The effects of winter vehicle exhaust, primarily from snowmobiles, on air quality became an issue in the later 1990's at Yellowstone National Park, WY. For the last several years, ambient air quality monitoring has been conducted at two locations in the Park, as part of the adaptive management plan. The plan is to determine the impact on air quality of implementing the 2004 temporary plan and now the 2007 Yellowstone Winter Use Plan (Federal Register 2007, NPS 2007). This report is an annual update that summarizes the carbon monoxide (CO) and particulate matter of 2.5 micrometers or less (PM2.5) monitoring data from winter 2007-2008 and provides a historical perspective of monitoring data at the park. The primary interest is trends in air quality that might reflect on winter use policy and the present conditions as compared to the national standards (EPA 2008) set by the Environmental Protection Agency (EPA).