Bhubaneswar: Normally, cities are regarded as pollution hotspots, with elevated pollution levels compared to surrounding non-urban regions — a pattern commonly known as the ‘urban pollution dome’ or ‘urban pollution island’ effect. However, a recent study by IIT Bhubaneswar found that this pattern does not hold true in many northern Indian cities.“Instead of a concentrated urban pollution dome, these cities display a ‘clean island’ effect, or what the researchers describe as a ‘punctured pollution dome’, where the city centres are, unexpectedly, relatively cleaner than the heavily polluted surrounding areas,” stated the study, published in the scientific journal Nature.Researchers V Vinoj, asssociate professor at the school of earth, ocean and climate sciences, IIT Bhubaneswar, and research scholar Soumya Satyakanta Sethi attributed this unexpected pattern to an ‘invisible barrier’ formed by city’s tall buildings and uneven structures, which slows down wind.“This limits pollutant dispersion, causing pollution to accumulate within the city and form a typical urban pollution dome. However, this same barrier can also prevent polluted air from outside the city from entering. As a result, in some cases, pollution builds up in areas surrounding the city, making the city centre appear relatively cleaner,” said Vinoj.Based on two decades of high-resolution aerosol data across 141 Indian cities, the study found that southern cities — less affected by pollution transported from afar — exhibit classic domes with more pollution inside. In contrast, cities in northern and northwestern India, particularly those in the Indo-Gangetic plain, experience heavy regional and long-range pollution, such as dust. There, the city’s barrier blocks incoming pollutants, causing them to accumulate in surrounding non-urban areas and forming what the researchers describe as “clean air domes.“The study stated that around 57% of cities exhibited urban aerosol pollution islands, while the remaining 43% showed urban aerosol clean islands. “Delhi and Mumbai are pollution islands normally, but they become clean islands whenever high dust is transported to the surrounding areas of these cities due to various reasons,” said Vinoj.Bhubaneswar, on the other hand, remains a pollution island even when high dust is transported to surrounding areas, he added.These findings challenge long-standing assumptions about urban air pollution — particularly the notion that transported aerosols simply add up over cities and uniformly degrade air quality. The study also highlights that monitoring air pollution solely at city boundaries may provide an incomplete picture, as the actual dynamics involve a complex interplay of local emissions, regional transport, microclimatic effects, and atmospheric processes.“Uncovering these hidden atmospheric dynamics is only the beginning. Achieving truly sustainable and climate-resilient cities requires a deeper, integrated understanding of how urban environments interact with atmospheric processes,” the researchers concluded.
