The winter air pollution season just ended, and it turns out it was one of off-the-charts numbers of air alerts and advisories.
The next air pollution season arrives with warmer weather when the heat creates ozone, bad for people with lung problems.
However, we have a reprieve from one dirty air season as we wait for summer the next one.
Minnesota Pollution Control Agency air quality specialist Cassie McMahon said most years from November to March, Twin Cities residents can expect perhaps two days where the agency issues air alerts or advisories because of air pollution.
Last winter there were 28 days with most of the alerts and advisories bunched in January and February.
"That is the most we have ever had," McMahon said. "The reason for that [is] that January through February was very calm, stagnant; and we require active weather to clean us out."
The best clean out comes from westerly winds bringing fresh air from our less polluting neighbors.
"We're pretty fortunate that we have the Dakotas to our west and a lot of clean air masses that come through to clean everything out," she said.
Monitors measuring Minnesota air pollution are arrayed around the state. There are 30 of them in greater Minnesota. The largest concentration -- 26 monitors -- is in the Twin Cities.
One of the monitor sites is on the roof of Harding High School in St. Paul. Through the hallways, past the students, through an unmarked door and up the steps to the school roof are five white metal boxes on metal stands.
The instruments at Harding High and at the other Twin Cities locations measure fine particles, nitrogen oxide, and volatile organic compounds among other pollutants.
The day is bright and sunny with lots of fresh clean air from the Dakotas blowing across the roof.
At a machine measuring fine particles, Cassie McMahon checked the color of white circles passing under the stack sucking air in.
"Right now the circle is very light," she said. "On an air quality alert day that circle gets very dark."
The major sources of air pollution in the Twin Cities are from our own smokestacks and vehicles. And on those days, when we don't get cleansing winds from the west, southerly breezes blow in pollutants from the industrial Midwest.
Even from overseas.
"There might be something happening hundreds and thousands of miles away that might impact the air pollution here where I live," said Gabriele Pfister, who lives in Boulder, Colorado.
Pfister is a scientist at the National Center for Atmospheric Research in Boulder. Ten years ago, Pfister said many believed air pollution was mostly a local issue.
She said more recent research shows how the winds aloft carry pollution from, say, Asia, to North America. There's still plenty of air pollution being created in the United States.
However, experts say because of limits imposed by the 1970 federal Clean Air Act, the air pollution levels in most areas of the United States are vastly lower than 30 years ago.
Gabriele Pfister said air pollution levels have increased elsewhere.
"Most of the studies that look on global scale see that you have pollution in Asia going up while you have pollution in the western world, Europe and North America, is somewhat going down," Pfister said.
One of Gabriele Pfister's specialties is research into ozone. Ozone is created by adding heat - sunlight - to emissions from our vehicles or from industry.
Pfister said ozone's life span is a few days, but because air pollution can travel on the winds from continent to continent, emissions from China can lead to ozone in North America.
The MPCA's Cassie McMahon said we didn't have many ozone air pollution problems in the Twin Cities last summer because of mild temperatures.
That could change if this summer is hotter.
Also the federal government may tighten ozone standards because of evidence showing its harm to human health. In fact, the federal government over the years has tightened standards for nearly all air pollutants.
So even though our air is cleaner, the tighter standards end up triggering more air pollution alerts and advisories. Tighter ozone standards, McMahon said, would probably push the Twin Cities out of compliance.
That would set in a motion a years-long process, she said, to find ways to reduce ozone levels.
For the moment, Twin Cities air specifically and Minnesota generally is relatively clean and meets all federal standards. Thank you Dakotas.