Why a temperature inversion traps atmospheric pollutants on the valley floor?

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.

On still cold nights (and even days), and especially in valleys, cold air can become trapped below a layer of warmer air. Any pollution is then trapped in the local area and cannot escape. Pollution can be 10s, even 100s or 1000s of times higher under an inversion than at times when there is no inversion.

The Boral / Allens application say temperature inversions were not considered

Also see how inversions magnify sound click here

This phenomenon of cold air trapped below warm air is known as a temperature “inversion”.

It is common along the Eudlo creek valley and surroundings especially during winter.

An inversion can lead to pollution such as smog being trapped close to the ground, with possible adverse effects on health....

Actual temperature inversion at Kunda valley (courtesy of Rowan, local Diddillibah resident )

Residents say they are frequent

  More information and photo from Commonwealth of Australia 2008, Bureau of Meteorology (ABN 92 637 533 532)

A temperature inversion traps pollutants

Temperature inversions
One of the more significant changes is a temperature inversion, in which the temperature increases with increasing height. Low-level inversions often occur with fogs, and they also act like a lid to trap pollutants, resulting in smog in our cities. A strong inversion forms at night at the surface due to cooling of the ground through loss of heat by radiation. This nocturnal or surface inversion is best developed on clear calm nights.

Air settling in the subtropical high pressure belts is compressed and warmed as it subsides. This often results in inversions. These inversions, typically 300 to 3000 metres above the surface, put a lid on convection and restrict cloud growth. Above the inversion the air is warm and dry. This is a major reason for the arid weather of the subtropical high pressure belt.

The boundary between the troposphere and the stratosphere, the tropopause, is also an inversion. It restricts movement of air between the troposphere and the stratosphere, and is the effective limit of even the largest thunderstorm clouds.

After sunrise, radiation from the sun is absorbed by the earth's surface, but scarcely any is absorbed by the air. Conduction of heat from the ground warms the air in the lowest few centimetres, but not higher up - air is a poor conductor. If there is no wind, this sets up a large vertical temperature difference in the lowest metre or so of the atmosphere. Cricketers fielding on a hot day may experience temperatures of over 45°C at their feet but only 35°C at their waist.

On windy days the situation is completely different. The air in contact with the ground is constantly being replaced; this distributes the heat through a deeper layer and prevents a large temperature difference from being established close to the ground.

Why are temperature inversions common in valleys?

Why are temperature inversions important?

How temperature inversions play a significant role in air pollution meteorology?

What is a temperature inversion and how can it affect outdoor air pollution levels?