YELLOWSTONE NATIONAL PARK, Wyo. (AP) - Four-stroke snowmobiles and snowcoaches are cleaner than the two-stroke snowmobiles that have been popular and widely used here for years, a study shows.

The study, funded by the park, was conducted by a team of university scientists last February, when there were virtually no limits on snowmobiles entering the park. Results were announced Monday, just weeks after a federal judge ordered that snowmobiles be phased out in favor of snowcoaches. Snowmobiles are still allowed in the park this winter, with limitations.

Cheryl Matthews, a Yellowstone spokeswoman, said the Park Service wanted to make the study available "as part of our ongoing winter use studies. We have been monitoring and looking at winter use for some time."

For the study, more than 200 air samples were taken from sites throughout the park from Feb. 12-16, 2003.

Exhaust samples also were taken from snowmobiles, a snowplow and snowcoach to get "fingerprints" from each and were used to compare relative emission levels of volatile organic compounds from the various machines, officials said.

The project's lead investigator, Barkley Sive, of the University of New Hampshire, said two-stroke machines are "much more polluting." Even in more remote areas of the park, he said, high levels of emissions consistent with two-stroke snowmobile exhaust were found.

"One of our recommendations was to reduce or phase-out the use of two-strokes in the park," said Sive, an atmospheric chemist with the Climate Change Research Center at UNH's Institute for the Study of Earth, Oceans and Space. "It was sort of painfully obvious, when you look at the measurements, that they are sort of the significant or the main polluter."