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The Wasatch Front is entering its first big inversion of winter, but state officials aren't waiting for the air quality to get unhealthy to declare the Salt Lake Valley's first no-burn day of the season.

Under new rules aimed at reducing the amount of particulate pollution that concentrates above the valley floor, the Division of Air Quality imposed mandatory emission restrictions Wednesday in five counties — Cache, Salt Lake, Davis, Utah and Weber — and asked motorists to curb their driving, even though pollution levels have yet to breach federal standards.

The burning of wood and other solid fuels will be largely banned along much of the Wasatch Front until the inversion breaks.

"We are not seeing any weather pattern showing a storm coming through anytime soon," said Donna Kemp Spangler, spokeswoman for the Utah Department of Environmental Quality.

The rules target concentrations of fine particulate matter, PM2.5, or particles smaller than 2.5 micrometers in diameter. This is the stuff that infiltrates pulmonary tissue and is implicated in numerous diseases, including asthma, stroke and cancer.

The U.S. Environmental Protection Agency has set the 24-hour threshold for this pollutant at 35.5 micrograms per cubic meter of air. A monitor at Hawthorne Elementary in Salt Lake City registered levels around 20 micrograms Tuesday, squarely in the moderate range.

But between winter storms, Salt Lake and other Utah valleys are subject to "inverted" temperatures, where cold air pools near the ground and prevents the atmospheric mixing that would dilute pollution and allow it to be blown away. Snow cover and ice exacerbate the situation by reflecting radiant energy that otherwise would warm the ground and spur convection.

"Utah Lake is starting to freeze and that's having an impact on socking in the inversion and increasing the numbers," Spangler said.

"Our inversions start right after the snow storm ends," she explained. "The pressure starts building. If we don't have a storm it's just going to build. We know that the [PM2.5] numbers are going to climb, potentially to the federal standard."

Recent research suggests wood smoke, primarily from fireplaces, wood stoves and restaurant grills, plays a much larger role in the particulate problem than previously believed.

This is because of its high content of volatile organic compounds, or VOCs, which act as a precursor to particle formation, according to Kerry Kelly, a chemical engineer with the University of Utah.

"Compared with burning natural gas or gasoline, burning wood produces orders of magnitude more VOC, hundreds of thousands more VOC per unit of energy output," said Kelly, who serves on the Utah Air Quality Board.

The board recently approved federally mandated plans to cut the number of days that Salt Lake and Utah county airsheds breach the 35.5 microgram limit. These plans are geared toward reducing the emissions that lead to PM2.5 accumulation, but weather and topography play a critical role in the problem.

Cold, stagnant air prolongs high-pollution events and can even spur the chemical reactions that enable VOCs, nitrous oxides and other emissions to form particles in the atmosphere.

Temperatures are expected to warm slightly over the next few days, with highs in Salt Lake City hitting 24 Wednesday and 32 by Thursday. Skies will remain mostly sunny with slight breezes out of the west.

The mandatory no-burn restriction is expected to remain in place through Friday, and possibly beyond.

The division had declared a no-burn day Nov. 27 for Utah County, but the order was lifted before an inversion took hold.