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Appalachian National Scenic Trail Visibility Summary

Figure 1. IMPROVE visibility observation locations in close proximity to the Appalachian National Scenic Trail used for analysis.
Figure 1. IMPROVE visibility observation locations in close proximity to the Appalachian National Scenic Trail used for analysis.

Introduction

Particles in the air (e.g., dust and smoke) from natural as well as anthropogenic sources (e.g., automobiles and industrial operations), in combination with humidity, are the primary causes of visibility impairment. Airborne particles not only decrease visibility but can alter the color at scenic vistas (Malm 1999), and may adversely affect human health (U.S. Environmental Protection Agency 2006; Figures 1 and 2).

In 1977, Congress wrote legislation into the Clean Air Act designed to address existing visibility problems in Class I areas, and to prevent further degredation. A Class I area is defined by the Clean Air Act to "…include national parks greater than 6,000 acres, wilderness areas and national memorial parks greater than 5,000 acres, and international parks that existed as of August 1977…"

In 1987 a collective of Federal, state, and regional agencies and organizations launched the Interagency Monitoring of Protected Visual Environments (IMPROVE) program for the purpose of monitoring visibility in National Parks and Wilderness Areas (WA). The network began with 20 long-term monitoring sites and now includes over 200 sites in parks and wilderness areas across the nation. Of these, eight are located within the HUC10 shell (see methods), placing them in close proximity to the Appalachian National Scenic Trail (APPA): the Great Gulf Wilderness Area in New Hampshire; Lye Brook Wilderness Area in Vermont; Mohawk Mountain in Connecticut; Arendtsville in Pennsylvania; Shenandoah National Park, Jefferson National Forest, and the James River Face Wilderness Area in Virginia; and, Great Smoky Mountains National Park in Tennessee and North Carolina.

A number of methods have been devoloped to quantify visibilty and to address the perception that visibility has deteriorted during the industrialization period. Standard Visual Range (SVR; a visual estimate) and measurement by instrument are both used by IMPROVE to quantify visibility. SVR is the distance a prominent landmark can be seen from a set location using familiar reporting terms like miles or kilometers. Visual estimates are generally easy to comprehend but can be subjective, whereas instrument driven measures may be more accurate but use unfamiliar terms like "light extinction" reported in "inverse megameters (Mm⁻¹)" that are not as easy to comprehend. These two measures are negatively correlated with increased light extinction representing visual degradation, so, the greater the light extinction, the worse the visibility. Typically, IMPROVE sites in the northeast report better visibility than in the southeast, and western sites report better visibility than sites in the east.

While assessing the status and trend of visibility might be relatively easy, finding ways to remediate impairment are not. Typically, the cause of impaired visibility is beyond "immediate" control, but instead must be addressed at a state or regional level. In 1999, Congress passed the Regional Haze Rule, which requires states to develop and implement plans to reduce pollutants that contribute to visibility impairment in Class I areas. Improvements are supposed to occur on the days with worst visibility as well as on the days with best visibility. The worst and best days categories are defined to mean the 20% of days in a given year (73 days) with the worst and best visibility, respectively.

Visibility was identified as a high priority "vital sign" in both the Appalachian Trail Vital Signs techical report (Shriver et. al. 2005) and the Appalachian National Scenic Trail Vital Signs Monitoring Plan (Dieffenbach 2011). Both reports identified the importance of existing program data, primarily from the IMPROVE network, for monitoring the status of visibility throughout the Appalachian region. Shriver et. al. (2005) presented images (Figures 2 and 4) to show the average visibility on the best days and worst days, respectively, for the period 1999-2003. Then, in two accompanying images (Figures 3 and 5), Shriver et. al (2005) showed how visibility had changed between two discrete time periods (1995-1999 and 1999-2003). They concluded that, in general, visibility on the best and worst days is better in the northeast than the southeast, but that while the average visibility had improved in the northeast on the best days (Figure 3) it had declined on the worst days (Figure 5).

Figure 2. Average visibility (Mm-1) on best days (from Shriver et. al. 2006).
Figure 2. Average visibility (Mm-1) on best days (from Shriver et. al. 2006).
Figure 3.  Difference in average best day visibility comparing the periods 1995-1999 versus 1999-2003 (from Shriver et. al. 2006).
Figure 3. Difference in average best day visibility comparing the periods 1995-1999 versus 1999-2003 (from Shriver et. al. 2006).

Shriver et. al. (2005) also presented a table similar to Figure 6 that contrasted the average light extinction at select locations near the APPA with other IMPROVE sites elsewhere in the country. Figure 6 depicts the same set of observation locations as Shriver et. al. (2005) except that it includes data from 2005-2009, whereas Shriver looked at 2001-2003 data. The order of observation locations from most impaired to least is nearly identical, with the primary difference between the two Figures being higher maximum extinction levels across the entire 2005-2009 continuum than in Shriver et. al. (2005).

Figure 4. Average visibility (Mm-1) on worst days (from Shriver et. al. 2006).
Figure 4. Average visibility (Mm-1) on worst days (from Shriver et. al. 2006).
Figure 5.  Difference in average worst day visibility comparing the periods 1995-1999 versus 1999-2003 (from Shriver et. al. 2006).
Figure 5. Difference in average worst day visibility comparing the periods 1995-1999 versus 1999-2003 (from Shriver et. al. 2006).

Methods

A few visibility observation locations are located very close to the APPA, but only in the south. To overcome the limitation on observation locations, we identified visibility monitoring stations located within the HUC10 shell in the same way we have identified observation locations and datasets for other natural resource focus areas. The HUC10 shell is based on watersheds defined by the USGS at the fifth level of the Hydrologic Unit Code system, with each being given a discrete 10-digit code (HUC10). The hydrologic unit system was developed by the USGS and subsequently modified by the Natural Resource Conservation Service (NRCS). The HUC10 shell, or the general frame of reference used to establish an area of interest around the APPA, is the "outer" boundary of all HUC10 hydrologic units that are within 5 miles of the APPA land base. There are 177 individual HUC10 hydrologic units within this shell. Though they are termed watersheds, Omernik (2003) explains that hydrologic units are not always true watersheds and that some hydrologic elements contained within the HUC10 shell may not include all upstream components of a true watershed. However, for the purpose of defining an area of interest around the APPA we believe the hydrologic unit system is satisfactory.

Figure 6. Average light extinction (Mm⁻¹) on days with the worst visibility at select IMPROVE sites. Red bars are sites within the HUC10 shell.

Data reviewed on this web page were obtained from the IMPROVE web site for the years 1990-2012, though not all stations reported data for this entire period. For example, the Lye Brook WA in Vermont lists 2 observation stations with one reporting data from 1990-2011, while the second Lye Brook station reported data only for 2012 (it is not clear if the second Lye Brook station is intended to replace the first). Datasets available from the IMPROVE web site are summarized so extensive processing is not required to view the results.

Results

Figure 7. Grey scale imagery showing regionally diverse visibility during a 'best' period (light shading depicts area of greater visibilty; dark shading depicts areas of lesser visibility).
Figure 7. Grey scale imagery showing regionally diverse visibility during a 'best' period (light shading depicts area of greater visibilty; dark shading depicts areas of lesser visibility).

Visibility along the APPA is generally more impaired in the south than it is in the north (Figures 2, 4, 6 - 8), but the precise cause for this cannot be established from these data. Figures 7 and 8, which capture a slightly larger regional perpective than data shown in the other figures, seem to suggest that the increased impairment reported in the south is part of a larger regional pattern. The generally darker shading (darker corresponds to more visibility impairment) shown in these two figures suggests that reduced visibilities characterize the area to the west of the southern and central portions of the APPA. Conversely, New England and areas to the west are comparatively less impaired. Natural and anthropogenic sources of visibility limiting materials are probably both involved, but again none of the data or information presented on this page point to a specific source or cause. Higher humidity, which is characteristic of the southeast, is almost certainly a factor.

Figure 8. Grey scale imagery showing regionally diverse visibility during a 'worst' period (light shading depicts area of greater visibilty; dark shading depicts areas of lesser visibility).
Figure 8. Grey scale imagery showing regionally diverse visibility during a 'worst' period (light shading depicts area of greater visibilty; dark shading depicts areas of lesser visibility).

As mentioned above, the results shown in Figure 6 are similar to a previous analysis of visibility performed for the APPA using data from 2001-2003, though the maximum extinction values for the period 2005-2009 are generally higher than previously reported. Figure 6 further shows that southern and mid-Atlantic observation sites are more deteriorated than northeastern sites and eastern observation sites are generally more impaired than western locations. However, while the 2005-2009 data are generally consistant with the findings by Shriver et. al. (2005) and the general notion that visibility has declined over the last century, on initial view they seem at odds with the results shown in Figure 9 (a through i). Figure 9 depicts data from IMPROVE as early as 1990 and through 2012 (station dependent), but instead of reporting light extinction on just the worst days, figure 9 presents average annual observations for all days, in the best, middle, and worst 20% groupings (73 days per group) expressed as SVT in Kilometers (km). In each case except Figure 9g which presents data from only a single year, the average annual visibility trend is positive throughout the monitoring period. Even Jefferson National Forest (Figure 9e), the most impaired site depicted in Figure 6, is also positive. There are two possible explanations. First, the units of measure and observation methods are different, and while the two methods are correlated there will be differences owing to the different methodologies. The second reason, and more likely explanation, is that IMPROVE started at a "low" point for visibility and that although visibility is getting better it is still not comparable to historic levels. Figures 3 and 5, which present visibility change over two discrete time periods, suggest that the visibility groupings might present differently and that the second explanation may be correct. In Figure 3, "clear" day visibility improved or remained unchanged, while "worst" day visibility deteriorated or remained unchanged in the northeast and improved in the southeast. Consequently, it isn't possible to generalize by saying that visibility has improved, declined, or remained unchanged for all groupings at any particular location. However, it is possible to say that on average all locations have seen improvement (Figure 9), in some cases dramatically, but during certain periods of (e.g., poor visibility) there may be deterioration (Figures 3, 5 - 6).

(a)
Visibility (km) at Arendtsville (AREN1)
(b)
Visibility (km) at Great Gulf Wilderness (GRGU1)

(c)
Visibility (km) at Great Smoky Mountains NP (GRSM1)
(d)
Visibility (km) at James River Face Wilderness (JARI1)

(e)
Visibility (km) at Jefferson NF (JEFF1)
(f)
Visibility (km) at Lye Brook Wilderness (LYBR1)

(g)
Visibility (km) at Lye Brook Wilderness (LYEB1)
(h)
Visibility (km) at Mohawk Mt. (MOMO1)

Figure 9 (a-i). Average visibilities (SVR) for each observation site in the HUC10 shell. Best to worst visibility in 20% increments. R2 can range from 0 to 1, with higher values suggesting that the line does a good job of describing the depicted trend. Values above 0.25 are generally considered good.
a) Trend: SVR = Year * 2.95 + 42.5; R2 = 0.84
b) Trend: SVR = Year * 4.24 + 104.55; R2 = 0.9
c) Trend: SVR = Year * 1.35 + 42.03; R2 = 0.58
d) Trend: SVR = Year * 2.43 + 45.82; R2 = 0.69
e) Trend: SVR = Year * 0.53 + 42.14; R2 = 0.74
f) Trend: SVR = Year * 3.27 + 94.46; R2 = 0.81
g) Trend: SVR = Year * + ; R2 =
h) Trend: SVR = Year * 4.63 + 76.65; R2 = 0.79
i) Trend: SVR = Year * 1.92 + 47.64; R2 = 0.8

(i)
Visibility (km) at Mohawk Mt. (MOMO1)

Sources cited

Dieffenbach, F, 2011. Appalachian National Scenic Trail vital signs monitoring plan. Natural Resource Technical Report NPS/NETN/NRR—2011/XXX. National Park Service, Northeast Temperate Network, Woodstock, VT.

Malm. W.C. 1999. Introduction to Visibility. Cooperative Institute for Research in the Atmosphere, NPS Visibility Program, Colorado State University, Ft. Collins, Co.

Omernik, J.M. 2003. The Misuse of Hydrologic Unit Maps for Extrapolation, Reporting, and Ecosystem management. Journal of the American Water Resources Association. (JAWRA) 39(3):563-573.

Shriver, G., T. Maniero, K. Schwarzkopf, D. Lambert, F. Dieffenbach, D. Owen, Y. Q. Wang, J. Nugranad-Marzilli, G. Tierney, C. Reese, T. Moore. November 2005. Appalachian Trail Vital Signs. Technical Report NPS/NER/NRTR--2005/026. National Park Service. Boston, MA.

U.S. Environmental Protection Agency. 2006. How Air Pollution Affects the View. EPA-456/F-06-001, Research Triangle Park, NC

Last Updated: December 30, 2016 Contact Webmaster