Some cities in northwest, northern Indo-Gangetic Plain shielded from long-range aerosol pollution

The Hindu

17-07-2025

A study of 141 cities in India from 2003 to 2020 using satellite-retrieved aerosol data reveals a surprise — aerosol levels were significantly high in 57% of cities compared with the surrounding areas outside the city in south and southeast India, while the converse was true in the case of 43% of cities in northwest and northern Indo-Gangetic Plain (IGP) region where the aerosol levels were relatively lower in the cities compared with the surrounding areas.
Researchers at the School of Earth, Ocean and Climate Sciences, IIT Bhubaneswar, refer to the cities in south and southeast India that show higher aerosol levels than the surrounding areas as Urban Aerosol Pollution Islands. And cities that show relatively lower aerosol levels compared with the surrounding areas are referred to as Urban Aerosol Clean Islands.
In the case of cities referred to as urban aerosol clean islands, the aerosol levels were not uniformly higher in the surrounding areas compared with the city. Instead, the levels were higher in the areas southwest of the city, which are located upstream of the dust flow, while the northeast side of the city located downstream of the dust flow showed less aerosol levels which almost matched the levels seen in the city.
'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 author of a paper published 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 said.
Wind stilling effect
Wind stilling effect 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), partially blocking the entry of long-range aerosol pollution, especially mineral dust from the nearby arid regions.
Also, cities located in regions with high background pollution tend to slow down the transport of pollutants from outside the city, like the dust from Thar desert or aerosol from biomass burning that are transported from elsewhere. This results in cities having relatively 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 author of the paper, the study makes it clear that the proposed barrier effect seen in cities does not eliminate pollutant transport into the city but only slows down the transport. In the process, the cities in the northwest and northern Indo-Gangetic Plain have relatively less aerosol load while the surrounding areas witness an increase in pollutant build up.
The reason why cities in southern India do not have urban aerosol clean islands is because southern cities don't have any large source of pollutants coming from elsewhere to create a large aerosol background, which will allow us to see this rather invisible dome effect. Instead, we see the urban pollution island because cities are still the dominant source of pollutants, says Dr. Vinoj.
The urban clean island effect was not studied during monsoon due to non-availability of data due to clouds and rain. The study looked at data for other seasons but found the urban clean islands effect was pronounced and clearly observable only during the pre-monsoon period. During other seasons, a large amount of dust or other sources of aerosols being transported long distances is not seen, making it difficult to see the urban aerosol clean islands effect clearly.
'The pre-monsoon time is when the signature is clear, and the urban clean islands effect becomes clearly visible,' he said. The urban clean island effect normally disappears during the post-monsoon but is again seen during winter because of dry conditions but not to the extent that is seen during the pre-monsoon period.
Invisible barrier
The study looked at the high dust case and no dust scenario and found the urban clean island effect became pronounced in many cities in northwest and northern Indo-Gangetic Plain in high dust case but not in the no dust scenario.
'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 said.
'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 fact, a few studies carried out over global megacities such as Shanghai, Atlanta and a few European cities have observed clean islands, but attributed it to emissions in suburban regions.
'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.