While there are many, many more cars, trucks, and motorcycles traveling through Yellowstone National Park in summer than snowmobiles in winter, the summer traffic is not as polluting, according to park studies. In fact, a greater problem with Yellowstone's air quality in summer stems from forest fires.
As part of the park's never-ending winter-use research, a study of Winter Air Quality in Yellowstone National Park, 2007-2008, showed a somewhat slight difference in pollution levels during summer and winter when it came to carbon monoxide and particulate matter.
The air quality at Yellowstone changes with the seasons in a way that follows the amount and type of mobile sources and the seasonal weather. Changes are most evident at the West Entrance station. Summer CO (carbon monoxide) is lower for maximum hourly and maximum 8-hour concentrations than the winter concentrations despite the large difference in traffic volume between summer and winter seasons. The seasonal averages include long periods of night time CO concentrations that are low, so that metric shows little difference at the resolution of the monitoring instruments. During the spring and fall season, when traffic is low or the park is closed, the CO concentration averages go down to the background regional concentrations. Carbon monoxide air quality is below the standard in all seasons and at its lowest in the fall when the park roads are closed to the public.
While the park reports "very large" particulate matter (PM2.5) concentrations in summer and early fall, those are attributed to the smoke from wildfires, not from traffic, the study said.
Air-quality monitoring in the park, though, is limited, and in some cases could be described as in its infancy; "No trend in the data is evident for the summer season; the number of available seasons of data is still quite small since summer monitoring started only in 2006," noted the authors. Too, there were only two monitoring stations used for this study. Air quality in winter was measured at West Yellowstone, where the bulk of the winter snowmobile traffic enters the park, and at Old Faithful, a destination for most of this traffic.
There are two other air-quality monitoring stations in the park, one near the Lake maintenance facility and another near Tower Junction. But neither of these monitors tracks CO emissions, and winter traffic past these areas is very light, according to the study.
Looking more closely at the 2007-2008 monitoring, West Yellowstone logged a maximum one-hour carbon monoxide concentration of 6.1 parts per million and a maximum eight-hour concentration of 1.6 ppm. During the summer of 2007, those numbers were 1.5 ppm and 3.1 ppm; again, the somewhat high 3.1 ppm measurement stemmed from a forest fire, the report's authors noted.
All that said, air quality is improving in Yellowstone, according to the study's authors, and remains below national standards for carbon monoxide and particulate matter. Here's the conclusion reached by those authors:
The air quality has stabilized at the monitoring stations in Yellowstone National Park over the last 4-5 years. This is primarily from the requirement for BAT snowmobiles (NPS 2008) and a much lower number of snowmobiles entering the park. The observed air quality is dependent on several meteorological conditions that dominate changes in the CO concentrations when there are just small changes in the snowmobile traffic. Overall, the current CO and PM2.5 air quality is well below the national standards during the winter.
Higher CO and PM2.5 concentrations are observed in locations where there is a higher density of vehicles. Total winter vehicles per day or per season are less of a factor than the vehicle density and
time of day when they are operating. Early morning operations and high volumes of traffic within the 9-10 am hour contribute to maximum CO and PM2.5 concentrations at the entrance stations more so than if the traffic were more spread out.
Entrance gate procedures contribute greatly to the concentration of traffic. That was even more evident in the past when individual entrance passes were sold to winter visitor traffic. Since most snowmobile visits are by guided groups, streamlined entrance pass checking is done that reduces backups at the gate. Other procedures that might be used include assignment of entrance times to the guided groups. This could spread out the traffic so that extremely long lines of snowmobiles didn’t develop at the entrance station or along the route into Old Faithful. It would also spread out the traffic at Madison Junction so that fewer snowmobiles were there at any one time. This procedure may conflict with soundscape or wildlife interaction management, but might be considered.
Some consideration should be made as to whether the national standards are really the best indicators of good air quality in a natural area such as Yellowstone. At current winter traffic levels the day-time maximum CO continues to be higher than the regional background and higher than the summertime concentrations. This reflects the higher emission rates of the BAT snowmobiles and the older snowcoaches (Bishop et al. 2007) relative to the wheeled vehicles used during the summer, as well as the reduced atmospheric mixing during the winter.
John Ray, an atmospheric chemist for the National Park Service and one of the report's authors, says the arrival of four-stroke snowmobiles and the requirements for "best available technology" in snowmobiles is bridging the pollution gap between winter and summer.
"Keep in mind that the CO at the 'hotspots' in Yellowstone is about what one would have in a small town. It is less that what (the town of) West Yellowstone has," says Mr. Ray. "A half-mile from the road and farther and the CO will be the regional pollutant value. There is little evidence to show that the current CO concentrations cause health problems or affect natural resources. The main difference is that there are short-term higher CO concentrations in winter, but they are still below the EPA health standard."
You can find the 2007-2008 report here: http://www.nps.gov/yell/parkmgmt/upload/aq70-08__final.pdf