Under normal atmospheric conditions, air is warmer near the ground and colder at higher altitudes. In a temperature inversion, the situation “inverts,” and cold air at the surface gets trapped under a layer of warmer air. During the winter, snow-covered valley floors reflect rather absorb heat, preventing the normal vertical mixing of warm and cold air that keeps pollutants from building up to unhealthy levels at the surface.
Calm winds, clear skies, and long nights prevent air at higher altitudes from mixing with air closer to the ground.
- Calm winds reduce the natural mixing of cold and warm air.
- Clear skies increase the rate of cooling of the air close to the ground.
- Long nights allow the cooling of the ground to continue over a longer period of time, resulting in a greater decrease in temperature near the surface.
- The sun is lower on the horizon during the winter, so it supplies less warmth to the earth’s surface and more to the atmosphere.
Mountains can also increase the strength of inversions in the valleys. The Wasatch Mountains, Oquirrh Mountains, and Traverse Mountain, for example, form a basin that traps cold air in the Salt Lake Valley and shields it from the stronger winds aloft that could clear out inversions.
Utah inversions often occur after a snowstorm. The snow cover makes the air colder near the ground, and the clear skies warm the upper atmosphere. If a high pressure system moves in, the gradual sinking of the warmer air acts as a cap over the cooler air, much like a lid over the valley bowl. The longer a high pressure system lasts, the longer and stronger the inversion.
The strength and duration of the inversion will control air pollution levels near the ground as measured by the Air Quality Index (AQI) levels. The AQI correlates daily air-quality levels with levels of health concern on a color-coded scale of six values that range from good to hazardous. The higher the AQI value, the greater the health concern. A strong inversion will confine pollutants to a shallow vertical layer, leading to high AQI values, while a weak inversion will lead to lower AQI values.
A typical Utah winter sees about five to six multi-day inversion episodes and on average, 18 days with high PM2.5 levels exceeding the National Ambient Air Quality Standard (NAAQS). A strong storm or low-pressure system is often needed to clear out the inversion.
Impact on Air Quality
Surface temperature inversions play a major role in air quality, especially during the winter when these inversions are the strongest. Pollutants from vehicles, wood burning, area sources, and industry become trapped near the ground during inversions, leading to poor air quality. PM2.5 concentrations build the longer the inversion lasts and can reach unhealthy levels.
Most of the PM2.5 particles in Utah’s air pollution are formed through chemical and photochemical reactions in the atmosphere rather than from direct emissions. Precursor emissions that contribute to this secondary formation of fine particulates include nitrogen oxides (NOx), volatile organic compounds (VOCs), sulfur dioxide (SO2) and ammonia (NH3). These chemicals are highly reactive in the atmosphere, breaking apart and combining with other gaseous chemicals, particularly ammonia, to form ammonium nitrate and ammonium sulfate. Secondary ammonium nitrate is the primary constituent in regional particulate matter and is responsible for up to 70 percent of PM mass during inversions and 40 percent during non-inversion periods.
While Utah’s unique topography, geography, and meteorology are important factors in the buildup of fine particulates during inversions, PM2.5 emissions and their chemical precursors are the primary cause of these pollution episodes. Better understanding of the mechanisms that drive these pollution episodes and improved identification of the most important chemical species for the formation of PM2.5 are needed to develop effective control strategies to reduce fine particulate levels.