YELLOWSTONE NATIONAL PARK, Wyo. — The air quality at two of the most popular winter spots here continues to improve, a new study finds.

Levels of carbon monoxide and certain particulate at the park’s west entrance at West Yellowstone, the most popular winter gate, and Old Faithful, the most popular destination, still are “well below” national ambient air-quality standards and the air quality is good, the study by the National Park Service concluded.

Those pollutants have decreased in recent winters because fewer snowmobiles travel into the park and because of new requirements that all snowmobiles meet standards for noise and emissions, park officials said.

The study analyzes air quality at Old Faithful and the West Yellowstone entrance during the winter of 2005-06. It’s the latest in a series of studies aimed at helping park officials decide how best to manage winter traffic in Yellowstone.

Late last year the Park Service released, for technical review, a draft plan to several counties and public agencies. A draft plan is expected to be released this spring for public comment, and a final decision is expected before next winter.

The latest proposal is to allow up to 720 snowmobiles and 78 snowcoaches into Yellowstone each day as long as they meet the “cleaner and quieter” standards. Snowmobilers would have to travel with a commercial guide.

The proposal is similar to a temporary plan enacted several years ago. Since then, the average number of snowmobiles entering the park each day has been 250 to 260.

Last winter, more than 13,000 snowmobiles entered the park, along with a record 1,401 snowcoaches.

John Ray, an atmospheric chemist for the Park Service who analyzed the 2005-06 data, said in his report that air quality has been improving the past three winters and that snowmobiles and snowcoaches have had similar carbon monoxide concentrations in the past two years.

Carbon monoxide levels stayed roughly the same last winter, despite an increase in the number of vehicles, and particulate readings were slightly higher than the previous year, the report said.

“At present concentrations, these pollutant concentrations are not thought to represent a health hazard to park staff or visitors,” the report said.

The report also looked at tiny specks of visible pollution, called particulate matter, often associated with smoke, dust and haze.

During the winter at Old Faithful, the particulate matter concentrations were highest in the early morning or at night when snowmobiles are not running. The increase is from other sources, probably wood-burning stoves and kitchen exhaust, the report said.

Particulate levels were higher during the summer in Yellowstone and were attributed not to more traffic but to smoke from wildfires, the report said.