Urban Aerosol Pollution and Clean Islands in Indian Cities
Recent studies from 2003 to 2020 reveal contrasting aerosol pollution patterns in Indian cities. Researchers at IIT Bhubaneswar identified two distinct phenomena – Urban Aerosol Pollution Islands (UAPIs) in southern and southeastern cities, and Urban Aerosol Clean Islands (UACIs) in northern and northwestern cities, particularly in the Indo-Gangetic Plain (IGP). These findings challenge traditional views on urban air pollution and offer new vital information about aerosol dynamics.
Urban Aerosol Pollution Islands (UAPIs)
Cities in south and southeast India show higher aerosol levels inside the city compared to surrounding areas. This occurs due to local pollution sources dominating aerosol presence. These cities lack external aerosol transport from dust or biomass burning. The result is a typical “pollution dome” where urban areas are more polluted than their rural surroundings.
Urban Aerosol Clean Islands (UACIs)
In contrast, 43% of cities in northwest and northern India display lower aerosol levels within city limits than in surrounding areas. Here, aerosols from external sources such as dust from the Thar Desert and biomass burning contribute heavily to background pollution. Cities act as barriers, reducing aerosol entry and creating cleaner urban zones compared to their surroundings.
Wind Stilling Effect
The phenomenon behind UACIs is the urban wind stilling effect. Urban infrastructure weakens surface winds, causing atmospheric stagnation zones. These zones disrupt the flow of long-range aerosols, especially mineral dust. The cities partially block or divert incoming aerosols, leading to higher pollution levels upstream and cleaner air downstream and inside the city.
Seasonal Variations in Aerosol Patterns
The urban clean island effect is most pronounced during the pre-monsoon season. During this period, dust transport and biomass burning aerosols are at their peak. In monsoon, cloud cover and rain limit data availability and reduce aerosol transport. Post-monsoon and winter show weaker clean island effects due to less dust movement and wetter conditions.
Implications for Urban Air Quality Management
The study marks that external aerosol transport does not always increase urban pollution. Instead, complex interactions between urban form, wind patterns, and aerosol sources shape pollution distribution. Southern cities remain pollution hotspots due to dominant local emissions. Northern cities may appear cleaner but suffer from polluted surroundings. These insights are crucial for designing climate-resilient, sustainable cities.
Global Context and Future Research
Similar clean island effects have been observed in global megacities like Shanghai and Atlanta but attributed to suburban emissions. The Indian study suggests urban growth and microclimates play roles in aerosol dynamics. Deeper scientific understanding is needed to predict pollution patterns as cities expand and climate changes.
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