Air pollution is a silent, invisible killer, a global issue that affects millions of lives and costs trillions of dollars worldwide. In 2021 alone, it was listed as the second deadliest health risk globally. India, a nation undergoing rapid urbanisation and projected to have the highest global rise in urban population by 2050, faces a particularly severe challenge. In 2019, air pollution was responsible for approximately 1.67 million deaths in India, accounting for nearly 18% of the country's total fatalities that year. Many Indian cities are also consistently ranked among the most polluted in the world.
Just as cities can become urban heat islands due to trapped warmth, they can also form urban pollution islands, where pollutants are concentrated over the city compared to surrounding areas. This phenomenon, often called the urban pollution or turbidity island effect, is crucial to understand as our cities continue to grow. A recent study from Indian Institute of Technology (IIT) Bhubaneswar has shed new light on these urban pollution patterns across India. The study reveals a surprising north-south divide in how pollution domes form over cities.
Aerosols are everywhere! They're tiny solid or liquid particles suspended in the air. Think of dust, smoke, pollen, or even tiny salt crystals from the ocean – they're all aerosols! |
Researchers found that while many cities in southern and southeastern India exhibit what they call Urban Aerosol Pollution Islands (UAPIs), meaning pollution levels are higher directly over the city, a different and counterintuitive pattern emerges in the north. Cities in the northern Indo-Gangetic Plain (IGP) region, despite being notoriously polluted, often show Urban Aerosol Clean Islands (UACIs). This means that, paradoxically, the air within these northern cities can be relatively cleaner than the air in their surrounding non-urban areas.
The study highlights that these pollution domes, especially the UACIs in the north, are significantly influenced, or even punctured, by aerosols transported from regional or long-range sources, particularly dust. They discovered a clear negative relationship: when dust levels are high, UACIs (where the city is relatively cleaner) are more common and more intense. This suggests that the presence of abundant dust in the surrounding environment plays a crucial role in shaping the pollution landscape of Indian cities.
To study these patterns, the researchers analysed satellite data collected over 141 Indian cities from 2003 to 2020. They primarily used Aerosol Optical Depth (AOD) data, a measurement derived from satellites that essentially tells us how much sunlight is blocked by tiny particles (aerosols) in the atmosphere. A higher AOD indicates a greater concentration of aerosols, resulting in higher air pollution.
Cities often create an urban wind-stilling effect, where buildings and urban structures slow down wind speeds. This reduced wind speed can act like a barrier. If the pollution is mainly generated within the city, this barrier traps it, leading to a UAPI (higher pollution over the city). However, suppose a significant amount of pollution is already present in the background environment, such as dust storms from the Thar Desert or smoke from biomass burning. In that case, this same wind-stilling barrier can actually impede the entry of these outside pollutants into the city. This leads to a UACI, where the city appears relatively cleaner than its hazy surroundings. The northern Indian region, being densely populated with numerous towns and industries, and also heavily impacted by long-range dust transport, experiences high background aerosol levels, which explains the prevalence of UACIs there.
The research significantly advances our understanding of urban air pollution in India. While previous studies have examined pollution islands in a few Asian or global cities, this is the first comprehensive attempt to characterise and quantify the spatiotemporal aspects of Urban Aerosol Islands. However, the study also notes some limitations. The observed UAI strength in Indian cities is generally weaker compared to some global megacities, such as Berlin or Beijing, suggesting that pollution levels are more uniformly high across both urban and rural areas in India. Crucially, a UACI doesn't mean a city is clean; it simply means the pollution inside the city is less intense than the extreme pollution outside. The high AOD levels in UACI cities remain a significant concern, and other types of aerosols, besides dust, could also play a role. Furthermore, the barrier effect proposed by the researchers only slows down the transport of regional pollutants; it doesn't completely block them.
By highlighting the dominant role of regional and long-range transported aerosols, especially dust, in shaping urban pollution patterns, the research underscores the need for a shift in air pollution mitigation strategies. Current efforts in India are often city-centric, focusing on local emissions. However, this study strongly advocates for synergistic regional and local measures to mitigate aerosol pollution. Understanding these complex aerosol loading patterns is also vital for predicting their impact on urban climate, temperature, and even rainfall, ultimately contributing to healthier and more sustainable cities for everyone.
This article was written with the help of generative AI and edited by an editor at Research Matters