METHODOLOGY FOR CODE RED RANKINGS:
We obtained air-quality data from the National Park Service for 13 parks with extensive monitoring programs for haze, ground-level ozone, and acid precipitation. Monitoring occurs at these and other parks because of the special protection national parks receive under the Clean Air Act.
This analysis evaluated Park Service data from all of the national parks that use the Interagency Monitoring of Protected Visual Environments (IMPROVE-visibility data), Park Service Gaseous Air Pollutant Monitoring Network (ozone data), and the National Atmospheric Deposition Program (NADP-acid precipitation data) to monitor the three pollutants reviewed in this report.
The formula for calculating the overall pollution index of the parks was revised for this report to ensure that each of the three pollution measures-visibility, ozone, and acid precipitation-were weighted equally in deriving the overall index score for each park. To do this, the 5-year averages for each pollution measure were standardized across the 13 parks by subtracting the mean and dividing by the standard deviation. The three scores for each park (also called Z-scores) could then be averaged to provide an overall score where no pollution measure was over- or under-weighted in relation to any other. In order to translate the resulting average Z-scores back into familiar numbers between 0 and 100 (similar to previous versions of this report), we looked up their associated proportions from a table of the normal distribution and multiplied by 100.
As in past versions of this report, we used data on visibility between the months of June and August provided by the IMPROVE program. Since the last report in 2002, however, IMPROVE changed their methods for calculating visibility. Thus, the improvements in visibility figures as compared to the 2002 Code Red report do not necessarily reflect actual visibility improvements, but rather reflect the changes in methods used by the monitoring agencies. Since the changes were applied to all parks and all years used in this report, however, the changes did not affect our ability to calculate park rankings and visibility trends in a consistent manner.
We also changed our measure of ozone pollution for this year's report. While past Code Red reports used a plant-based measure of ozone pollution, SUM60GS, we found this led to some confusion among the public and the media and have accordingly changed to a more frequently used human health-based measure. The measure we used is the federal Clean Air Act human health standard for ozone, which declares a site in non-attainment if the fourth highest 8-hour ozone average for a year is 85 ppb or more-the so-called 8-hour standard. It should be noted, however, that even if the 8-hour standard is not violated, ozone-sensitive plants and human health may still be damaged. While the federal fourth highest standard is used in computing a park's rankings, this report also uses the total number of unhealthy air days (days when the 8-hour average ozone is greater than 85 ppb) over a five-year period for graphic and other purposes because this measure is more meaningful and understandable to the average person.
In this report, as in past reports, we use sulfate and nitrate as measures of acid deposition in the belief that these reflect the most relevant chemical additions to soils and streams as a result of human-induced acid precipitation. While the numbers do not account for all sulfate and nitrate additions (we only have access to wet deposition data from most parks), nor do we know that all of the nitrate and sulfate represented was originally derived from sulfuric and nitric acid, there is no perfect way to account for human-induced acid precipitation from the available data. These measures represent the most relevant information for the long-term health of the parks from human-induced acid precipitation.
We did, however, make a minor change from previous reports in terms of how we combined sulfate and nitrate deposition numbers to account for acid precipitation. In the past, the amount of sulfate and nitrate in terms of lbs/acre were simply added together. Since this is really adding "apples and oranges," we chose this year to scale the two in relation to their potential addition of acidity to soils (assuming they were originally derived from nitric and sulfuric acids respectively). To do this, we multiplied the numbers in terms of Kg/ha by the number of charges associated with the molecule (one for nitrate, two for sulfate) and divided by the formula weight of each molecule (62g/mole for nitrate, 96 g/mole for sulfate). This provides a value in terms of Keqv/ha*year of potential acidity and is justified because the H+ ions associated with both nitric and sulfuric acid will be more than 99 percent dissociated in almost every realistic acidity level of soils, streamwater and rainwater that they will encounter (the pKa[2] of sulfuric acid is 1.92, while the pH of soil, stream or rain water at any park will rarely be below 3.92).
An addition to the report this year was to look at the trends for each pollutant starting in 1991-the year after the most recent amendments to the Clean Air Act-to monitor progress since the act was last strengthened. To calculate these trends, we used the same methods employed by EPA in a recent report to Congress on air pollution in Federal Class I areas (EPA 2001). This method, also known as the Thiel Method, is a non-
parametric, least squares technique for determining the direction and statistical significance of time series data.
It is a robust and conservative method for determining whether pollution is increasing or decreasing significantly at national parks over time. Methods for calculating Kendall's Tau (for determining whether a trend was increasing or decreasing), and an approximation of a Z value for determining the statistical significance of the 13-year trends, were taken directly from the USGS handbook, "Statistical Methods in Water Resources," (Helsel and Hirsch, 2002). In determining statistical significance, 2-tailed Z-tests were used because there was no a priori reason to assume the trends would be increasing or decreasing.
References:
EPA, 2001. "Visibility in Mandatory Federal Class I Areas, 1994-1998: A Report to Congress.
Helsel and Hirsch, 2002. "Chapter A3: Statistical Methods in Water Resources." In "Book 4, Hydrologic Analysis and Interpretation." United States Geological Survey. |
