A team of specialists was assembled to gather information currently known about the effects of air pollution to forest and aquatic resources in the Southern Appalachians. Public concerns in recent years led to extensive research on the impacts of acidic deposition (acid rain) and impacts of ground-level ozone on forests. Previously reported pollution trends and studies on air pollution effects on visibility, aquatic resources, and terrestrial resources were important sources of information for the team. Air quality monitoring data were also used either as the measured values, or as input into mathematical models to predict pollution concentrations across the landscape.
Not all of the questions could be answered with complete certainty from available information. In its work, therefore, the team identified new information that would have been helpful in answering the questions. Filling these information gaps will be useful if another assessment is conducted in the future.
Many pollutants are released into the atmosphere from both natural sources and human activities. The chemicals that are released are called "primary pollutants." The primary pollutants discussed in the report are the major ones that could impact natural resources of the Southern Appalachians. There are others not reported that affect human health. Many of these primary pollutants impacting natural resources go through chemical reactions in the atmosphere and form "secondary pollutants." The primary pollutants of greatest concern in the Southern Appalachians are sulfur dioxide, nitrogen oxides, volatile organic compounds, and particulate matter. Secondary pollutants formed from these reduce visibility, acidify soils and streams, and injure vegetation.
Sulfur dioxide is a gas released into the atmosphere during the combustion of fossil fuels that contain sulfur. In the atmosphere, sulfur dioxide is transformed into sulfate particles that reduce visibility and acidify soils and streams in the study area. For visitors and residents, spectacular views are major attractions in the Southern Appalachians, and many people are concerned because they think visibility in the region is declining. Long-term measurements show that they are correct. Visibility has deteriorated considerably since the 1940s with the poorest visibility in the summer, which is the major tourist season (fig. 64). Studies have shown there is a strong correlation between the emission of sulfur dioxide and haziness (fig. 65).
Figure 64
Haze in the James River Face Wilderness, and throughout the
Southern Appalachians, is worse in summer (top) than in winter
(bottom). Haze reduces the distance a person can see and the
clarity of an object being viewed.
Figure 66
In 1994, electric utilities were the largest emitters of
sulfur dioxide in the United States. (Source: EPA 1995a)
In the assessment, various measures of visibility were analyzed. Visibility in winter deteriorated slightly between 1960 and 1992. Summertime visibility decreased considerably between 1960 and the early 1970s, improved somewhat by 1980, and remained fairly stable since then. During the summer, increasing use of air conditioning has driven up power consumption. Scientists have suggested that the changes from a winter maximum in haze in the 1960s to a summer maximum in the 1980s can be attributed in part to increased sulfate from increased sulfur dioxide emissions at coal-fired electricity generating plants. Another cause may be more complete conversion of precursors (nitrogen oxides, sulfur dioxides, and organics) to particulate matter during the summer. Other changes in trends and patterns are due to the complex interplay between emissions and meteorology.
Aerosol samples collected twice a week for several years show that sulfates are the largest contributors to haziness in the region. On an average day, sulfates account for 60 percent of haze. On days with the worst visibility, sulfates account for nearly 80 percent of the fine particulate mass. Analysis of fine particulate data from the Shenandoah and Great Smoky Mountains National Parks shows an annual increase in sulfate of 2 to 3 percent each year between 1982 and 1992. This increasing trend was even more pronounced in the summer, when sulfate concentration increased 4 percent each year. Based on this information, the apparent lack of improvement in visibility conditions since the early 1980s is understandable. What is more elusive is why sulfate would be increasing at such a steady rate when sulfur dioxide emissions are stable or only increasing slightly.
Poor visibility in the summer is also a function of the weather. In that season, stagnant air masses often hang over the Southeast, trapping pollution and allowing concentrations to increase. High concentrations of pollutants, high temperatures, and high humidity interact to increase haziness. In particular, high relative humidity has a significant impact on visibility. At high relative humidity, sulfate aerosols are more likely to grow to the size fraction most likely to cause haziness. The same sulfate particulate mass will have greater impact on visibility at higher humidities.
Sulfur dioxide emissions are expected to decrease in the Southern Appalachians in the years ahead. Nationally, the 1990 Clean Air Act (CAA) Amendments will reduce sulfur dioxide emissions by 10 million tons below the 1980 level, and there will be a cap on emissions from utilities and industrial sources. Reductions in and near the assessment area are uncertain, however, because local emitters could choose to purchase emission credits from other regions of the country.
Once they are fully implemented, the CAA Amendments of 1990 should lead to reduced haziness (improved visibility by 2 to 3 deciviews - roughly 4 miles) in the summertime in the Southern Appalachians (fig. 68). For comparison, an example of current median visibility and what visibility is predicted to be like after implementation of the CAA are shown in figure 68. Will the predicted improvement in visibility as a result of CAA regulations be noticeable to the public and will the public be satisfied? If the public is not satisfied with these improvements, further analysis will be needed to determine the technical feasibility and economic reality of further improvements. Continued monitoring of visibility and public reactions to what is observed will provide answers to these questions.
Figure 68
These photographs depict what a 3-deciview decrease in
haziness (visibility improvement) would look like compared with
the current median summer condition and natural background
visibility. The view is James River Face Wilderness in
Virginia.
Natural Background
In addition to improving visibility, reductions in sulfur dioxide emissions are predicted to reduce the amount of acid deposition in the Southern Appalachians. Acid rain became a prominent news story in the 1980s. Forests at high elevations in the Southern Appalachians were among those thought to be at risk, and considerable research was done there in the National Acid Precipitation Assessment Program. That program's National Stream Survey was a primary source of information about effects of acid deposition on aquatic systems. Another was the National Atmospheric Deposition Program's National Trends Network. Six of its deposition measuring sites are in the assessment area.
The primary acidifying chemicals in rainfall are sulfates and nitrates. Technical problems make measurement of cloudwater and dry deposition difficult. As a result, estimates of the amounts of acid-forming chemicals entering high-elevation ecosystems that receive significant cloudwater deposition are consistently low.
Sulfate deposition is highest at the highest elevations and in the northern portion of the Southern Appalachians (fig. 69). Unfortunately, portions of streams at high elevations are probably least able to neutralize or "buffer" incoming acidity. Sulfate concentrations in precipitation seem to be decreasing in the Southern Appalachians, but so are the concentrations of buffering chemicals that can offset the acid effects of sulfates and nitrates. Consequently, acidity of rainfall has not improved.
Figure 69
Modeled distribution of wet sulfate deposition in the Southern
Appalachians, 1983-1990. Deposition of sulfate in rainfall is
greatest at high elevations and in the northern part of the
Southern Appalachians.
Figure 70
Modeled distribution of wet nitrate deposition in the Southern
Appalachians, 1983-1990. As with sulfate, deposition in rainfall
is greatest at high elevations and in the northern part of the
Southern Appalachians.
Decreases in acid deposition are expected as the CAA Amendments of 1990 are fully implemented. The regulations will decrease emissions of both sulfates and nitrates from electricity-generating plants. Vehicle emissions are a second major source of nitrogen compounds. However, the importance of that source is expected to grow as the population of the study area increases. In the northern portion of the assessment area, implementation of the CAA Amendments should maintain the same proportion of chronically acidic streams as in 1985, unless nitrogen saturation occurs. Under current deposition levels, streams in the Southern Blue Ridge are susceptible to acidification. Streams in the northern portion of the Southern Appalachians and upper reaches of the southern portion of the assessment area, particularly in wildernesses, are more sensitive than those surveyed by the National Stream Survey. The Direct Delay Response Program estimated that a 30 to 50 percent reduction in sulfate deposition would prevent further acidification of streams in the Southern Blue Ridge. The 1990 CAA Amendments are predicted to accomplish a reduction of sulfate in that range. However, even with reduced sulfate deposition, streams may continue to acidify in watersheds that are losing the capacity to buffer incoming sulfur.
Ozone, a chemical composed of three oxygen atoms linked together, is highly beneficial in the upper atmosphere. At ground level, however, it is a powerful oxidizing agent that is capable of killing tissues. Small amounts of ozone occur naturally, but the large quantities measured at ground level are formed primarily through chemical reactions between nitrogen oxides, volatile organic compounds, and sunlight.
The two largest emission sources of oxides of nitrogen are motor vehicles and electricity-generating plants that burn fossil fuels (fig. 71). These two sources have approximately equal annual emissions. Various oxides of nitrogen quickly react in the atmosphere to form nitrogen dioxide, which can be seen as a brownish haze when it is sufficiently concentrated. Nationally, emissions of nitrogen oxides in the United States rose between 1940 and 1994. Part of the increase in the Southern Appalachians is attributable to a rise in the number of vehicle miles driven and part to increases in electricity generation. Title IV of the 1990 CAA Amendments requires a reduction in nitrogen oxide emissions from utility boilers by 2 million tons from the 1980 level, but there is no cap on emissions to keep emissions at or below levels in 1980. Emissions of nitrogen oxides in the Southern Appalachians are projected to increase by 2010 as vehicle miles increase and as electrical power demand rises with an increasing population.
Figure 71
The two largest emission sources of oxides of nitrogen are
motor vehicles and electricity-generating plants that burn fossil
fuels. (Source: EPA 1995a)
Figure 72
Many types of volatile organic compunds are emitted into the
atmosphere. (Source: EPA 1995a)
Figure 73
Areas with the greatest frequency of potential ozone damage to
vegetation, 1983-1990.
What are the implications of ozone exposures on the health of forests in the Southern Appalachians? The forest products industry may be concerned if ozone reduces tree growth and impacts future timber supplies. Ozone exposures could also be reducing the genetic diversity of some species, such as white pine. Furthermore, little is known about the effect ozone exposures may have on rare and endangered plant species.
Particles in the atmosphere include wind-blown soil, soot, smoke, and liquid droplets. Nationally, the major source of particulate matter 10 microns or smaller are fugitive dust sources from roads, construction, and agriculture (fig. 74). Nationally, between 1940 and 1994, particulate matter emissions from stationary sources have decreased significantly, but point sources only comprise 8 percent of the total emission. Overall, particulate matter emissions are expected to remain constant to the year 2010.
Figure 74
Nationally, the major source of particulate matter, 10 microns
or smaller, are fugitive dust from roads, construction, and
agriculture. (Source: EPA 1995a)
Between 1985 and 1994, average annual particulate matter concentrations appear to have declined in the SAA area. For particles of 10 microns or less, the annual mean for the region declined from 44 to 24 microns per cubic meter. It should be noted, however, that during the period the focus of measurement shifted from total suspended particulates to particles of 10 microns or less. This shift led to the replacement of sampling equipment. As a result, measurements of long-term change may not be completely reliable.
At most monitoring stations, particulate matter concentrations are currently well below NAAQS values. New sources that emit modest amounts of particulate matter, therefore, will not cause violations of the annual standard. Annual average concentrations of particulate matter do not differ substantially by state, and urban monitored values do not differ greatly from those for rural sites. In the area as a whole, averages for spring and summer are about 12 percent higher than the annual mean.
Like annual averages, maximum 24-hour concentrations appear to be declining. Between 1985 and 1994, 24-hour concentrations at specific locations seldom exceeded 90 percent of the NAAQS maximum. Land managers must be concerned about exceeding 24-hour particulate standards in the vicinity of prescribed forest burns. Stationary air sampling equipment is seldom close enough to a prescribed forest burn to be helpful. Some special sampling has been done near such fires in Florida and Texas. In nine-tenths of the cases, particulate matter concentration was below 150 microns per cubic meter 1 mile away from the outer edge of the fire. In two-thirds of the cases, the standard was maintained as close as 0.5 mile from the outer edge.
Some forest ecologists believe there is a need to return fire to its role in the ecosystem, reducing combustible fuel and enhancing wildlife and plant habitat, especially for fire-dependent pine ecosystems. This policy would be accomplished through increased use of prescribed fire. Is there an upper level of prescribed fire over a given time period that would exceed NAAQS for particulate matter? To answer this question, additional monitoring of particulate concentrations would be needed in rural areas; most particulate monitors are currently located in urban areas.
As the Atmospheric Team was answering the questions that were posed, we noted the kinds of information that would help answer the questions more fully or help decision makers select appropriate responses to the problems that were identified. These research and monitoring needs are:
Return to SAA Summary Report - Table of
Contents