Boulder, CO Air Quality Index (AQI) Today

Boulder County is inside the Denver Metro/North Front Range ozone nonattainment area, along with Denver, Adams, Arapahoe, Broomfield, Douglas, Jefferson and parts of Larimer and Weld. The mechanism behind that designation is partly geographic. On weak-wind summer days, thermally driven mountain-valley circulation pulls air upslope toward the foothills through the afternoon and can recirculate it rather than flush it out, so the terrain at the western edge of the metro helps hold the air it receives.

What rides in on that flow has an unusual source split. Urban nitrogen oxides from the US 36 and I-25 corridors combine with volatile organic compounds from the Denver-Julesburg oil and gas field spread across Weld County to the northeast. The FRAPPÉ and DISCOVER-AQ field campaigns found that on individual days the oil and gas precursors alone can add more than 30 parts per billion of ozone, which the work linked directly to exceedances of the federal standard.

The season runs roughly May through September, peaks in the middle and late afternoon, and foothills monitors often read higher than downtown Denver. Winter works against a buildup rather than for one here. The downslope windstorms that hit this stretch of foothills are severe ventilators: in the December 2021 event, NCAR’s Mesa Lab clocked a 112 mph gust.