Resource Feb 13, 2024

Polluted Parks Report 2024: Resources and Analytical Methods

Our methodologies and further explanation of our analysis of data

Air & Climate Report

Polluted Parks

National parks are home to some of America’s most iconic landscapes and best-preserved cultural and historic sites, and these places need clean air and a healthy climate to flourish. Yet…

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Air Quality

The national park air quality conditions in this report rely on data from the Air Resources Division (ARD) at the National Park Service (NPS). The NPS participates in several national, multi-agency air quality monitoring networks. These monitors collect data on ozone, visibility, particulate matter and atmospheric deposition of nitrogen, sulfur and mercury. Data from these networks was used to evaluate current air quality conditions and provided a snapshot of the air quality conditions in 399 national parks.

NPS obtained the air quality values using either an estimation method or by directly reporting monitoring data from in-park or representative monitoring sites. Because air quality monitoring sites are not evenly distributed across the U.S., NPS used spatial interpolation to estimate air quality measures in non-monitored locations or locations with multiple monitors.

Hazy Skies: Visibility is monitored throughout the U.S. by the Interagency Monitoring of Protected Visual Environments (IMPROVE) network. Particulate samples are collected and analyzed for chemical composition. This data is used to calculate annual visibility impairment expressed in deciviews (dv). NPS calculated its visibility estimates by averaging data between 2017 and 2021 from each visibility monitoring site with at least three years of complete annual data. Through analysis of these data sets, NPS evaluated how far people can see in national parks without haze pollution versus actual air quality conditions between 2017 to 2021. The “floor” is the state of natural visibility.

For our updated Polluted Parks report, we reinterpreted the NPS data to determine whether visibility in a particular park is of significant concern, unsatisfactory or a low Concern, depending on the difference between the floor and the monitored or estimated visibility data using visibility ranges. If the difference between the floor and the monitored data is greater than 8 dv between an average day and a hazy day, it is listed as significant concern; between 2 and 8 dv is unsatisfactory and less than 2 dv is low concern.

Unhealthy Air: Ozone is monitored across the U.S. through air quality monitoring networks operated by the NPS, Environmental Protection Agency (EPA) and state agencies. Aggregated hourly ozone concentration data are acquired from the EPA Air Quality System database. NPS compiles information about the levels of ozone—also known as smog—in the parks. This report uses the NPS’s interpolation to estimate the five-year average fourth-highest maximum eight-hour average ozone concentrations in parts per billion (ppb), a standard measure that has been developed to reflect the level of threat to human health. The EPA national ambient air quality standard for ozone is 70 ppb—meaning that if ozone levels are above those concentrations, ozone is deemed unhealthy.

In NPCA’s updated Polluted Parks analysis, we used the ozone ranges considered by NPS; if the five-year average value is greater than or equal to 71 ppb it is listed as significant concern; between 55 and 70 is unsatisfactory; and less than or equal to 54 is low concern.

Harm to Nature: Atmospheric wet nitrogen and sulfur deposition are monitored across the U.S. as part of the National Atmospheric Deposition Program/National Trends Network (NADP/NTN). Weekly samples are analyzed for nitrate, ammonium and sulfate concentration which are then used to calculate wet deposition by annual precipitation in kilogram per hectare (Kg/ha) and classified based on NPS ranges. Moreover, the nitrogen and sulfur deposition conditions were adjusted based on results from national assessment reports that identified ecosystems and resources at risk of acidification and nitrogen enrichment in national parks. If park ecosystems are ranked “very high” in sensitivity to acidification or nutrient enrichment effects from atmospheric deposition, the condition category was adjusted to the next most conservative concern level.

For our updated Polluted Parks analysis, wet deposition greater than 3 kg/ha/yr is assigned as significant concern. Wet deposition from 1–3 kg/ha/yr is assigned unsatisfactory concern. And low concern levels are assigned when wet deposition is less than less than 1 kg/ha/yr.

For the impacts of ozone on vegetation, the W126 metric was used. The W126 metric is a biologically relevant measure that focuses on plant response to continuous ozone exposure. This measure is a better predictor of vegetation response to ozone exposure than the metric used for the human health standard (4th-highest daily maximum 8-hour average ozone concentration) because damage develops slowly under continuous exposure. The W126 metric equation preferentially weights the higher ozone concentrations that are more likely to cause plant damage. It sums all the weighted concentrations during daylight hours because this is when most of the gas exchange occurs between plants and the atmosphere. The highest 3-month period that occurs during the growing season (March–September) is reported in parts per million-hours (ppm-hrs).

Using ranges identified by NPS, for NPCA’s updated Polluted Parks report, W126 index greater than 13 ppm-hrs is assigned as significant concern. A W126 index from 7–13 ppm-hrs is assigned as unsatisfactory concern level. A low concern level is assigned when the W126 index is less than 7 ppm-hrs.

In this report, the Harm to Nature rating is a summary compilation of these three data sets and reflects the worst rating of the park in any of these three categories. For example, if a park is listed as significant concern for nitrogen wet deposition, but unsatisfactory or low concern for sulfur wet deposition and ozone, it is listed as significant concern for the Harm to Nature category.

For more detailed information about the analytical methods for the air quality conditions visit the NPS’s Air Quality Analysis Methods article.

Climate Change Threats

The climate change analysis in this report relies on data for 397 parks from the study entitled: A Strategic Analysis of Climate Vulnerability of National Park Resources and Values produced by the Natural Resource Stewardship and Science Directorate at NPS.

NPS identified potential high risk vulnerability factors relevant to the cultural, natural, or physical infrastructure of parks from available documentation. For each aspect of climate vulnerability, potential indicators were identified by examining existing relevant datasets that could be evaluated relative to each park or region. Datasets for these analyses needed at a minimum to include the entire continental United States, be publicly available and well documented, be of consistent quality and completeness, and be directly relevant to evaluating factors contributing to climate change vulnerability.

A total of 47 indicators were identified by NPS. Examples include projected changes in mean annual temperature, changes in precipitation, fire hazards and at-risk species. Each indicator could be relevant to one or more resource groups: terrestrial-living resources, aquatic-living resources, or non-living resources. NPS defined exposure indicators as those that measure a projected change in climate or climate-sensitive disturbance from a baseline condition compared to projected conditions at a future time. Sensitivity indicators were defined as those that measure current or recent historical conditions within or surrounding the park unit that affect the degree of impact climate changes may have. Finally, adaptive capacity indicators were those that measured the degree to which a site facilitates or impedes adaptation to climate impacts.

Two main approaches were used to assess the vulnerability of a park to the impacts of climate change. First, combined indicators were used to calculate a total potential vulnerability score for each park. Parks with the highest score – either nationally or within their ecoregional groups – were identified as priorities for further study. Second, parks that met certain thresholds for selected high-impact vulnerability factors – namely, wildfires, drought, invasive species (insect pest and disease damage) and sea-level rise were identified. These high-impact vulnerabilities have the potential to transform park resources dramatically and rapidly and have available quantitative indicators. These four high-impact vulnerability factors are the foundational data upon which our updated Polluted Parks report is built for the climate change section. In the report, we refer to them simply as “Climate Threats”.

Drought refers to the mean projected change in the number of moderate or worse drought events per park over a 50-year period (2006-2055).

Invasive Species include insects, pathogens, or diseases that affect trees and vegetation in natural ecosystems in national parks. NPS used two indicators of insect pest and disease risk: 1) modeled percent basal area mortality for forested areas within the park, and 2) percent park area designated as being “at-risk” for damage. Some of the invasive insects affecting national parks include the Asian Longhorned Beetle, Balsam Woolly Adelgid, Beech Bark Disease, Chestnut Blight, Dogwood Anthracnose, Dutch Elm disease, Emerald Ash Borer, Gypsy Moth, Hemlock Woolly Adelgid, Sudden Oak Death, White Pine Blister Rust, and Winter Moth, among others.

Sea Level Rise focuses on the projected percentage of a park area inundated by seal level rise or by storm surge by 2050.

Wildfire pertains to the frequency and probability of wildfire episodes in parks. To identify priority parks for wildfire impacts, NPS used three indicators: a USDA Forest Service map of current fire hazard and two global-scale projections of changes in future fire activity.

For more detailed information about the analytical methods for the climate vulnerability analysis visit the NPS Data store to access the full study.

Please contact Daniel Orozco at with any questions.

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