Colorado air quality today

Nine counties share one ozone designation along the Front Range: Adams, Arapahoe, Boulder, Broomfield, Denver, Douglas, Jefferson, Weld and part of Larimer. EPA reclassified that area from serious to severe under the 2008 ozone standard in 2022, and it carries a serious classification under the tighter 2015 standard. This is a summer designation. The ozone is built in place, from precursors cooking under strong high-altitude sunlight over a corridor that runs north to south between the mountains and the plains.

The precursor mix is the part specific to Colorado. Weld County produces more oil and gas than any other county in the state and sits over the Denver-Julesburg Basin, upwind of the metro area on many summer days. The FRAPPÉ field campaign found vehicle emissions and oil-and-gas production emissions to be the largest local contributors to ozone formation along the northern Front Range, and the Air Quality Control Commission has since tightened requirements on upstream oil and gas operations with the stated aim of halving their ozone-forming emissions by 2030 against a 2017 baseline. Wildfire smoke stacks on top of all that: NOAA identified smoke as a major contributor to Front Range ground-level ozone during the summer of 2021.

The Air Pollution Control Division at the Colorado Department of Public Health and Environment, together with the Regional Air Quality Council, issues Ozone Action Day alerts for the Front Range urban corridor through the warm months. West of the Continental Divide the mechanisms change — mountain valleys, small towns, smoke arriving from fires in other states — and our page count there is low.