Parts of the Twin Cities area woke up to unhealthy air quality on Wednesday morning, as a classic wintertime inversion pushed pollution from vehicle tailpipes close to the ground.

Minnesota Pollution Control Agency air quality meteorologist David Brown said that stagnant air across the state and a relatively mild weather pattern allowed the fine particles to linger, when normally wind would have diluted the pollution and whisked it away.

"The emissions that cause this tend to happen most in the morning," Brown said, as people across the region drove to work.

A monitor in Minneapolis' Phillips neighborhood reached an air quality index (AQI) of 166 at 8 a.m. Wednesday, and another sensor on NE. 3rd Street in Minneapolis reached 151 at the same time. Both readings are in the red classification of AQI, meaning the air is unhealthy for all people to breathe.

But around 11 a.m., sensors in parts of south and northeast Minneapolis showed improving air in the orange AQI category — meaning it was unhealthy only for sensitive groups, including people with breathing and cardiovascular problems.

Wintertime inversions are not new in Minnesota. One extreme example in January 2023 sparked an air quality alert for much of the state and spread a yellowish haze across the metro area. But smoke drifting south from Canadian wildfires drove much of Minnesota's declining air quality last year, sparking a record number of bad air alerts from the MPCA.

It's not yet clear what kind of smoke season 2024 will bring, Brown said, but conditions in Canada are similar to this time last year — warm and dry, raising the risk that fires could spark before green plants emerge, which helped contribute to last year's rash of blazes.

Forecasters are also watching reports of "zombie fires" that burn in the peat of boreal forests, even under layers of snow. The BBC recently reported that residents of northern Canada are already reporting smoky smells, indicating a record number of overwintering fires.

Brown said he expects Canadian officials to start issuing fire season outlooks in March, and MPCA will release its own smoke forecast later in the spring.