Studies 141 cities in India from 2003 to 2020, using aerosol data recovered by satellite, reveals surprise-plans of aerosol in 57% of cities compared to the surrounding areas outside the city in the south and southeast India, while 43% of cities in northwestern and norther the area.
Scientists at School of Earth, Ocean and Climatien Sciences, Iit Bhubaneswar, refer to cities in South and Southeast India, showing higher levels of aerosol than the surrounding areas such as the islands of pollution of urban aerosols. And cities, which show relatively lower levels of aerosol compared to the surrounding areas, are referred to as urban aerosol net islands.
In the case of cities referred to as urban aerosol, clean islands were not evenly higher in the surrounding areas compared to the city. Instead, the levels were higher in areas southwest of the city, which are upstream from the dust, while the northeast side of the city is downstream from dust dust, which showed smaller levels of aerosol, which almost corresponded to the levels in the city.
“Aerosol coming from the outside did not contribute to pollution already in cities, which are called urban aerosols of clean islands. Instead, the city in the northwest and the northern Indo-Gangnetic plains stopped or diverted Aerosol and moved it around the city.”
“North Indian cities, although accused of poor air quality, have been found that they do not have consistent” pollution “. Instead, we observed urban clean islands – zones with relatively lower aerosol levels than the surrounding areas.
The effect of the wind
The effect of wind feeling concerns the weakening of surface winds across highly urbanized cities, where buildings and infrastructure transform the local climate and create atmospheric stagnation zones. These zones collectively lead to invisible barriers throughout the city (in the wind), which partly blocks the input of aerosol pollution with long reach, especially mineral dust from nearby dry areas.
Also, cities located in regions with high -background pollution tend to slow down the transport of pollutants from the outside of the city, such as the dust of Thar or Aerosol from the burning of biomass, which are transported from anywhere. This results in cities have relatively smaller loads of aerosol than the surrounding areas.
“While external sources still contribute to pollution, this barrier changes as the pollutants accumulate and disperse, leading to a deceptive pocket of cleaner air in the city and its mines in the wind. Vinoj.
Smaller aerosol load
According to Soumya Sethi, Ph.D. The study and the first author of the paper explains that the proposed barrier effect observed in cities does not rule out the transport of pollutants, but only slows transport. In this process, cities in the northwest and northern Indo-Gangnetic plain have relatively smaller aerosol, while the surrounding areas are witnessing an increase in the accumulation of pollutants.
The reason why cities in South India do not have urban aerosols clean islands is that southern cities have no large source of pollutants coming from anywhere to create a large aerosol background, allowing us to see this rather invisible copus effect. Instead, we see the island of urban pollution, because cities are still a dominant source of pollutants, says Dr. Vinoj.
Urban Clean Island Effect was not studied during the monsoon due to data unavailability due to clouds and rain. The study focused on data for the next season, but found that the effect of urban clean islands was significant and clearly observed only during the period before the monsoon. During other seasons, a large amount of dust or other sources of aerosols that are transported over long distances are not visible, making it difficult to see urban aerosols clear islands.
“Pre-Monsoon time is when the signature is clear and the effect of urban clean islands becomes clearly visible,” he said. Urban Clean Island Effect normally disappears during the post-monzun, but can be seen again in winter due to dry conditions, but not in the extent that can be seen during the monsoon period.
Invisible barrier
The study focused on a high dust case and no scenario of dust and found that the effect of the clean island of Urban, in many cities in the northwest and northern Indo-Gangnetic plain in the case of high dust, but not in a scenario without dust.
“Our hypothesis is that, regardless of the season, whenever there is a minor transmission of aerosol or pollution, you will see the islands of urban pollution. But whenever transportation is from outside, you will see a clean island effect,” Dr. Vinoj.
“There is an invisible barrier that has not been recorded earlier. This barrier is only seen when aerosols from combustion or dust are transported from anywhere.”
In fact, several studies performed on global megacites, such as Shanghai, Atlanta and several European cities, watched clean islands, but attributed them to emissions in suburban regions.
“These Findings, We Feel, Challenge Conventional UnderStanding That Long-Range Transported Aerosols Will Always Lead to More Pollutation Over the Cities and Underscore The Need For Deeper Scientific UnderStanding Ino How Growth and Evolving Micro-Climates Pollution and Its Spatial Patterns.
Published – July 17, 2025 09:20
