St. Paul, MN Air Quality Index (AQI) Today

Two conditions produce this city’s bad readings and they have nothing to do with each other. The first is local and small. The Mississippi runs through St. Paul in a trench between steep bluffs, and on a still, clear, subzero night cold air drains off the high ground into it while a surface inversion caps whatever has collected there, holding vehicle exhaust and residential wood smoke near the ground until the sun climbs far enough to break the layer. The neighborhoods down on the flat and the ones on top of the bluff line are not breathing the same air on those mornings. Flint Hills Resources’ Pine Bend refinery, the largest in Minnesota, sits down the same valley at Rosemount and Inver Grove Heights, which a southeasterly puts upwind.

The second condition is continental. Minnesota shares a border with Manitoba and Ontario and lies in the usual downwind path of Saskatchewan’s fires as well, and since 2023 the state’s air quality alert record has been dominated by smoke arriving from boreal forest outside the country. That year ran alerts over more days than any year before it.

Nothing about the two overlaps. An inversion morning is worst before dawn, usually gone by afternoon, and worse in the trench than a mile away on top of the bluff. Smoke keeps no hours, covers the whole state at once, and can hold for a week.