Why the Twin Cities' air quality is worst it's been all year

This is the first time in 2014 this map has featured an orange blob over the Twin Cities.

This is the first time in 2014 this map has featured an orange blob over the Twin Cities.

Today, for the first time in 2014, the Twin Cities is under an "orange" air quality alert. That means breathing the air could be unhealthy for sensitive groups.

Reached this morning, Cassie McMahon, air quality research analyst for the Minnesota Pollution Control Agency, said that unfortunately, relatively poor air quality is just a fact of life in the upper midwest this time of year.

"We typically consider our most intense fine particle season to be between November and March, but because it's been so cold this winter and we haven't had a lot of warm up we've actually had a very clean winter for fine particle pollution," McMahon said. "It's important to note today isn't a unique day for fine particle emissions, it's just a combination of normal emissions interacting with a weather condition that allows the pollution level to build up."

What weather condition is she referring to, you might be wondering? Basically, one where melting snow interacts with relatively warm air to create a type of lid effect.

"In winter when there's lots of snow, colder surface temperatures have a layer of warm air above, and that warm air acts like a lid," McMahon said. "What also happens is when the snow melts, it adds moisture to the air, and for fine particles it acts like a sponge and cold hold them in place and enable them to build up."

That "sponge effect" becomes especially pronounced when the wind isn't blowing, McMahon explained.

It should be noted that today's relatively poor quality isn't just impacting Minnesota, as Bob Moffitt, director of media relations for the American Lung Association in Minnesota, told us.

"This particular system is a huge one encompassing several states," said Moffitt, who we've normally talked to in the context of smoking-related stories. "It's part of the polar vortex that's been freezing us but keeping air clean, but it moved out and warmer air moved in carrying quite a bit of pollution from Oklahoma up to Fargo, all across the middle of the country."

On Twitter, Moffitt gave Minnesotans some practical advice:
-- Robert Moffitt (@justplainbob) March 7, 2014 -- Follow Aaron Rupar on Twitter at @atrupar. Got a tip? Drop him a line at [email protected]