Recent research analysing aerosol pollution in 141 Indian cities between 2003 and 2020 reveals contrasting patterns of air quality across regions. Cities in south and southeast India show higher aerosol levels than their surroundings, while many cities in northwest India and the Indo-Gangetic Plain (IGP) display lower aerosol levels compared to nearby rural areas. This phenomenon challenges traditional views on urban pollution and marks the role of local climate and geography in shaping air quality.
Urban Aerosol Pollution Islands and Clean Islands
Cities with higher aerosol pollution than their surroundings are termed Urban Aerosol Pollution Islands (UAPIs). These are mostly found in southern and southeastern India. Conversely, cities in northwest India and the IGP often act as Urban Aerosol Clean Islands (UACIs), where aerosol levels inside the city are lower than in adjacent rural zones. This unexpected pattern indicates complex interactions between urban environments and aerosol transport.
Role of Aerosol Transport and Wind Patterns
In UACIs, aerosol pollution is not uniformly distributed around cities. Areas upstream of dust and pollution flows have higher aerosol concentrations than the city itself. Downwind zones show lower aerosol levels, often matching those within the city. This suggests that cities in the northwest and IGP regions disrupt or divert aerosol transport, creating pockets of cleaner air downwind.
Wind Stilling Effect and Urban Microclimates
The wind stilling effect explains this phenomenon. Urban structures weaken surface winds, creating atmospheric stagnation zones. These zones act as invisible barriers that partially block or divert long-range aerosol transport, such as mineral dust from arid regions like the Thar Desert. Consequently, cities slow down incoming pollutants, resulting in cleaner urban air compared to surrounding rural areas.
Differences Between Northern and Southern Cities
Southern cities lack external aerosol sources like transported dust, so they do not exhibit the clean island effect. Instead, urban emissions dominate pollution, forming traditional pollution domes with higher aerosol levels inside cities. Northern cities, influenced by transported dust and biomass burning aerosols, show the clean island effect prominently during the pre-monsoon season when dust transport is highest.
Seasonal Variations in Aerosol Patterns
The urban clean island effect is most visible in the pre-monsoon season due to increased dust transport. It diminishes during the monsoon due to cloud cover and rain, which reduce aerosol levels. The effect reappears during winter but less strongly. This seasonal variation marks the influence of meteorological conditions on aerosol distribution.
Implications for Urban Air Quality Management
The discovery of urban aerosol clean islands challenges the assumption that cities always have higher pollution than their surroundings. It puts stress on the need to consider urban microclimates, wind patterns, and regional pollution transport in air quality management. Sustainable urban planning must incorporate these factors to effectively control pollution and protect public health.
Global Context and Future Research
Similar clean island phenomena have been observed in global megacities like Shanghai and Atlanta, though attributed to suburban emissions. The Indian findings suggest a broader need to understand how urban growth and climate interact to influence pollution patterns. Further research is crucial for developing climate-resilient cities worldwide.
Questions for UPSC:
- Point out the factors responsible for the formation of Urban Aerosol Pollution Islands and Urban Aerosol Clean Islands in Indian cities.
- Critically analyse the role of wind stilling effect in altering urban air quality with suitable examples from the Indo-Gangetic Plain.
- Estimate the impact of seasonal variations on aerosol pollution levels and their implications for public health policy in India.
- What are the challenges in managing air pollution in rapidly urbanising cities? How can understanding local microclimates improve pollution control strategies?
