NPS: Nature & Science» Air Resources Division

2005 Yellowstone Air Quality Study Summary - Air Quality Improves Due to Fewer Snowmobiles

Visitors enter Yellowstone National Park by snowcoach and snowmobiles
West Entrance of Yellowstone National Park, MT - Winter 2005

Yellowstone National Park is a popular place to visit in both summer and winter. Visitors have been traveling into the park for years during the winter in snowcoaches, and more recently on snowmobiles. In recent years, the use of winter travel vehicles in national parks has become a very controversial issue. Snowmobiles emit large amounts of carbon monoxide (CO) and concentrations of CO at park entrance stations have approached unhealthy levels. In order to protect park resources, the NPS designed a Winter Use Plan that requires 1) reductions in the number of snowmobiles allowed into the park each day, 2) guides to accompany the snowmobiles, and 3) that snowmobiles be equipped with cleaner 4-stroke engines.

Winter monitoring data from two locations in the park show that under the current Winter Use Plan, air quality has improved; concentrations of both carbon monoxide and particulate matter have decreased considerably.

Graph showing trends in west entrance total traffic and carbon monoxide concentrations

The figure to the right shows that the amount of winter snowmobile traffic and the maximum hourly CO concentrations have decreased simultaneously since 2002. The Winter Use Plan allows up to 720 snowmobiles per day to enter the park. During the 2004-2005 winter season, a markedly lower number of snowmobiles entered the park than in previous years (an average of 260 snowmobiles per day). The reduction in snowmobile traffic is the primary reason for improvements in air quality, but the cleaner 4-stroke engines that are now required and weather have also played a role.

As snowmobiles have gotten cleaner, the emissions from the snowcoaches have come under closer scrutiny. Direct measurements of snowcoach emissions by the University of Denver have shown that on a per-passenger basis the average emissions from snowcoaches are about the same as the cleaner 4-stroke engine snowmobiles. Most of the snowcoach fleet consists of vehicles that predate modern pollution controls on truck engines, so there is room for improvements as the fleet turns over to newer vehicles. The park is currently considering a Best Available Technology (BAT) standard for snowcoaches and other techniques for bringing down winter vehicle emissions in Yellowstone.

More information on this topic is available in the recently released Winter Air Quality Monitoring Report for 2004-2005.

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