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SALT LAKE CITY — When a high-pressure system moved into Utah earlier this month, meteorologists feared it could result in long-lasting inversion over the state.

Fortunately, a cold front came through over the weekend and cleared out the inversion and pollution trapped beneath it.

Inversions aren’t uncommon at all and occur throughout the year, but Utahns are prepared for the worst in winter when pollution is often trapped under the inversion and air quality declines.

The University of Utah defined these as “persistent cold pools,” which Bryce Bird, director of the Utah Division of Air Quality, explained is when an inversion doesn’t break during the day and lasts for multiple days.

These inversions, recorded by the U. from 1956 to 2010, ranged from a day to more than a week. As Bird noted, the length of inversions in a given year is unpredictable and based on weather patterns.

“It is variable from year to year, and that’s what leads to good years and bad years. It is reflected in the data that there is variability from storm to storm and year to year,” he said. “As for as predicting (inversions), that’s a little bit more difficult.”

Here is a list of Utah's the longest-recorded inversions documented by the U researchers:

Longest inversion event period (1956-2010):

Dec. 16, 1985 — 14.5 days

Jan. 28, 1984 — 10 days

Dec. 30, 1990 — 8.5 days

Nov. 29, 1963 / Jan. 2, 1984 / Jan. 8, 1986 — 7.5 days

Nov. 21, 1956 / Jan. 28, 1962 / Dec. 11, 1980 / Jan. 14, 1997 — 7 days

Dec. 10, 1962 — 6.5 days

Air quality correlation

Of course inversions, especially in the winter, are often associated with poor air quality because pollution is trapped underneath an inversion. The Department of Environmental Quality has issued air quality alerts for years, though those alerts have changed during that span.

In 2000, the state began recording particulate matter 2.5 in addition to PM 10 — the standards regarding pollution in the air. The difference between the two, according to U.S. Environmental Protection Agency, is that PM 2.5 records particles that come from combustion, organic compounds and metals specifically; PM 10, records all that PM 2.5 records and also includes dust, pollen and mold.

Smog in the Salt Lake Valley in 1942 (Photo: Utah State History)

The EPA revised its PM 2.5 standard in 2006, and the agency and the state adopted a 12 micrograms-per-cubic-meter standard in 2012. The previous acceptable measurement was 15 micrograms per cubic meter. The state also switched “red” and “yellow” alert days to “voluntary” and “mandatory” alert days that year.

With the new particle standard implemented in 2000, the amount of red and yellow days rose significantly that year. From 1992 through 2000, there was an average of 4.25 red days per year in Salt Lake County. From 2000 through 2012, that number jumped to 10.75 red days per year, according to Utah Division of Air Quality data.

From 2012 through 2017, when the state switched to mandatory and voluntary days, Salt Lake County averaged to 27.5 mandatory days. Similar numbers were recorded in neighboring counties. However, Cache County led all counties with an average of 31 mandatory days during that span.

Highest amount of red days per year, from 1992-2000:

Utah County — 23 (1992-93)

Davis County / Salt Lake County — 17 (1992-93)

Davis County / Salt Lake County — 7 (1994-95, 1995-96)

Highest amount of red days per year, from 2000-2012:

Davis County / Salt Lake County — 30 (2006-07)

Cache County — 29 (2009-10)

Utah County — 26 (2006-07)

Cache County — 25 (2003-04)

Davis County / Salt Lake County — 23 (2003-04)

Highest amount of mandatory days per year, from 2012-2017

Cache County — 42 (2012-13)

Cache County — 38 (2015-16)

Weber County — 36 (2012-13)

Cache County — 36 (2013-14)

Davis County / Salt Lake County — 35 (2012-13)

Photo: Devon Dewey, KSL.com

Improving air quality

While the numbers rose with time, Utah Department of Environmental Quality spokeswoman Donna Spangler said air quality has still improved overall in the state. One reason is legislative intervention, which has become a bipartisan issue in the state.

In January 2017, Gov. Gary Herbert noted statewide emissions dropped by 30 percent from 2002 to 2014.

“The air quality has gotten better,” Spangler said. “People are no longer burning coal. We’re trying to move away from people burning wood. … All of those things actually add up to improvement.”

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