Temperature Inversions, City Pollution and Defunct Jet Engines

A photograph showing a typical temperature inversion layer over the Scottish city of Glasgow. The Sun is shining bright, and the sky is clear and blue, above a thick blanket of smog, located at ground level. Photograph: NaturPhilosophiePollution City

Temperature inversions are meteorological phenomena which can occur over busy cities under particular environmental conditions.  Retired jet engines could be used as “virtual chimneys”, and draw upwards the resulting smog that clouds the air over some of the World’s most polluted cities. 

This photograph shows an episode of temperature inversion that occurred a few weeks ago over my home city of Glasgow in Scotland.  It was taken on 25th November 2016, at around 15:00 GMT.

Above the clouds, the Sun is shining, and the air feels unseasonably warm.  At ground level, however, the weather conditions are very different.  The sky is obstructed by a very dense freezing fog.


A diagram comparing the difference between the atmospheric layers above a city agglomeration under normal conditions and during a temperature inversion: Arrows show air flow in normal conditions on the left and during temperature inversion on the right. In normal conditions, warm air rises and normal convective patterns persist. During temperature inversion, the warm air acts as a cap, effectively shutting down convection and trapping smog over the city.
Source: Understoryweather

Temperature Inversions

Thermal inversion layers or temperature inversions are areas where the normal decrease in air temperature with increasing altitude is reversed and the air above the ground is warmer than the air below it.

Temperature inversions can occur anywhere from close to ground level up to thousands of feet up into the Earth’s atmosphere.

Normally, air temperature decreases at a rate of 6.4°C for every kilometre (roughly 3.5°F for every 1000 feet) you climb up into the atmosphere.  When this normal cycle is present, it is considered an unstable air mass and air constantly flows between the warm and cool areas.  As such, the air is better able to mix and spread around pollutants.

At times of temperature inversions, temperatures increase with increasing altitude.  The warm inversion layer then acts as a cap and stops atmospheric mixing.

This is the reason why inversion layers are called stable air masses.


Impact of Inversion Layers on Micro-Climates and Smog

A photograph taken from the rooftops of a big city.
According to data from the WHO, Glasgow is among 11 urban areas in the UK and Ireland which have breached air pollution safety levels.  Source: Thinkstock

Temperature inversion layers are significant to meteorology because they block the atmospheric flow which in turn causes the air over an area experiencing an inversion to become stable.  This can result in various types of weather patterns.

More importantly, areas with heavy pollution are prone to unhealthy air and to an increase in smog when an inversion is present, as they trap pollutants at ground level, instead of letting them circulate away.

Although freezing rain, thunderstorms, and tornadoes are significant weather events, one of the most important things to be impacted by temperature inversions is smog – the brownish gray haze that covers many of the World’s largest cities as a result of dust, auto exhaust, and industrial manufacturing.

Smog is impacted by the inversion layer because it is in essence, capped, when the warm air mass moves over an area.  This happens because the warmer air layer sits over a city and prevents the normal mixing of cooler, denser air.  Instead, the air becomes still and over time the lack of mixing causes pollutants to become trapped under the inversion, developing significant amounts of smog.

During severe inversions that last over long periods, smog can cover entire metropolitan areas and cause respiratory problems for the local inhabitants.  And urban air pollution is rising.


A map showing the global statistics for the percentage of deaths due to urban air pollution: 5% of deaths (or 2.6 million of the World's population) are caused by the urban air pollution. Source: World Health OrganizationRising Air Pollution

The World Health Organization (WHO) was able to compare a total of 795 cities in 67 countries for levels of small and fine particulate matter (PM10 and PM2.5) during the five-year period, 2008-2013.  PM10 and PM2.5 include pollutants such as sulfate, nitrates and black carbon, which penetrate deep into the lungs and into the cardiovascular system, posing the greatest risks to human health.


WHO Ambient Air Quality Guidelines


10 μg/m3 annual mean
25 μg/m3 24-hour mean

20 μg/m3 annual mean
50 μg/m3 24-hour mean


Data Analysis

Data was then analysed to develop regional trends.

Key trends from 2008-2013

  • Global urban air pollution levels increased by 8%, despite improvements in some regions.
  • In general, urban air pollution levels were lowest in high-income countries, with lower levels most prevalent in Europe, the Americas, and the Western Pacific Region.
  • The highest urban air pollution levels were experienced in low-and middle-income countries in WHO’s Eastern Mediterranean and South-East Asia Regions, with annual mean levels often exceeding 5-10 times WHO limits, followed by low-income cities in the Western Pacific Region.
  • In the Eastern Mediterranean and South-East Asia Regions and low-income countries in the Western Pacific Region, levels of urban air pollution has increased by more than 5% in more than two-thirds of the cities.
  • In the African Region urban air pollution data remains very sparse, however available data revealed particulate matter (PM) levels above the median.  The database now contains PM measurements for more than twice as many cities than previous versions.


Polluted Capital Cities and the Importance of Topography


A black-and-white photograph showing the effect of the dense smog in a London street in the winter of 1952.
The Great Smog which wrapped London in December 1952 is the worst urban air pollution event in the history of the UK. Source: Pinterest

In December 1952, such a temperature inversion occurred in London.  Due to the cold December weather, Londoners began to burn more coal, which increased air pollution in the city.  Since the inversion was present over the city at the same time, these pollutants became trapped and increased London’s air pollution.

The resulting Great Smog of 1952 caused thousands of deaths, and it remains the worst recorded urban air pollution event in the history of the United Kingdom.


Mexico City

An aerial photograph showing the highly polluted skyline of Mexico City, with the heights of local strato-volcano Popocatépetl seen in the background.
Mexico City remains highly polluted, on par with Los Angeles, but it has been a model of pollution management, and air quality has vastly improved since the 1990s.  Source: Myspinny

Mexico City has over 21 million residents and is almost always battling with high pollution levels since the valley in which it is located is surrounded by volcanic mountains.

The population of the capital lives in the shadow of Popocatépetl, the country’s most active volcano.  As recently as March 28, 2016, the strato-volcano released a 2,000-metre high ash column, prompting the establishment of a 12-kilometre “security ring” around the summit.


Topography Effect

Topography can also play a role in creating a temperature inversion since it can sometimes cause cold air to flow from mountain peaks down into valleys.  This cold air pushes under the warmer air rising from the valley, giving rise to the inversion.

Additionally, inversions can also form in areas with significant snow cover because the snow at ground level is cold and its white colour reflects almost all heat coming in – a phenomenon that is known as the albedo effect.  The air above the snow is thus often warmer because it holds the reflected energy.

London’s Great Smog and Mexico’s similar problems are extreme examples of smog being impacted by the presence of an inversion layer.

This is a global problem although cities like Los Angeles (California, United States), Mumbai and Dehli (India), Santiago (Chile) and Tehran (Iran) frequently experience intense smog when an inversion layer develops over them.

For this reason, many of these cities are working towards a reduction of their air pollution.


Daily Pollution

A photograph showing a view of Dehli's crowded road infrastructure, with a monumental gateway in the background almost completely obscured by traffic fumes.
Delhi air pollution is causing a major public health disaster In India. Source: Millenium Report

The Indian city of Dehli has some of the most toxic air in the World.

The widespread use of festival fireworks, the burning of rubbish by the poorest of the city population, the farm waste that surrounds the city, the motorised vehicle emissions, and the construction and demolition dust, all contribute to the city’s disastrous air quality and excessively thick smog.

Additionally, coal now accounts for over 60% of the country’s power generation.  Within two years, India could overtake China as the biggest importer of thermal coal.

Coal-fired energy may be linked to more than 100,000 premature deaths, and millions of cases of asthma and respiratory illnesses.  As environmental conditions worsen during the winter months, schools have to shut, construction and demolition works are suspended, and people need to wear face masks.

Emissions from a 1,000 MW coal power plant are equivalent to emissions from roughly 500,000 cars.


And as such, Dehli is the ideal candidate for a unique new experiment.


Making the Most of Retired Jet Engines

An annotated diagram illustrating the structure and function of a jet engine. And an artist's impression of what the moveable equipment might look like.
A jet engine is a reaction engine discharging a fast-moving jet that generates thrust by jet propulsion. the term jet engine loosely refers to an internal combustion airbreathing jet engine. These typically feature a rotating air compressor powered by a turbine, with the leftover power providing thrust via a propelling nozzle — this process is known as the Brayton thermodynamic cycle. Source: Wikipedia/Alamaro2016

Next year, a retired jet engine will be mounted on a flatbed trailer, and taken to a coal-fired power plant in Dehli.

With the exhaust nozzle pointed towards the sky, the combustion reaction engine will be placed near the smokestack, and switched on.

The jet engine exhaust will act as a “virtual chimney”, creating powerful updrafts that will in theory blast the emissions from the power plant to higher altitudes, above the temperature inversion layer, thereby driving the smog upwards.

This is the first time anybody has tried using jet engines for smog mitigation.  The decision comes after levels of PM2.5 particles soared to over 90 times what is considered to be safe by the WHO, and 15 times the federal government’s norms.

To begin with, the jet engines will be tested in remote locations, to observe their properties and for the sake of optimization.  As it roars to life, the engine will generate a nozzle speed of 400 metres per second (1,440km/h or 900mph), approximately the speed of sound.

Although there are concerns about the potential noise and critics are skeptical about the use of such technology on a large scale, carrying out the jet engine experiment outside a coal-powered electricity plant makes a lot of sense.


Smog Mitigation

A single jet engine could deal with emissions from a 1,000 megawatt power plant.  However, the jet systems could also be located near busy highways where vehicle emissions are high.

In future, the program could use retired military and commercial jet engines, and has the possibility of adding new value to numerous defunct propulsion systems.

The research team from the United States, India and Singapore – including lead researcher Moshe Alamaro, an aeronautical engineer and atmospheric scientist from M.I.T. – believe this experiment could lead to a successful implementation of new technology for smog mitigation all over the World.