The Positive Impacts of Policy Implementation on Wintertime Air Quality in Yellowstone National Park through 2024

• Barkley C. Sive, Chemist, Air Resources Division, National Park Service
• Lisa M. Devore, Air Quality Specialist, Intermountain Region, National Park Service
• Munkhzaya (Mooji) Boldbaatar, GIP Intern, Air Resources Division, National Park Service
• Ann Rodman, Yellowstone Center for Resources, Yellowstone National Park
• Hillary Robison, Deputy Chief of Yellowstone Center for Resources, Yellowstone National Park

Abstract

Motorized access to Yellowstone National Park during the winter season has been historically contentious, with continual concerns regarding potentially affected resources, including air quality. As seasonal motorized usage increased, the National Park Service (NPS) began air quality monitoring at two locations in the park (West Entrance and Old Faithful) during the 2002-2003 winter season (December 15 - March 15) as part of the adaptive management program on the use of over-snow vehicles (OSVs, i.e., snowmobiles and snow coaches). Criteria pollutant monitoring was conducted, including ambient concentrations of carbon monoxide (CO), particulate matter of 2.5 microns or smaller (PM_2.5), and nitrogen oxides (NO_x = NO+NO₂), to quantify OSV emissions and assess their impacts on air quality, visibility, human health, and other park resources. Fundamental to the decision-making process, robust data and scientifically sound information were required in order to ultimately reduce the impairment of park resources resulting from OSV usage.

For this work, we will explain the historical background of wintertime OSV usage and air quality changes after the allowed usage was implemented. The 2013 long-term winter use management rule that established transportation event limits and standards has resulted in dramatic air quality improvements for CO and PM_2.5 levels in the park. This implementation also positively affected wildlife response, soundscape, and acoustic environments. The rule also ensured that historic data exists to understand and quantify changes in overall air quality data and to evaluate pollutant trends. The culmination of the air quality monitoring aimed at understanding the impacts of OSVs in Yellowstone National Park provides an excellent example of how balancing policy and public interests can result in positive environmental impacts.

The management of over-snow vehicles (OSVs; snowmobiles and snow coaches) has been a contentious issue. But over time, with advancement in technology and transportation planning, the park can maintain this winter use and reasonable air quality.

NPS/Jacob Frank

Motorized Winter Use in Yellowstone

The story of the emergence of the Yellowstone National Park snowmobile issue and its transcendence to a high-profile issue commanding national attention is an interesting but complex one, the telling of which, in a manner that captures the nuances of the public policymaking process, lies beyond the scope of this article. We summarize here the key events that allowed for a compromise that improved air quality and for a continued positive wintertime experience in the park.

The main driver of the wintertime air quality issues stem from the numbers and types of OSVs entering and touring the park during the winter use season, which spans from December 15 to March 15. The use of snowmobiles in Yellowstone grew substantially in the 1990s. Starting in the mid- to late-1990s through the 2001-2002 season, there were approximately 60,000 OSVs entering the park (Figure 1).

Figure 1. Over-snow vehicle count timeline in Yellowstone National Park.

The fleet at the time was 2-stroke snowmobiles and snowcoaches that did not use clean-burning 4-cycle engine technology. This, coupled with cold temperatures at the surface, created an ideal scenario for pollutant concentrations to rise to potentially harmful levels. Figure 2 illustrates the blue haze exhaust emitted from 2-stroke snowmobiles that was regularly observed during this time, causing air quality and human health concerns.

Figure 2. Blue haze from engine exhaust builds up as snowmobiles idle in groups like this or around entrance stations.

In 1999, there was a petition from environmental groups to ban snowmobiling in the park. In ­2000, the NPS decided to phase out most snowmobile use in Yellowstone and Grand Teton national parks to protect air quality related and other issues. The notion of eliminating snowmobile use as part of the wintertime experience caused strong backlash from local vendors and businesses that relied on winter use access to Yellowstone for their livelihood along with visitor recreational use concerns. At the same time, visible exhaust plumes and elevated pollutant levels emitted from snowmobiles in a protected, pristine place spawned widespread public criticism.

To snowmobile or not to snowmobile; this was the key question in 2002. In November 2002, the Bush administration overturned the ruling by the Clinton administration to ban snowmobiles in the Greater Yellowstone Area (GYA). In 2003, the NPS commenced a winter use program with guided only entry, implementation of best available technologies (BATs), and OSV limits.

During the 2003-2004 season, there were approximately 40,000 fewer OSVs entering the park; however, 25,000 is still a lot of OSVs. Although there was significant reduction of OSVs and a transition to cleaner 4-cycle engines, it is worth noting that the park was litigated for every winter use program it put forth and public concerns continued.

In February 2013, the NPS published a final Winter Use Plan and Supplemental Environmental Impact Statement (SEIS) to guide the future of winter use in Yellowstone National Park. This plan proposed to manage OSVs based on “transportation events” rather than the number of snowmobiles and snow coaches allowed in the park each day. By packaging traffic into transportation events (i.e., groups) and limiting the total number of transportation events, the park was able to improve air quality conditions while allowing more visitors to see the park in winter. Within the allowable number of transportation events, commercial tour operators have the flexibility to combine snow coach and snowmobile trips in a way that ultimately protects park resources and responds to fluctuations in visitation demand.

Table 1 shows allowable OSVs per day by transportation events, with a maximum of 110 events. Voluntary “Enhanced BAT” (E-BAT) certification allows commercial tour operators to increase average snowmobiles per event from 7 to 8 and snowcoaches from 1 to 1.5 across the season (Table 1, column 4). Currently, snowmobiles do not meet E-BAT standards.

Table 1. Allowable OSVs per day each winter season by transportation events.

	Maximum number of transportation events daily	Maximum number of OSVs allowed daily1	Average number of OSVs allowed daily (each winter season)	Maximum average number of OSVs (if E_BAT) daily
Snow coaches	60	60	60	120
Commercially guided snowmobiles	46	460	322	368
Non-commercially guided snowmobiles	4	20	20	20
Total	110	540	402	488

¹Although the maximum number of OSVs is much greater than the number of events, there is a limit on the number of vehicle types in a group. For example, non-commercially guided groups are limited to 5 snowmobiles to enter through each of the 4 park entrances every day. For commercially guided snowmobiles, there is a cap of 10 snowmobiles per transportation event.

Air Quality Standards

Yellowstone National Park is a designated Class I area under the federal Clean Air Act (CAA), and as such, is afforded the highest levels of protection from visibility and other associated air quality impairments. Impacts of OSV emissions on park air quality are evaluated using the Environmental Protection Agency’s (EPA) National Ambient Air Quality Standards, or the NAAQS. The CAA requires the EPA to set standards for 6 principal pollutants, or the “criteria” pollutants; of the 6 criteria pollutants, the 3 of concern for OSV emissions are carbon monoxide (CO), particulate matter with diameters less than 2.5 microns (PM_2.5), and nitrogen dioxide (NO₂). There are 2 types of standards, primary and secondary, that are set to protect human health and welfare. Table 2 shows the different NAAQS levels by pollutant applicable for the adaptive winter use management plan.

Table 2. National Ambient Air Quality Standards for CO, PM_2.5, and NO_2.

NAAQS Pollutant	Standard	Standard
CO	35 ppm (1-hr CO)	9 ppm (8-hr CO)
PM2.5	35 µg/m3 (24-hr PM2.5)	12 µg/m3 (Annual PM2.5)
NO2	100 ppb (1-hr NO2)	53 ppb (Annual NO2)

Source: NAAQS Table | US EPA

While the NAAQS values are used to address violations of the regulatory thresholds, more recent research has shown that health effects can occur at lower levels. Additionally, short-term elevated emissions when OSVs were operating nearby showed increased levels of key criteria pollutants and air toxics to well above background levels, to the point that levels more closely resembled an urban area as opposed to a pristine environment.

Monitoring Overview

To fully understand the impact of OSV emissions in the park, long-term measurements for associated pollutants were not only necessary, but fundamental to the NPS mission. The NPS has an affirmative legal responsibility to protect clean air in national parks. The 1916 NPS Organic Act created the agency with the mandate to conserve the scenery, natural and cultural resources, and other values of parks in a way that will leave them unimpaired for the enjoyment of future generations. Under this umbrella, the NPS Air Resources Division is tasked with identifying air pollution sources and potential risks affecting park resources by providing scientifically robust data to support mitigation and future protection of resources. As such, air quality monitoring achieves the following objectives:

• Identify air pollutants with the potential to injure or damage park natural resources.
• Establish existing, or baseline concentrations in NPS units and monitor over time to assess air quality conditions and trends.
• Assist in development and revision of national and regional air pollution control policies affecting park resources.

The Yellowstone winter-use air quality monitoring program was fully implemented during the 2002-2003 season, which, unfortunately, did not capture the maximum impacts associated with higher visitation experienced during the previous years. From the 2003-2004 season onward, the average number of OSVs entering the park leveled off at approximately 21,000, a 61% decrease from the preceding 6 years, with a slight uptick in numbers during the last 3 seasons (Figure 1). However, by simply reducing numbers of OSVs and using the best available technology for the cleanest burning snowmobiles, a dramatic change occurred in the overall air quality with reductions in concentrations of the criteria pollutants CO and PM_2.5. Monitoring data from the West Entrance is used as a comparison to Old Faithful because of its proximity to West Yellowstone, Montana. The air quality monitoring at the West Entrance is crucial because it serves as the indicator for understanding air quality impacts from incoming vehicles and other city traffic located upwind of the park. Old Faithful is the primary destination for most of the winter-use vehicles within the park and provides additional vehicle usage impacts as well as background concentrations at times of lower usage or during other seasons.

As a robust marker of combustion processes, as in OSV exhaust, CO levels are examined first. The CO levels measured at the West Entrance and Old Faithful sites are shown in Figure 3. The top panel in Figure 3 shows the maximum 8-hour average CO concentration for each winter season and the bottom panel shows the corresponding maximum 1-hour average. The dates on the plots show the “end year” of each winter season. For reference, background atmospheric CO levels are about 0.1-0.2 ppm and are roughly 1 to 2 orders of magnitude below the standard. As illustrated in Figure 3, both sites are below the NAAQS thresholds but experience concentrations well above background atmospheric levels, attributed to the type and quantity of OSVs and their associated emissions.

Figure 3. CO levels at the Yellowstone’s West Entrance and Old Faithful.

Notably, both graphs show that decreasing CO levels at both the West Entrance and Old Faithful sites with concentrations leveling off since the 2002-2003 season. Although there is data variability, these observed decreases are significant and demonstrate tangible results based on policy-driven visitation and vehicle changes. At the West Entrance, during the 2002-2003 season, the max 1-hr CO was 8.6 ppm; during the 2023-2024 season, it was 0.8 ppm, resulting in a decrease of approximately 91% in the 1-hr max value. For Old Faithful, the 2002-2003 max 1-hr CO was 2.9 ppm and the 2023-2024 value was 0.25 ppm; the corresponding decrease was about 91% with Old Faithful achieving close to background levels of CO. Because these are the highest CO levels for the season, we have effectively moved into a regime where the OSV emissions are not influencing the ambient data at Old Faithful. For the 8-hr standard at the West Entrance, during the 2002-2003 season, the max 8-hr CO was 3.3 ppm; during the 2023-2024 season, it was 0.33 ppm, a decrease of about 90%. For Old Faithful, the 2002-2003 max 8-hr CO was 1.2 ppm and the 2023-2024 value was 0.17 ppm; the corresponding decrease was about 86% with Old Faithful achieving close to background levels of CO, for both the 1-hour and 8-hour max values. The key point is that by changing the fleet composition and numbers of over-snow vehicles has dramatically reduced the maximum levels of CO in the park and is either approaching or achieving background levels (based on averaging times) at both sites.

For PM_2.5, there were decreases in concentrations similar to observed CO levels over time. Figure 4 shows the 98^th percentile of the 24-hr average PM_2.5 concentrations beginning at the 2002-2003 winter season. At the West Entrance, during the 2002-2003 season, the 98^th percentile of the 24‑hr average PM_2.5 concentration was 15.4 micrograms per cubic meter (µg/m³); during the 2023-2024 season, it was 3.4 µg/m³, a decrease of about 78%. For Old Faithful, the 2002-2003 the 98^th percentile of the 24-hr average PM_2.5 concentration was 23.0 µg/m³ and the 2023-2024 value was 3.4 µg/m³; the corresponding decrease was about 85%. Like CO, this drop was a direct result of changing OSV fleet type and numbers entering the park.

Figure 4. PM2.5 levels at Yellowstone’s West Entrance and Old Faithful.

However, for NO₂ emissions, the implications of the technology tradeoff are observed, that is, switching from 2-stroke to 4-stroke snowmobiles. While 2-stroke engines used in snowmobiles emit high levels of CO, PM_2.5, and hydrocarbons, they emit lower levels of NO_x than cleaner-burning 4-stroke engines. Therefore, the decreases observed in CO and PM_2.5 levels corroborate the changeover in engine types allowed into the park. Figure 5 shows the 98^th percentile of the 1-hour daily maximum NO₂ concentrations. Unlike CO and PM_2.5, there is no statistically significant decreasing trend (p > 0.5) in the NO₂ data at the West Entrance (Figure 5). For Old Faithful, the NO₂ levels have been highly variable; however, over the last two winter seasons, they have been trending towards those of the West Entrance (Figure 5). These observations are attributed to the fact that 4-stroke OSVs produce more NO₂ emissions. The nitrogen oxides measurements, which include NO₂, commenced for the 2009-2010 season at the West Entrance and the 2019-2020 season at Old Faithful. Unfortunately, these measurements did not begin until after the fleet turnover had been implemented, but they still provide critical insight related to OSV emissions. To put these values into perspective, the hourly NO₂ concentrations measured at both sites, not just the 98^th percentile levels of the 1-hour daily max, are representative of urban or highly polluted areas. Therefore, the concurrent NO₂ measurements at the West Entrance and Old Faithful will enable us to assess 4-stroke snowmobile emissions on ambient NO₂ levels moving forward.

Figure 5. NO2 levels at Yellowstone’s West Entrance and Old Faithful.

Summary

Since the inception of air quality monitoring in 2002-2003, we note measurable improvements in the overall air quality at Old Faithful and the West Entrance. Decreases observed in CO and PM_2.5 related to OSV usage highlight the positive effects on air quality stemming from the winter use policies implemented in the park. The tradeoff on pollutant emissions from OSV engine type (2-stroke vs. 4-stroke) is clear, with significant decreases in CO and PM_2.5 levels resulting from the fleet type changeover coupled with controlling the number of transportation events. The newly deployed NO_x measurements at Old Faithful are crucial to comprehensibly assess park air quality impacts. To fully understand the impact of OSV emissions in the park, including emissions from other regional sources, long-term measurements for all pollutants continue to be necessary at both monitoring sites, and potentially at other locations throughout the park. Additional positive aspects of the winter use policies are that wildlife response to OSVs is significantly reduced and acoustic impacts have improved resulting from the stricter OSV standards. Working with the public to implement amicable policies over time has allowed for the Yellowstone wintertime visitation experience to continue in a way that is truly Americana.

A snowmobile group gets ready for a ride at Falls Ranch.

NPS/Jacob Frank

Acknowledgements

We thank past and present Yellowstone site operators and park staff, particularly John Klaptosky, and our contractor Air Resource Specialists, including Mike Slate, Dave Beichely, Jessica Ward, Scott Cismoski, Frank Schreiner, and Joe Adlhoch; the NPS Air Resources Division, especially Ksienya Taylor; and Jonathan Nicholson at the Yellowstone Center for Resources.

Further Reading

National Park Service. 2021.
Yellowstone Center for Resources: YCR Winter Use Monitoring Summary Findings 2014-2020, prepared by Environmental Quality and Compliance, 2021. Winter Use Monitoring Summary of Findings 2014-2020

National Park Service. 2020.
Winter Use Adaptive Management Adjustments - Change to East Entrances Dates/South Entrance Times, Yellowstone National Park, Mammoth, Wyoming, USA.

National Park Service. 2016.
Winter Use Adaptive Management Plan, Yellowstone National Park, Mammoth, Wyoming, USA. ParkPlanning - Winter Use Adaptive Management Plan

National Park Service. 2013a.
Yellowstone National Park Final Winter Use Rule. U. S. Department of the Interior, National Park Service, Yellowstone National Park, Mammoth, Wyoming, USA.

National Park Service. 2013b.
Yellowstone National Park Winter Use Plan/Supplemental Environmental Impact Statement. U. S. Department of the Interior, National Park Service, Yellowstone National Park, Mammoth, Wyoming, USA.

National Park Service. 2011.
Scientific Assessment of Yellowstone National Park Winter Use March 2011.

Shively, D. D., B. M. C. Pape, R. Mower, Y. Zhou, R. Russo, and B. C. Sive. 2008.
Blowing Smoke in Yellowstone, Air Quality Impacts Resulting from Oversnow Motorized Recreation in Yellowstone National Park. Environmental Management 41(2): 183-199.

Zhou, Y., D. Shively, H. Mao, R. Russo, B. Papa, R. N. Mower, R. Talbot and B. Sive. 2010.
Air Toxic Emissions from Snowmobiles in Yellowstone National Park. Environmental Science & Technology. 44: 222-228.

Cite this article as:

Sive, B. C., L. M. Devore, M. Boldbaatar, A. Rodman, and H. Robison. 2025. The positive impacts of policy implementation on wintertime air quality in Yellowstone National Park through 2024. National Park Service, Intermountain Park Science, 3(1), 2025.