Urban Aerosol Pollution

Mains: GS III - Environmental Pollution and Degradation

Why in the news?

A recent study on aerosols levels of 141 cities in India from 2003 to 2020 by IIT at Bhubaneswar, has shed new light on urban aerosol pollution patterns across India, revealing a north-south divide in how pollution domes form over cities.

What is urban aerosol pollution?

Aerosols

It is a suspension of fine solid particles or liquid droplets in a gas, most commonly in air.
Sources
- Natural cause – Volcanoes
- Anthropogenic cause - Burning fossil fuels.
Size – They are typically small, with diameters often less than 1 micrometre, allowing them to remain suspended for extended periods.
Role – They play a role in atmospheric processes, influencing air quality, weather, and climate.

Urban aerosol pollution – It refers to the presence of small, solid, or liquid particles suspended in the atmosphere within cities.
Two types
- Urban aerosol pollution island
- Urban aerosol clean island
Urban aerosol Pollution Island – It refers to the cities that show higher aerosol levels than the surrounding areas.
It occurs, whenever less transport of aerosol or pollution is happening irrespective of the season.
The aerosol levels were not uniformly higher in the surrounding areas compared with the city.
Urban aerosol clean island – It refers to the cities that show relatively lower aerosol levels compared with the surrounding areas.

What is the level of urban aerosol pollution Island in India?

Regional variation – 57% Cities in south and southeast India show higher aerosol levels inside the city compared to neighbouring parts.
Causes – This occurs due to local pollution sources which dominates aerosol presence and lack any large external source of pollutants coming from elsewhere.
It became distinct in many cities during no dust scenario.

Urban aerosol 2

Consequences – This results in a typical “pollution dome” where urban areas are more polluted than their rural surroundings.

A pollution dome is a phenomenon where pollutants, such as smog and particulate matter, become concentrated and trapped in the atmosphere usually above a city or industrial area, forming a dome-like layer of polluted air.

Higher level – It is in the areas southwest of the city, which are located upstream of the dust flow.
Lower level – It is in the northeast side of the city situated downstream of the dust flow that exhibited less aerosol levels which almost matched the levels seen in the city.
It is a trail that continues downwind for some distance.

What is the level of urban aerosol clean Island in India?

Regional variation – 43% of cities in northwest and northern Indo-Gangetic Plain display lower aerosol levels within city limits than in surrounding areas.
North Indian cities, despite being blamed for poor air quality, are found to have no consistent pollution domes.
External aerosol source - Aerosols from external sources such as dust from the Thar Desert and biomass burning contribute heavily to background pollution.
It became pronounced in many cities in high dust case.

Urban aerosol 3

Clean island effect is whenever the pollution transport from outside is enhanced.
Cities as barriers – The infrastructures of the cities block aerosol entry and creating cleaner urban zones compared to their surroundings.
This unexpected pattern is linked to a phenomenon known as urban wind stilling effect.

Wind Stilling Effect

It refers to a weakening of surface winds across highly urbanised cities where the buildings and infrastructure reshape local climates, creating zones of atmospheric stagnation.
These zones collectively lead to invisible barriers around the city in the upwind regions.
They partially block the entry of long-range aerosol pollution, especially mineral dust from the nearby arid regions.
This results in cities having relatively less aerosol loading than the surrounding areas.
Global megacities such as Shanghai, Atlanta and a few European cities have urban aerosol clean islands.

Seasonal variation in India
- During monsoon – Not studied due to non-availability of data due to clouds and rain.
- Pre-monsoon – Clearly observable.
- Post-monsoon – Normally disappears but seen again in winter due to dry conditions.

What lies ahead?

Deeper scientific understanding could help to evolve an idea on how urban growth and emerging micro-climates influence air pollution and its spatial patterns.
Creation of truly sustainable, climate-resilient cities will depend on our capability to understand and anticipate these multifaceted interactions as cities continue to grow.

Reference

The Hindu| Urban Aerosol Pollution

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