Air Quality and Smoke Monitoring
The Clean Air Act Amendment of 1977 designates Yosemite National Park as a Class I area, setting high standards for air quality and visibility in and around the park. While protected, the park’s air quality is far from perfect. Smoke and emissions, both local and regional, often degrade air quality. Particulate matter and increased ground ozone levels negatively affect visitors’ enjoyment of the park by reducing visibility and potentially affecting respiratory health. A robust air quality monitoring program throughout the park monitors visibility, particulate matter, ground level ozone, and atmospheric deposition.
Smoke in and near Yosemite National park is often responsible for elevated particulate levels. Smoke is often present during fire season, as Yosemite and the surrounding area is adapted to burn between July and November, or until the rains arrive. During the cooler, wetter season, it is not uncommon for short periods of smoky conditions, as park fire managers may conduct controlled burns or pile burning for forest management. Smoke emitted from campground campfires can also degrade local air quality. In Yosemite Valley, the nighttime inversion can trap campfire smoke and particulates, sometimes causing air quality to be unhealthy for sensitive groups.
Particulates in smoke can impact visibility and negatively affect respiratory health. Visit AirNow for a color-coded report of the current Air Quality Index and any health recommendations. Check the above webcams to monitor current smoke conditions, keeping in mind that wind and weather can change conditions quickly and dramatically.
Elevated levels of ground level ozone pollution are typically correlated with warm days, with the highest concentrations recorded between May and October. In 2020, Yosemite National Park recorded 12 days of ground level ozone in excess of federal standards. Days of unhealthy ozone levels in the park generally coincide with days of poor air quality in California’s Central Valley, with pollutants carried to the park on afternoon westerly winds. Mid-elevation west-facing slopes in Yosemite such as El Portal, Wawona, and Crane Flat record higher ozone levels, with the highest concentrations of ozone usually occurring in the afternoon and evening. Yosemite Valley and Tuolumne Meadows are more sheltered areas with generally low ozone levels.
Elevated ozone levels can reduce visibility, irritate the human respiratory system or cause other health issues, and stress plants and animals. View current ground level ozone levels at Turtleback Dome, situated just above the landmark Tunnel View.
Air Quality Monitoring Program
Yosemite National Park is a host site to many long-term air quality monitoring projects, including Clean Air Status And Trends Network (CASTNET), the National Atmospheric Deposition Program (NADP) and the Interagency Monitoring of Protected Visual Environments (IMPROVE). Two beta-attenuated monitors (BAMs) monitor concentrations of particulate matter 2.5 and PM10 at Yosemite Village, and automatically update the AirNow map. The map also reports PM2.5 sensors throughout the park that belong to various local organizations and residents. When conditions are smoky, the National Park Service deploys portable BAMs throughout the park to monitor conditions and provide advisories for areas of high visitation or residency. Additional PM2.5 points will automatically appear on the map when EBAMs are deployed.