Utah’s Winter Haze a Pollution Problem

The Salt Lake Valley’s topography lends itself to a natural phenomenon called an inversion, which traps air pollution.

The view of Salt Lake Valley from just outside the Snowbird Ski Resort in Utah on a January afternoon in 2014. Photo by Chau Tu

To outsiders, Salt Lake City, Utah, during the winter may seem like a land of clear skies and fresh air. But Utah’s capital has been experiencing poor air quality for many decades, according to John Horel, a professor in the University of Utah’s department of atmospheric sciences.

The problem is particularly pronounced in Salt Lake City during the wintertime, when a phenomenon called an inversion commonly occurs. While air is usually warmer when it’s close to the earth, snow and frigid air can refrigerate the air closer to the ground, making it cooler than what’s higher up, according to Horel, who, with colleagues, just finished a four-year study on the weather conditions associated with Utah’s inversions. Shorter winter days mean less available sunlight to heat up the low-lying cold air, which can lead to inversions that last two weeks.

The Salt Lake Valley—which sprawls about 500 square miles and includes Salt Lake City—has a bowl-like topography that makes it a prime spot for inversions, says Kerry Kelly, a chemical engineering researcher at the University of Utah. After winter storms pass through the area and cold air sinks into the valley, ridges of high pressure and warmer air slide over the cold, trapping it like a lid, she says.

The problem is, that lid not only caps the cold air, but pollution in the valley as well, leading to conditions like the one in the picture above.

Cars, home heating, and refineries are just some of the sources responsible for the Salt Lake Valley’s air pollution. These emissions, including volatile organic compounds (VOCs) and nitrogen oxides, react in the atmosphere to form secondary pollutants including ammonium nitrate, which has been linked to respiratory problems and increased risk of pre-term birth. During a winter inversion, the amount of ammonium nitrate alone rises from about 45 percent of the Salt Lake Valley’s fine particle pollution to as much as 75 percent, according to Patrick Barickman, a technical analysis manager at the Utah Division of Air Quality.

The EPA measures air quality based on the size of pollutant particles. Those measuring 2.5 micrometers—many fractions the width of a human hair—or smaller can be hazardous to health. “That size can be transported quite deeply into the lungs, where it can have a number of adverse health effects,” such as exacerbating asthma, says Kelly.

Horel estimates that the Salt Lake Valley—home to more than a million residents—experienced four inversion events from December 2013 to January 2014. During that span, pollution levels exceeded the standard set for fine particles for a total of 30 days.

Utahns may soon be in the clear, however—inversions generally only occur during the winter months of December through February, says Kelly. Plus, increased public frustration over air quality has grabbed the state government’s attention. There are about 20 air quality bills floating around the state legislature right now, says Kelly, entailing proposals ranging from stricter vehicle fuel standards to funding for a program to convert home heating fuel from wood to natural gas.

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About Chau Tu

Chau Tu is an associate editor at Slate Plus. She was formerly Science Friday’s story producer/reporter.

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