Sulfur in the Southern Appalachians

Large Reductions in Regional Sulfur Dioxide Emissions by TVA and Others Have Not Yet Had the Expected Beneficial Impact on Particle Sulfates and Haze in the Southern Appalachians

Trends in Atmospheric Sulfur Over the Southern Appalachian Mountains

The Science of Haze

The scenic vistas in the southern Appalachian Mountains are a major tourist attraction. Nearly ten million people visit the Great Smoky Mountains National Park (GSMNP) annually, in part, for the panoramic mountain-top views. However, visitors sometimes cannot fully appreciate these views because of regional haze. Regional haze is caused by very small particles that scatter and absorb light, reduce visual range, and alter the color of landscapes. The degree of impairment depends on a number of factor the concentration, size, and chemical composition of these fine particles, relative humidity, and the angle at which the sunlight penetrates the haze.

In rural parts of the eastern United States, fine particle sulfate can account for most of the visibility impairment. Two-hundred times smaller in diameter than a human hair, these particles are just the right size to scatter visible light.

The Sulfur Dioxide-Sulfate Link

In the eastern United States, particle sulfate (PS) originates from both natural and human-caused emissions of sulfur especially sulfur dioxide (SO2). The conversion of SO2 to PS usually is very slow. But once created, PS removal from the air is a slow process, unless sufficient rainfall occurs to wash it from the atmosphere. Thus, a string of hot, humid, and stagnant summer days provides ideal conditions for the production and accumulation of PS and, consequently, poor visibility. In general, a much greater fraction of SO2 converts to PS in summer than in winter.

Given the typical, relatively slow conversion rate, summer PS found in the southern Appalachians could have been influenced by very large source regions up to 1,000 km away. This would include the Ohio Valley, the Tennessee Valley, and the Deep South (Alabama, Georgia, and Mississippi).

Observed Fine Particle Sulfate Trends

Two independent networks have measured PS levels in the southern Appalachians since the late 1980s. The Clean Air Status and Trends Network (CASTNet) sites, sponsored by the Environmental Protection Agency, measure both SO2 and PS. Each CASTNet site collects samples continuously for one week. CASTNet monitoring sites are usually in rural areas.

The Interagency Monitoring of Protected Visual Environments (IMPROVE) network focuses on visibility-related measurements and operates in national parks and wilderness areas where visibility is a highly-valued environmental factor. IMPROVE sponsoring agencies include the National Park Service, Forest Service, Fish and Wildlife Service, and the Bureau of Land Management. Around-the-clock IMPROVE measurements are made twice each week. Figure 1 shows the locations of CASTNet and IMPROVE monitoring sites in the region.

The trend of average summertime (June-September) PS levels at the CASTNet and IMPROVE sites are depicted for the 1989-98 period in Figure 2. The CASTNet trend is probably best described as being downward from 1989 through about 1994 and then either flat or slightly upward after that. In contrast to the CASTNet results, IMPROVE data suggest either little change or a slight increase in PS concentrations in the vicinity of the GSMNP.

Sulfate Dependence on Anthropogenic SO2 Emissions

There is no question that anthropogenic sulfur emissions do lead to the production of PS, but there are any number of questions as to when and where these emissions have their greatest impact. A comparison of summertime SO2 emission data from the four easternmost TVA fossil plants (Bull Run, Kingston, John Sevier, and Widows Creek) with PS data from the nearest IMPROVE station (Look Rock) shows an apparent association (Figure 3). It is tempting to link these PS trends with the pattern of SO2 emissions to infer a cause-effect relationship based on association alone. In fact, such data have been used to contend that the four TVA eastern fossil plants are largely responsible for poor visibility in the GSMNP. However, this argument is weak in that conversion from conversion from SO2 to PS is a very slow process that occurs over a wide ranging area. In fact, PS at Look Rock is influenced by a much larger source region than just the four nearby TVA plants. Changes in regional SO2 emissions dwarf the small changes that have occurred at the four TVA plants. Therefore, it is reasonable to expect that these larger regional SO2 changes would be reflected in observed PS.

Figure 4 contains two plots comparing SO2 emissions from various groupings of nearby SO2 sources with downwind IMPROVE PS levels. The top plot compares summer SO2 emissions from utility boilers in central and eastern Kentucky south through northern Alabama with PS averaged for Mammoth Cave National Park and Sipsey River Wilderness. Sulfur-dioxide emissions decreased by 28 percent (170,000 tons) across the Kentucky-Tennessee-Alabama region between 1993 and 1995, while, IMPROVE PS increased nearly 25 percent during the same period. The lower plot compares emissions from eastern Tennessee and northern Georgia with Look Rock PS. In the vicinity of the GSMNP, the SO2 emissions decline was similar to that across Kentucky-Tennessee-Alabama (28 percent, 260,000 tons) while PS increased.

These figures illustrate that no simple, consistent, linkages exist between SO2 emissions and PS production in the southern Appalachians. The data also highlight the problems inherent in drawing meaningful conclusions about the benefits of local versus distant SO2 emission controls.

Conclusions

Following are highlights of this analysis of summertime SO2 emissions and atmospheric PS in the southern Appalachian region:

CASTNet data from 1989 to the present indicate an overall decline in PS early in the period followed by a more recent leveling off.
IMPROVE data show little change or a slight increase in PS over the same decade.
The association that exists between Great Smoky Mountain IMPROVE PS data and TVA SO2 emission data from the four eastern-most fossil fuel plants does not establish any cause and effect relationship. In contract, examination of relationships between PS data and SO2 emission data across the larger region make it evident that the large SO2 emission reductions of the mid-1990s have not been reflected in reductions in PS concentrations.
Despite large reductions in SO2 emissions in and around the southern Appalachian region, there appears to have been little short-term improvement in PS, which contributes greatly to regional haze.

It is evident that PS trends in space and time can only be understood in the larger context of emission changes occurring over geographical regions covering much of the eastern United States. Perhaps, the biggest unknown is the influence of changing weather patterns. The importance of long-term environmental monitoring programs is also underscored. In working for the long-term understanding of the consequences of human activities, only long-term, consistent air quality monitoring programs (such as the National Atmospheric Deposition Program) can establish the benefits of various environmental control strategies.

Information Contacts

Stephen F. Mueller
(256) 386-3643
sfmueller@tva.gov

William J. Parkhurst
(256) 386-2793
wjparkhurst@tva.gov

Frances P. Weatherford
(256) 386-2344
fpweatherford@tva.gov

Inquiries and comments should be sent to wjparkhurst@tva.gov.