A comprehensive study analyzing satellite-derived aerosol data across 141 Indian cities from 2003 to 2020 has revealed unexpected patterns in pollution distribution. Researchers from the School of Earth, Ocean and Climate Sciences at IIT Bhubaneswar found that 57% of cities in southern and southeastern India exhibited higher aerosol levels than their surrounding areas — a phenomenon they have labeled as Urban Aerosol Pollution Islands . In contrast, 43% of cities in the northwestern and northern Indo-Gangetic Plain (IGP) region showed Urban Aerosol Clean Islands, where city air was actually cleaner than adjacent regions.

 

Interestingly, the aerosol concentration in these clean island cities was not uniformly higher around the entire periphery. Instead, it was particularly elevated in the southwest — the direction from which dust-laden winds typically arrive — while the northeast, located downstream of this flow, had aerosol levels similar to the city itself. According to lead researcher Dr. V. Vinoj, this suggests that cities in these regions are not accumulating more pollution but rather obstructing and redirecting incoming aerosols. The effect was unexpected, with cities effectively diverting or halting airborne particles, especially dust.

 

Dr. Vinoj explained that, despite frequent criticism for poor air quality, northern Indian cities don't exhibit consistent "pollution domes." Instead, they display relatively clean air pockets — a surprising pattern attributed to the urban wind stilling effect. This effect involves the slowing of surface winds due to dense urban infrastructure, leading to stagnant atmospheric zones that act as invisible barriers. These barriers limit the infiltration of long-range pollution, such as dust from arid zones or biomass-burning aerosols.

 

While these barriers don’t entirely block pollutants, they slow their transport, resulting in less aerosol accumulation within cities. This contrasts sharply with southern cities, where local sources dominate pollution and there’s little incoming pollution to block, thus forming traditional pollution domes.

 

Ph.D. scholar Soumya Sethi, the study's first author, noted that this "barrier effect" doesn’t eliminate external pollution but makes its movement into cities more sluggish. As a result, northern cities appear cleaner, while surrounding areas bear the brunt of accumulation.

 

The clean island effect was most visible during the pre-monsoon season, when conditions like dry weather and active dust transport amplify the phenomenon. It faded during monsoon due to cloud cover and rain, and reappeared in winter, albeit less strongly.

 

By comparing high-dust and no-dust scenarios, the researchers confirmed that the clean island effect becomes most prominent during heavy dust events. Whenever there is significant pollution transport from external sources, the effect surfaces. In cleaner conditions, cities themselves emerge as pollution hotspots instead.

 

Dr. Vinoj emphasized that similar urban clean islands have been observed in cities like Shanghai and Atlanta, though often attributed to suburban emissions rather than wind stilling. The study challenges the traditional assumption that long-distance aerosol transport always increases urban pollution. Instead, it shows that city growth, infrastructure, and evolving local climates can significantly alter pollution dynamics.

 

The findings underscore the need for more nuanced, location-specific strategies to design climate-resilient and sustainable urban spaces by understanding how cities interact with their broader environments.