Please note: We are no longer updating this webpage and results are only available for 2000 through 2011.
High ozone exposures can negatively impact sensitive plants by reducing the amount of photosynthesis produced by the leaves. A reduction in photosynthesis means a reduction in food needed for healthy growth. In the United States, ambient ozone monitoring is typically conducted near urban areas in order to evaluate if large populations are being exposed to unhealthy ozone concentrations. There are fewer ozone monitors located in rural areas of the United States. For this reason, statistical models have been utilized (Knudsen and Lefohn, 1988, and Lefohn et al., 1988) to estimate the amount of ozone found in rural areas where our National Forests and Wilderness areas are located. Click here to learn more about how ozone impacts sensitive vegetation.
The purpose of this webpage is to provide a summary of the seasonal amount of ozone exposure and how two ozone exposure indices (W126 and N100) have changed over time for an individual wilderness or National Forest in the eastern United States. By selecting the State and Forest or Wilderness, a time series of the number of hours greater than or equal to 0.100 parts per million (ppm)(N100, top), and the W126 (bottom) are displayed. Adequate soil moisture is also needed in order for sensitive plants to take ozone into the leaves (Lefohn et al., 1997). If present, the red lines in the graphs below are the concern thresholds that (when both are exceeded for a specific year) experimental trials have predicted a 10 percent or greater loss in biomass (Lefohn, 1998).
The annual results in the graphs below are based upon using the 24-hours of available data for the months of April through September when vegetation is actively growing in most of the continental United States. The W126 was developed as a biologically meaningful way to summarize hourly average ozone data because it is cumulative and concentration weighted. The W126 places a greater weight on the measured values as the concentrations increase (see the calculator below). Thus, it is possible for a high seasonal W126 to occur with few to no hours above 0.100 ppm. Therefore, it is also necessary to determine the number of hours the ozone concentrations are greater than or equal to 0.100 ppm. The N100 is calculated because experimental trials with a frequent number of peaks (hourly averages greater than or equal to 0.100 ppm) have been demonstrated to cause greater growth loss to vegetation than trials with no peaks in the exposure regime (Hogsett et al., 1985; Musselman et al., 1983; Musselman et al., 2006; and Musselman et al., 1986). It should also be noted the lack of N100 values does not mean ozone symptoms will not be present when field surveys are conducted. The use of both the N100 and W126 is consistent with the recommendations of the first Federal Land Manager Air Quality Related Values Workgroup report (FLAG, 2002).
The ozone estimates are for an area (or grid cell) representing 0.5 degrees latitude by 0.5 degrees longitude (about 910 square miles). There could be more than one grid cell estimate for the Shining Rock Wilderness and the mean was computed for the N100 and W126. The 95 percent confidence intervals for the mean was computed by averaging the estimates for the grid cell values. The maximum and minimum was the grid cell with the highest and lowest values, respectively. The last five years of available statistical modeling results had a weighted mean N100 of 0.1 (range: 0.0 to 0.4) hours, while the weighted mean W126 was 25.0867 (range: 12.17 to 37.09) ppm-hours. tulip poplar is found within this location and controlled experiments have shown this ozone sensitive species is predicted to have a 10 percent biomass reduction if both the N100 and W126 thresholds are exceeded (Lefohn, 1998). The N100 threshold is 4 hours and the W126 threshold is 14.5 ppm-hours. The combined N100 and W126 estimates for each year between 2007 and 2011 the threshold was exceeded 0 times.
The X- and Y- axes can be adjusted. Click and Drag your mouse along the axes to scroll through the values. To adjust the scale of the axes, Click and Drag your mouse along the axes while holding the shift key and left mouse button.
The X-axis adjusts on each graph in unison with other graph, but each graph's Y-axes adjust independent of other graph.
Please note that values below zero for the N100 and W126 lower 95% confidence interval estimates are not displayed.
Below is a calculator to help you understand the relationship between an hourly ozone value and the equivalent W126 value. For a season worth of data, the weight is calculated for each hourly value and then the weight is multiplied by the hourly average ozone value. The seasonal W126 is then calculated by adding together all of the weighted hourly average ozone values. Enter an hourly ozone concentration with a unit of parts per million (ppm) in the box below (such as 0.070) and then click the Calculate button to see the result.
FLAG. 2002. Federal Land Managers' Air Quality Related Values Workgroup. Phase I Report. December 2000.
Hogsett, W. E.; Plocher M; Wildman V.; Tingey, D. T. and Bennett, J. P. 1985. Growth response of two varieties of slash pine seedlings to chronic ozone exposures. Can. J. Botany 63:2369-2376.
Knudsen, H.P. and A.S. Lefohn. 1988. The Use of Spatial Statistics to Characterize Regional Ozone Exposures, pp 91-105. In: Assessment of Crop Loss from Air Pollutants. W.W. Heck, O.C. Taylor, D.T. Tingey (eds). Elsevier Applied Science Publishing, London, U.K.
Lefohn, A.S. 1998. The identification of ozone exposures that result in vegetation visible injury and growth loss for specific species in the southern Appalachian mountain region. Report on file at: Southern Appalachian Mountains Initiative, 9 Woodfin Place, Asheville, NC 28801.
Lefohn, A.S., H.P. Knudsen, and L.R. McEvoy, Jr. 1988. The Use of Kriging to Estimate Monthly Ozone Exposure Parameters for the Southeastern United States. Environmental Pollution. 53:27-42.
Lefohn, A.S.; Jackson, W.; Shadwick, D.S.; Knudsen, H.P. 1997. Effect of surface ozone exposures on vegetation grown in the Southern Appalachian Mountains: Identification of possible areas of concern. Atmos. Environ. 31: 695-1708.
Musselman, R. C.; Oshima, R. J. and Gallavan, R. E. 1983. Significance of pollutant concentration distribution in the response of 'red kidney' beans to ozone. J. Am. Soc. Hort. Sci. 108:347-351.
Musselman, R. C.; Huerta, A. J.; McCool, P. M.; and Oshima, R. J. 1986. Response of beans to simulated ambient and uniform ozone distribution with equal peak concentrations. J. Am. Soc. Hort. Sci. 111:470-473.
Musselman, R. C.; Lefohn, A. S. ; Massman, W. J.; and Heath, R. L. 2006. A critical review and analysis of the use of exposure- and flux-based ozone indices for predicting vegetation effects. Atmos. Environ. 40:1869-1888.