Why Was The Twin Cities Under An Air Quality Alert For So Long?

MINNEAPOLIS (WCCO) -- An air quality alert has expired in the Twin Cities. The stubborn air mass wouldn't budge for three days. But why have we seen such dirty air lately?

The calm winds we've recently had is just one of the factors leading to poor air quality. In Minnesota, poor air quality can come from Canadian wildfires, but the stalled atmospheric set-up that led to this week's unhealthy air isn't as common.

Daniel Dix of the Minnesota Pollution Control Agency has been tracking the stagnant air mass causing the poor air quality all week. He says it's been four years since the Twin Cities have seen a stagnation event like this.

"We've had an inversion, and the clouds and the fog we've all experienced since last Friday. It's just been a build-up day after day," Dix said.

An inversion is where warm air is trapped between two areas of cold air, which acts like a lid suppressing pollutants down into the lower atmosphere.

"The good news is we are seeing an improving trend with air quality out in the Dakotas and western Minnesota, [and] there's a front that's moving through," Dix said. "Clouds are starting to break up and the air quality numbers are already on the improvement."

It's a welcome change for Minnesotans who want to get back outside.