Temperature inversion and Air Pollution

Temperature inversion, as the name suggests, is the inversion of the behavior of the earth’s normal temperature(T°). It is also known as thermal inversion. Air pollution refers to the contamination of air we breathe by natural, meteorological, or anthropogenic reasons. But how exactly does it affect air pollution? Let us have a look.

Temperature Inversion

A temperature inversion is an increase in the earth’s T° with an elevation in the earth’s lower atmosphere. The closest layer to the earth’s surface is the troposphere where the majority of our weather occurs. Normally, when we ascend into the troposphere, the T° of the air drops. A temperature inversion happens when this is altered.

Warm air has a lower density than cold air. When cold air is above, this causes warm air to bounce, resulting in rising thermals. 

An inversion acts as a boundary for that warm air, stopping it from expanding and rising upwards. When convective clouds hit an inversion, they stop growing. Rather than rising vertically, these clouds begin to spread horizontally beneath an inversion. Convection fog clouds will not produce rain if the inversion is low. It can also trap mist and fog, as well as smog and pollutants within the atmosphere.

There are several types of inversion, depending on the nature of the air and the T°. These include:

Ground Inversion- 

When the air near cold earth’s surface cools faster than the air above, a ground inversion occurs. This is common on a clear and quiet night when heat radiates away from the earth and swiftly cools. Fog can form near the ground. Occasionally remaining until the earth warms up again during the day.

Frontal inversion- 

A frontal inversion happens when a mass of cold air collides with warm air and undercuts it. As warm air is forced upwards it then cools down and condenses into horizontal layers of mist or vapor. These form clouds. When these cloud layers are thick enough, they give drizzle, rain or snow.

Subsidence inversion-

A subsidence inversion happens when a high-pressure area causes a layer of air to sink or subside. As it falls, the air compresses and dries out, allowing it to warm up. In some cases, such as land masses in winter, the air near the earth’s surface remains colder. The resulting inversion can cause shallow layers of cloud to trap haze and hinder visibility.

Air pollution

Air pollution is the contamination of air quality via natural or anthropogenic reasons. This alters the chemical composition of the air present in the atmosphere. Air pollution can exist both indoors and outdoors. But what is the reason behind contamination of air? There are many natural, and anthropogenic reasons that contribute to the deteriorating air quality. Some of these include:

Natural causes- 

These include many meteorological, and weather conditions that affect the concentration of air pollution in an area. These include:

• Wind speed and wind direction- wind speed and wind direction determine the flow of air pollution. A popular example of this can be stubble burning residuals carried by the winds to Delhi and other northern states of India, that deteriorates the air quality. AQI exceeds the “hazardous” category in Delhi around those activities.

• Temperature- Hot air is less dense than the cold air. T° in the atmosphere influences this hotness and coldness of the air. Dispersion of air pollution depends on if the air is cold or hot. That is why smog is very famous and persistent around winters in Delhi.

• Humidity-High humidity in the air traps the dust and PM particles suspended in the air. This makes these particles heavy to be suspended in the air further, making them fall onto the ground. This reduces the pollution levels.

• Rains- Rain is proven to decrease the pollution levels. When it rains in a highly polluted area, the improved air quality is very much visible in that area.

• Volcanic activity and Wildfires– When volcanoes erupt, they release various poisonous gasses such as S8 and Cl2. Whereas ash and smoke are released in case of wildfires. 

• Dust storms- Dust storms are responsible for increased dust particles to remain suspended in the air for days. If not followed by rain or strong winds.

• Temperature inversion- This phenomenon prevents the air pollutants from traveling high up in the sky. Instead, they invert the natural state of hot and cold air, trapping the air contaminants near the earth’s surface.

Anthropogenic causes- 

Air pollution causes due to anthropogenic or man made reasons are much more responsible for air quality deterioration as compared to natural causes. Some of these include:

• Household emissions- Emissions from households play a major role in contributing to the ambient pollution levels. 
• Emissions from vehicles- As the number of vehicles are increasing on the roads, so is the air pollution levels.
• Industrial emissions- Various toxic gasses and dust emissions are produced through industries into the air.  
• Open garbage burning- Open garbage burning releases smoke and ash into the air.
• Unpaved or unmaintained roads- Dust and PM pollution is very common when talking about unpaved or unmaintained roads.
• Fossil fuel burning- Burning fossil fuels such as coal and petroleum release various toxins into the air.
• Construction and destruction activities- PM pollution (PM2.5 and PM10) are released when C&D activities take place.
• Mining activities- Unpaved roads, blasting, and dust explosions during transport are major causes of pollution in mining operations. Aside from air pollution caused by mining activities, significant fine coal particulate emissions occur.

Role of inversion in Air Pollution

When an inversion occurs, the air near earth’s surface does not mix with higher air, pollutants can build up when an advection inversion is present. It plays a significant role in determining the air pollution dispersion and diffusion. 

• Temperature inversion affects formation of clouds.
• An inversion restricts the upward flow of air.
• As a result, convection fog caused by downward heating is limited to levels below the inversion.
• The dispersion of air pollutants, particulates, and smoke is also restricted when there is inversion.
• Convective clouds cannot develop high enough to rain in places with substantial low-level inversions.
• Visibility may be greatly diminished below the inversion, even in the absence of fog. This is due to the accumulation of dust and smoke particles.
• Fog is very common here as cold air is present near the base of inversion.
• The longer the inversion lasts, the worse the air quality gets.
• Temperature inversion affects rain patterns.

Sources:

1. https://www.who.int/health-topics/air-pollution#tab=tab_1
2. https://www.insightsonindia.com/world-geography/physical-geography-of-the-world/climatology/heat-temperature/temperature-inversion/

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