North Carolina, specifically total sulfate and nitrate deposition, has been intensely studied on forty-four sites for over a century. Study sites demonstrate a significant increase in total sulfate deposition from 1860-1970 followed by a significant decline after the Clean Air Act of 1970 (CAA). The CAA mandates pollution control devices and other techniques be adopted to meet targeted reduction levels. In 1977, an amendment to the CAA set a national goal to prevent any future, and remedy any existing, impairment of visibility in mandatory Class I areas which are a result of human caused air pollution.
There are four federally mandated Class I areas in Western North Carolina: Joyce Kilmer-Slickrock Wilderness, Linville Gorge Wilderness, Shining Rock Wilderness, and the Great Smoky Mountains National Park. To achieve the visibility goal set by the CAA, a Regional Haze Rule has been implemented to achieve natural background visibility (following reasonable progress) by the year 2064.
The first year to determine if reasonable progress is being achieved is 2018. The initial emissions reductions will focus on reducing sulfur dioxide emissions in order to improve visibility by 2018 in comparison to the 2000–2004 baseline conditions. Information collected from study sites predict a decrease in total sulfate deposition between 2010 and 2018, and it is reasonable to assume that further sulfur dioxide emission reductions will be required to achieve natural background visibility at the four Class I areas by 2064.
Man-made emissions of sulfur dioxide, nitrogen oxides, and ammonia are converted in the atmosphere and deposited on the ground. The deposition of sulfates, nitrates, and ammonia can occur in three forms: wet, dry, and cloud water. Before dry deposition can occur, sulfates and nitrates, along with other pollutants, contribute to a uniform haze that obscures scenic views in Western North Carolina.
At most low elevation locations in the eastern United States, the annual amount of wet and dry deposition is similar. Fog may be an additional source of deposition below 3,500 feet elevation, but is considered a minor component, contributing less than 10 percent of the annual total. Cloud water, however, is a significant contributor at elevations above 3,500 feet, and the region’s forests may be immersed in clouds 30-50 percent of the year. Moreover, elevations above 3,500 feet may have twice as much deposition as lower elevation sites as a result of the larger amount of acid compounds deposited from the cloud water.
Tiny particles of matter originating from land and sea are continually emitted directly into the atmosphere and suspended in either gas or liquid form. Particulates are also formed in the atmosphere – for example, when sulfur dioxide is converted to ammonium sulfates.
Fine particles are responsible for visibility impairment, but they can also negatively impact people’s health. High concentrations of fine particulates on a daily or annual basis can increase the likelihood of respiratory or cardiovascular disease, especially for children and the elderly.
On most days in rural areas of Western North Carolina, especially when visibility is poor, ammonium sulfates make up the majority of fine particles suspended in the atmosphere. In urban areas, the addition of organic particles (sodium chloride, magnesium, sulfates, nitrates, calcium, ammonia) makes adverse health effects greater in urban areas.
The Environmental Protection Agency (EPA) has established two National Ambient Air Quality Standards (NAAQS) to protect people’s health. In Western North Carolina, there are six locations measuring fine particulates. In all six locations, neither the daily average nor the annual average has exceeded EPA’s standards. Furthermore, the 3-year average trend in fine particulates has been decreasing at most of the monitoring sites.
Ozone is an oxygen molecule with three oxygen atoms (O3). A gas found high in the atmosphere, ozone protects the planet against the harmful effects of ultra-violet radiation. Ozone also naturally occurs at ground level. However, ground levels of ozone can increase substantially when nitrogen oxides emitted mostly from vehicles and coal-fired power plants combine with volatile organic compounds released primarily from trees.
The greatest amount of ground level ozone formation occurs on hot sunny days when wind speeds are low and the air becomes stagnant. Both chronic and acute ozone exposures at ground level can be harmful to sensitive humans and vegetation.
Ground level ozone is continuously monitored on eleven sites in Western North Carolina. The sites are distributed across low, medium, and high elevation sites. At low and medium elevation sites, ground level ozone varies throughout the day. Typically, ozone levels begin to increase at about 8:00 AM, reaching the highest concentrations at 11:00 AM and remaining elevated until about 5:00 PM. Ozone begins to decrease in the evening hours and continues to decrease until the early morning hours of the next day.
Ozone patterns mirror daily weather patterns of temperature and solar radiation, increasing during the day and decreasing during the night. The daily pattern also mirrors fossil fuel use. Nitrogen oxide emissions increase in the morning when people use electricity to get ready for work and gasoline to travel to work. Fossil fuel combustion, and thus nitrogen oxide generation, is also greatest at the end of the day; however, with decreasing solar radiation, the nitrogen oxides react with the ozone (instead of volatile organic compounds) and decrease the ozone concentrations in the atmosphere. Typically, the total amount of ozone exposure is less at lower elevation sites (except for the monitoring site adjacent to Asheville) when compared to the medium elevation site at Linville Falls.
Recent monitoring indicates that ground level ozone is currently not exceeding EPA standards at low and medium elevation sites, but standards are exceeded at most high elevation sites. Therefore, people outdoors and sensitive plant communities above 4000 feet elevation are likely to be negatively impacted by ground level ozone.
The EPA continually reviews the scientific literature and is considering a revision in ground level ozone standards. If EPA decides to lower the standard to 0.06 parts per million, all eleven sites in Western North Carolina will exceed the National Ambient Air Quality standard. EPA may also reduce secondary ozone standards, such as hourly averages over time and peak concentrations. If this change occurs, most of the study sites will exceed the new standard.
Ground Level Ozone and Growth Rate of Trees
Controlled studies indicate that reductions in vegetation biomass (the dry weight of stems, leaves and roots) occur with chronic exposures of peak concentrations at high elevation sites. Although scientists disagree on the effects of ozone exposure to vegetation during the night, it has been argued that vegetative defense mechanisms to ozone exposure may decrease during the night and, therefore, peak ozone concentrations could cause physiological damage to vegetation.
Other factors have to be considered as well. The Southern Appalachian Mountains have the highest number of plant species of any temperate forest in the United States. While only a few species have been studied, scientists have demonstrated that there is a continuum of plant tolerance to ozone exposure both between and within species. Recent studies have shown that Northern red oak was unresponsive to increases in ozone exposure, however, tulip poplar experienced reduced biomass formation. Overall, the total amount of area occupied by trees in a forest stand is likely to remain unchanged with variable ozone levels, but species composition may change.
Additional factors, such as the amount of sunlight hitting the leaves or the amount of available soil nitrogen, can influence the level of gaseous uptake from the atmosphere in sensitive species. Soil moisture, however, has the greatest influence on whether ozone will penetrate the leaves because plant cells used for gas exchange close during periods of drought.
NOX and SO2 Emissions
Nitrogen oxide emissions have been decreasing in states near Western North Carolina and it is anticipated that they will continue to decrease in the future. This reduction will lower ozone exposures, especially peak concentrations from ground level sources (primarily vehicles) and elevated sources (primarily coal-fired power plants).
In a recent six state analysis by the Southern Appalachian Mountain Initiative, nitrogen oxide reductions from ground level and atmospheric sources were studied, one state at a time. Resulting ozone concentrations were then predicted for Joyce Kilmer- Slickrock, Linville Gorge, and Shining Rock Wilderness Areas (these three Wilderness Areas represent the range of responses to ozone reduction that could occur throughout Western North Carolina). Findings indicate that ozone exposures will be lowered if nitrogen oxides are reduced from both sources. However, the greatest benefit would occur if ground level emissions of nitrogen oxides were reduced. Also, Western North Carolina would receive the most benefit if nitrogen oxide emission reductions occurred in eastern Tennessee, North Carolina, South Carolina, and Georgia.
The major source of sulfur dioxide emission is coal-fired power plants. In 1990, the utility industry contributed about one-half of all nitrogen oxide emissions. From 1980 - 2008, in states adjacent to the region, the implementation of various emission reduction strategies led to decreased emissions of both sulfur dioxide and nitrogen oxides. Additionally, from 1993 - 2008, the reduction in sulfur dioxide led to an overall reduction in wet sulfate deposition. In the same period, the annual wet nitrate deposition decreased, but the ammonia deposition from rainfall remained level or perhaps increased slightly.
The Southern Appalachian Mountains Initiative conducted a study in 2002 to find out where air pollution affecting the region originates. Estimates were provided for three locations in Western North Carolina: Joyce Kilmer, Linville Gorge and Shining Rock Wilderness Areas. Fossil fuel emissions from the states of Tennessee, North Carolina, Georgia, and Alabama are major contributors to visibility impairment and unsustainable acidic deposition in these areas of Western North Carolina.