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Smog trapped by the inversion hangs over Utah Valley Tuesday, Jan. 28, 2014. MARK JOHNSTON/Daily Herald

Mark Johnston

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Smog trapped by the inversion hangs over Utah Valley Tuesday, Jan. 28, 2014. MARK JOHNSTON/Daily Herald

Mark Johnston

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Smog trapped by the inversion hangs over Utah Valley Tuesday, Jan. 28, 2014. MARK JOHNSTON/Daily Herald

Mark Johnston

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The volume of pollution in the air over Utah Valley is regularly at least double that of Salt Lake Valley, according to local monitoring data. Yet Salt Lake County’s population is double Utah Valley’s, and the population per square mile is more than five times greater.

What gives?

The answer is likely a mixture of mystery, geography and ice, said Bo Call, manager of the Utah Air Monitoring Center.

Tuesday was a great example of the mysterious pollution pattern that regularly socks Utah Valley. At noon, Salt Lake’s small particle pollution level, called PM 2.5, was 11. Provo’s air contained more than twice as much pollution, at 23.5. Lindon’s was three times as much, at 34.6, making it far and away the most polluted air in Utah.

“It is a head-scratcher,” said Call. “We don’t know why, for sure.”

He theorizes that Utah Lake might be to blame. For the past two years, an unusually early freeze has lidded the lake with ice earlier than usual. Ice reflects light, and PM 2.5 is created through a chemical reaction that is believed to be triggered by sunlight interacting with what Call called “the chemical soup” in Utah Valley’s atmosphere.

Geography also plays a role. If there is even a slight breeze from the south — which is fairly typical in winter SEmD then it pushes the chemicals in the air northward, trapping and concentrating them in Utah Valley. The chemicals are like a bubble trapped in the bowl, formed by the surrounding mountains. The bubble is “sloshed” by the breeze, but only the power of a storm can break the bubble and clean away the chemical pollutants.

In Utah Valley, there is an air monitoring device in Spanish Fork, one in Provo, and one in Lindon. Many times officials can actually watch pollution being pushed northward by a gentle south breeze in winter, Call said.

“We can watch it slosh,” he said.

The exact forces at work are not obvious. Fully 57 percent of air pollution is caused by driving. Clearly, with a population twice Utah County’s, Salt Lake creates more pollution from driving. Salt Lake County also has more heavy industry, and more homes to heat, all of which add to air pollution.

PM 2.5 is also created from burning wood, but “during winter is not directly emitted from smokestacks,” Call said.

Rooftops, roads, parking lots — these are all things that actually absorb light. Snow and ice, however, reflect light, helping to drive the creation of PM 2.5.

The monitoring devices in Provo and Lindon are also near the freeway, which could also explain why the pollution levels they pick up are consistently high, he said.

Pollution particles “less than 10 micrometers in diameter are so small that they can get into the lungs, potentially causing serious health problems,” according to the Utah Division of Air Quality website. “Ten micrometers is smaller than the width of a single human hair.” Particles less than 2.5 micrometers in diameter “are so small they can be detected only with an electron microscope. Sources of fine particles include all types of combustion, including motor vehicles, power plants, residential wood burning, forest fires, agricultural burning, and some industrial processes.”