A research of 141 cities in India from 2003 to 2020 utilizing satellite-retrieved aerosol information reveals a shock — aerosol ranges have been considerably excessive in 57% of cities in contrast with the surrounding areas exterior the metropolis in south and southeast India, whereas the converse was true in the case of 43% of cities in northwest and northern Indo-Gangetic Plain (IGP) area the place the aerosol ranges have been comparatively decrease in the cities in contrast with the surrounding areas.
Researchers at the School of Earth, Ocean and Climate Sciences, IIT Bhubaneswar, seek advice from the cities in south and southeast India that present larger aerosol ranges than the surrounding areas as Urban Aerosol Pollution Islands. And cities that present comparatively decrease aerosol ranges in contrast with the surrounding areas are known as Urban Aerosol Clean Islands.
In the case of cities known as city aerosol clear islands, the aerosol ranges weren’t uniformly larger in the surrounding areas in contrast with the metropolis. Instead, the ranges have been larger in the areas southwest of the metropolis, that are situated upstream of the mud circulation, whereas the northeast facet of the metropolis situated downstream of the mud circulation confirmed much less aerosol ranges which just about matched the ranges seen in the metropolis.
“Aerosol coming from outside wasn’t adding to the pollution already seen in the cities that we refer to as urban aerosol clean islands. Instead, the cities in northwest and in the northern Indo-Gangetic Plain region were stopping or diverting the incoming aerosol and moving it around the city. We observed areas upstream of the aerosol flow had more aerosol load than the city, while the areas northeast of the city that were located downstream of the flow had less aerosol load. It is a trail going actually downwind for some distance,” says Dr. V. Vinoj from IIT Bhubaneswar, and the corresponding writer of a paper printed on July 16, 2025 in the journal Communications Earth & Environment. “We didn’t expect this.”
“North Indian cities, despite being blamed for poor air quality, are found to have no consistent ‘pollution domes’. Instead, we observed urban clean islands —zones with relatively lower aerosol levels than surrounding areas. We hypothesise this unexpected pattern to be linked to a phenomenon known as urban wind stilling effect,” he mentioned.
Wind stilling impact
Wind stilling impact refers to a weakening of floor winds throughout extremely urbanised cities the place the buildings and infrastructure reshape native climates, creating zones of atmospheric stagnation. These zones collectively result in invisible boundaries round the metropolis (in the upwind areas), partially blocking the entry of long-range aerosol pollution, particularly mineral mud from the close by arid areas.
Also, cities situated in areas with excessive background pollution are likely to decelerate the transport of pollution from exterior the metropolis, like the mud from Thar desert or aerosol from biomass burning which can be transported from elsewhere. This outcomes in cities having comparatively much less aerosol loading than the surrounding areas.
“While external sources still contribute to pollution, this barrier alters how pollutants accumulate and disperse, leading to a deceptive pocket of cleaner air within the city and its downwind regions. In contrast, southern Indian cities, with less influence from transported dust and different meteorology, show traditional pollution domes,” Dr. Vinoj explains.
Less aerosol load
According to Soumya Sethi, a Ph.D. scholar and the first writer of the paper, the research makes it clear that the proposed barrier impact seen in cities doesn’t remove pollutant transport into the metropolis however solely slows down the transport. In the course of, the cities in the northwest and northern Indo-Gangetic Plain have comparatively much less aerosol load whereas the surrounding areas witness a rise in pollutant construct up.
The motive why cities in southern India do not need city aerosol clear islands is as a result of southern cities don’t have any massive supply of pollution coming from elsewhere to create a big aerosol background, which is able to permit us to see this quite invisible dome impact. Instead, we see the city pollution island as a result of cities are nonetheless the dominant supply of pollution, says Dr. Vinoj.
The city clear island impact was not studied throughout monsoon attributable to non-availability of information attributable to clouds and rain. The research checked out information for different seasons however discovered the city clear islands impact was pronounced and clearly observable solely throughout the pre-monsoon interval. During different seasons, a considerable amount of mud or different sources of aerosols being transported lengthy distances is just not seen, making it troublesome to see the city aerosol clear islands impact clearly.
“The pre-monsoon time is when the signature is clear, and the urban clean islands effect becomes clearly visible,” he mentioned. The city clear island impact usually disappears throughout the post-monsoon however is once more seen throughout winter due to dry circumstances however to not the extent that’s seen throughout the pre-monsoon interval.
Invisible barrier
The research checked out the excessive mud case and no mud state of affairs and located the city clear island impact turned pronounced in many cities in northwest and northern Indo-Gangetic Plain in excessive mud case however not in the no mud state of affairs.
“Our hypothesis is that, irrespective of the season, whenever less transport of aerosol or pollution is happening, you will see urban pollution islands. But whenever the pollution transport from outside is enhanced, you will see a clean island effect,” Dr. Vinoj mentioned.
“There is an invisible barrier that was not noticed earlier. This barrier is seen only when aerosols from biomass burning or dust are transported from elsewhere.”
In reality, just a few research carried out over world megacities akin to Shanghai, Atlanta and some European cities have noticed clear islands, however attributed it to emissions in suburban areas.
“These findings, we feel, challenge conventional understanding that long-range transported aerosols will always lead to more pollution over the cities and underscore the need for deeper scientific understanding into how urban growth and evolving micro-climates influence air pollution and its spatial patterns. Therefore, building truly sustainable, climate-resilient cities will depend on our ability to understand and anticipate these complex interactions as cities continue to expand,” he added.
