National Park Service

Northeast Temperate Network (NETN)

Parks in this Network

NETN Network Map
Click to see Larger
Network Map
Find your park logo

Appalachian National Scenic Trail Ozone Summary

Figure 1. CASTNet ozone observation locations in close proximity to the Appalachian National Scenic Trail used for analysis (Blue = Active; Red = Inactive).
Figure 1. CASTNet ozone observation locations in close proximity to the Appalachian National Scenic Trail used for analysis (Blue = Active; Red = Inactive).


Ozone pollution, at ground level, is a known stressor of terrestrial vegetation with clear ecological relevance and human health implications (Shriver 2006), and has been identified as a high priority Vital Sign for the Appalachian National Scenic Trail (APPA; Dieffenbach 2011). The Environmental Protection Agency identifies a series of serious health consequences associated with a range of ozone exposure levels (USEPA 2014) as well as impacts to terrestrial plants and the marine food web (USEPA 2011). High ozone concentrations cause respiratory problems in humans, and are a particular threat to people who are engaging in strenuous aerobic activity, such as hiking. High ozone levels can temporarily reduce lung function in healthy individuals, and are particularly dangerous for people with respiratory problems like asthma. Environmental impacts include damage to sensitive plant species by causing a visible spotting or "stipple" on the upper surface of plant leaves, can cause reduced photosynthesis and growth, premature aging, and leaf loss with or without the occurrence of foliar injury. Some species are more sensitive to ozone exposure, and a list of ozone-sensitive species has been developed for the APPA (Table 1).

Figure 2. Counties (2010) in the vicinity of the APPA that maintain ozone monitoring capabilities (Green = below secondary threshold; Yellow = exceeds primary (7 ppm-h) standard; Red = exceeds secondary (15 ppm-hr)).
Figure 2. Counties (2010) in the vicinity of the APPA that maintain ozone monitoring capabilities (Green = below primary threshold; Yellow = exceeds primary (7 ppm-h) standard; Red = exceeds secondary (15 ppm-hr) standard).

Atmospheric ozone concentration data needed to characterize conditions near the APPA are available from the Clean Air Status and Trends Network (CASTNet) as well as other sources, and need only be acquired and summarized. While the existing array of monitoring stations are not entirely representative of conditions on the APPA because of differences in elevation, meteorology, and the fact that several stations are no longer active, data from these sites used on this page provide a meaningful review of regional ozone concentrations (Figure 1; NPS 2004).

Data collected from CASTNet stations proximate to APPA suggest that summertime ozone concentrations can reach levels that are harmful to humans and sensitive vegetation. In 2010, EPA published a list of counties that failed to attain the W126 criteria for the period 2006-2008 using the 2010 National Ambient Air Quality Standards (NAAQS). With the exception of Maine, New Hampshire, and Vermont, the APPA passes through ozone "nonattainment" counties in each of the other 11 states (Figure 2). Whether ozone levels in the counties identified in Figure 2 are uniform throughout cannot be determined, but the fact that the APPA passes through counties that exceed NAAQS cannot be overlooked. NOTE: The 2010 NAAQS were not finalized by the EPA leaving the 2008 standards, which the 2010 standards were intended to replace, as the legal standards currently in effect. We use the slightly more restrictive 2010 standards to make it possible to compare the county level attainment results.

Figure 3. Estimated W126 ozone exposure statistic.
Figure 3. Estimated W126 ozone exposure statistic.


Data used for this web page were downloaded from CASTNet and the U.S. Department of Agriculture (USDA) Air Resource Management (ARM) portal. The CASTNet and ARM sites make data available in a variety of formats, including "summarized," which satisfied our needs. The CASTNet data were subsequently filtered by proximity to the APPA by comparing data locations to the HUC10 shell. 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.

We use two exposure statistics to describe ozone on this page, W126 (ppm-hr) and N100 (ppb). NPS (2004) defines W126 and N100 to be "…the weighted sum of the 24 one-hour ozone concentrations daily from April through October, and the number of hours of exposure to concentrations >= 100 ppb (0.10 ppm) during that period. The W126 index uses a sigmoidal weighting function in producing the sum: the lower concentrations are given less weight than are the higher concentrations since the higher exposures play a greater role in producing injury. The significance of the higher concentrations is also reflected in the requirement that there be a specified minimum number of hours of exposure to concentrations >= 100 ppb. Thus, the W126 index has two criteria that must be realized to satisfy its thresholds: a minimum sum of weighted concentrations and a minimum number of hours >= 100 ppb…" N100 is the number of hours that ozone concentations exceed 100 ppb. The combination of two measures makes it possible to identify areas that experience periods of relatively high ozone concentrations (W126), as well as areas that endure long periods of slightly lower concentrations (N100).

Figure 4. Estimated N100 ozone exposure statistic.
Figure 4. Estimated N100 ozone exposure statistic.

After identifying CASTNet monitoring sites in close proximity to the APPA we discovered that of the 12 sites that had historically monitored Ozone, 7 were active (Figures 1-2, 5) and 5 were inactive (Figures 1, 2, and 6). Of the inactive sites, all were in the segment of the APPA between New York and Vermont and all had been inactive for at least 12 years (Figure 4).


Because no single approach to gathering and reporting ozone concentrations can comprehensively describe the conditions found on the APPA, we integrate summarized data obtained from a variety of sources. The results of different types of monitoring are illustrated in Figures 2-6. Figure 2 depicts county level monitoring results using the W126 measure, Figures 3 and 4 show regional level (small scale) W126 and N100 ozone estimations, and Figures 5 and 6 present results from a series of CASTNet sampling locations (W126). Each of these sources provide meaningful information, but not interchangeable information.

There are 38 counties that intersect the HUC10 shell, touch the footpath, and have some form of ozone monitoring program (Figure 2). Of these 38, 2 counties do not violate either of the proposed 2010 NAAQS standards (<7 ppm hours; green), 7 counties violate the secondary standards threshold (15 ppm hours; red) and 29 counties violate the primary standards threshold (7 ppm hours; yellow). The counties that maintain ozone monitoring capabilities are generally higher density (population) and lower elevation locations than counties that do not have such programs. Because of the way that county ozone programs are implemented, this could mean that the portion of each listed county that immediately surrounds the footpath may be somewhat less exposed to high ozone levels than the county as a whole because the footpath is generally described as being more "remote" than surrounding areas. However, these generalizations overlook the reality that there are portions of the APPA that are not isolated and are potentially just as susceptible to high ozone levels as the overall county through which the footpath passes. Combine this with the fact that high ozone levels are known to occur at high elevations, areas which are not well represented by county monitoring programs. Figure 3 illustrates the presense of increased ozone at higher elevations. Notably, the three darker areas along the footpath (NH, Va., NC), with darker shading representing higher ozone levels, are areas that are characterized by being higher elevation and generally less densely settled.

Figure 5. Annual W126 ozone statistic for active CASTNet monitoring locations (COW137 = Coweeta (NC); GRS420 = Great Smokey Mtn. NP (TN); PNF126 = Cranberry (NC); VPI120 = Horton Station (VA); SHN418 = Shenandoah NP (VA); ARE128 = Arendtsville (PA); WST109 = Woodstock (NH)).

Figure 5 shows active CASTNet ozone monitoring stations. During the late 1990's until the early 2000's, W126 ozone levels were relatively high, with several locations exceeding the 15 ppm threshold. Only the Woodstock, NH site (WST109) remained below the lower 7 ppm threshold througout the entire period. Figure 5 also shows that ozone levels dropped during the period 2003 through 2013, with no stations reaching the lower (7 ppm) threshold during 2009 or 2013.

Figure 6. Annual W126 ozone statistic for inactive CASTNet monitoring locations (ARE228 = Arendtsville (PA); WPB104 & WPA103 = West Point (NY); CAT175 = Claryville (NY); LYE145 = Lye Brook (VT)).

Figure 6 shows CASTNet stations that have been deactivated. At least 12 years has lapsed since any of these stations were active, but the Arendtsville, Pa. station was replaced.

Table 1 lists species found along the APPA that are also know to be sensitive to ozone. Some species have ranges that are limited (e.g., Doellingeria umbellata and Krigia montana are only reported for one location), while others are widespread (e.g., Prunus serotina is present at 28 locations, and 7 other species are present at 20 or more locations). If there is interest in investigating the effects of ozone on plants, this table can help. Species that are good indicators of high ozone levels, widespread and easily identifiable species may be best.

Table 1. Ozone sensitive species at monitoring locations in close proximity to the APPA.

Sources cited

Dieffenbach, F, 2011. Appalachian National Scenic Trail vital signs monitoring plan. Natural Resource Technical Report NPS/NETN/NRR2011/XXX. National Park Service, Northeast Temperate Network, Woodstock, VT.

National Park Service. 2004. Assessing the Risk of Foliar Injury from Ozone on Vegetation at Sites Along the Appalachian National Scenic Trail., accessed 2 July 2014.

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 (EPA). 2014. Ozone and Your Patients' Health Training for Health Care Providers., accessed 2 July 2014.

U.S. Environmental Protection Agency (EPA). 2011. Health and Environmental Effects of Ozone Layer Depletion., accessed 2 July 2014.

Last Updated: December 30, 2016 Contact Webmaster