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

A examine of 141 cities in India from 2003 to 2020 utilizing satellite-retrieved aerosol knowledge reveals a shock — aerosol ranges have been considerably excessive in 57% of cities in contrast with the encircling areas outdoors the town in south and southeast India, whereas the converse was true within the case of 43% of cities in northwest and northern Indo-Gangetic Plain (IGP) area the place the aerosol ranges have been comparatively decrease within the cities in contrast with the encircling areas.

Researchers on the College of Earth, Ocean and Local weather Sciences, IIT Bhubaneswar, check with the cities in south and southeast India that present greater aerosol ranges than the encircling areas as City Aerosol Air pollution Islands. And cities that present comparatively decrease aerosol ranges in contrast with the encircling areas are known as City Aerosol Clear Islands.

Within the case of cities known as city aerosol clear islands, the aerosol ranges weren’t uniformly greater within the surrounding areas in contrast with the town. As an alternative, the degrees have been greater within the areas southwest of the town, that are positioned upstream of the mud movement, whereas the northeast facet of the town positioned downstream of the mud movement confirmed much less aerosol ranges which just about matched the degrees seen within the metropolis. 

“Aerosol coming from outdoors wasn’t including to the air pollution already seen within the cities that we check with as city aerosol clear islands. As an alternative, the cities in northwest and within the northern Indo-Gangetic Plain area have been stopping or diverting the incoming aerosol and transferring it across the metropolis. We noticed areas upstream of the aerosol movement had extra aerosol load than the town, whereas the areas northeast of the town that have been positioned downstream of the movement had much less aerosol load. It’s a path going really downwind for far,” says Dr. V. Vinoj from IIT Bhubaneswar, and the corresponding writer of a paper printed on July 16, 2025 within the journal Communications Earth & Setting. “We didn’t count on this.”

“North Indian cities, regardless of being blamed for poor air high quality, are discovered to haven’t any constant ‘air pollution domes’. As an alternative, we noticed city clear islands —zones with comparatively decrease aerosol ranges than surrounding areas. We hypothesise this sudden sample to be linked to a phenomenon generally known as city wind stilling impact,” 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 obstacles across the metropolis (within the upwind areas), partially blocking the entry of long-range aerosol air pollution, particularly mineral mud from the close by arid areas.

Additionally, cities positioned in areas with excessive background air pollution are inclined to decelerate the transport of pollution from outdoors the town, just like the mud from Thar desert or aerosol from biomass burning which can be transported from elsewhere. This leads to cities having comparatively much less aerosol loading than the encircling areas.

“Whereas exterior sources nonetheless contribute to air pollution, this barrier alters how pollution accumulate and disperse, resulting in a misleading pocket of cleaner air throughout the metropolis and its downwind areas. In distinction, southern Indian cities, with much less affect from transported mud and totally different meteorology, present conventional air pollution domes,” Dr. Vinoj explains.

Much less aerosol load

Based on Soumya Sethi, a Ph.D. scholar and the primary writer of the paper, the examine makes it clear that the proposed barrier impact seen in cities doesn’t remove pollutant transport into the town however solely slows down the transport. Within the course of, the cities within the northwest and northern Indo-Gangetic Plain have comparatively much less aerosol load whereas the encircling areas witness a rise in pollutant construct up.

The rationale why cities in southern India should not have 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 can enable us to see this somewhat invisible dome impact. As an alternative, we see the city air 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 resulting from non-availability of knowledge resulting from clouds and rain. The examine checked out knowledge for different seasons however discovered the city clear islands impact was pronounced and clearly observable solely through the pre-monsoon interval. Throughout different seasons, a considerable amount of mud or different sources of aerosols being transported lengthy distances isn’t seen, making it troublesome to see the city aerosol clear islands impact clearly.

“The pre-monsoon time is when the signature is evident, and the city clear islands impact turns into clearly seen,” he mentioned. The city clear island impact usually disappears through the post-monsoon however is once more seen throughout winter due to dry situations however to not the extent that’s seen through the pre-monsoon interval.

Invisible barrier

The examine seemed on the excessive mud case and no mud situation and located the city clear island impact grew to become pronounced in lots of cities in northwest and northern Indo-Gangetic Plain in excessive mud case however not within the no mud situation.

“Our speculation is that, no matter the season, each time much less transport of aerosol or air pollution is going on, you will notice city air pollution islands. However each time the air pollution transport from outdoors is enhanced, you will notice a clear island impact,” Dr. Vinoj mentioned.

“There’s an invisible barrier that was not seen earlier. This barrier is seen solely when aerosols from biomass burning or mud are transported from elsewhere.”

In truth, a number of research carried out over world megacities reminiscent of Shanghai, Atlanta and some European cities have noticed clear islands, however attributed it to emissions in suburban areas. 

“These findings, we really feel, problem typical understanding that long-range transported aerosols will at all times result in extra air pollution over the cities and underscore the necessity for deeper scientific understanding into how city progress and evolving micro-climates affect air air pollution and its spatial patterns. Due to this fact, constructing actually sustainable, climate-resilient cities will rely upon our means to grasp and anticipate these complicated interactions as cities proceed to develop,” he added. 

Revealed – July 17, 2025 09:20 am IST