India’s Urban-Rural Air Quality Divide

A recent study finds that aerosol pollution in north Indian cities is lower than in their surrounding rural areas, while the pattern is reversed in south India — city air is more polluted than nearby rural areas. These patterns arise when the cityscape interacts with climatic processes in the broader landscape of its location, influencing the dispersion of pollutants. The study calls for a regional perspective for monitoring and mitigating air pollution.

Cities have a strong influence on their local environment. For example, cities are usually warmer than their surroundings due to urban heat island effects. This occurs when green cover decreases, built-up structures increase, and greenhouse gases are more widespread in cities compared to rural areas. Similarly, air pollutants can get trapped over cities, resulting in urban pollution islands. Aerosols or particulate matter (PM) are important components of air pollution; they consist of suspended solid or liquid particles of diverse origins and are classified based on their particle size. They enter our bodies when we breathe, posing a significant health risk in densely populated urban areas that may be enveloped by pollution domes.

“Aerosols are known to cause a range of health outcomes, from adverse birth outcomes to non-communicable diseases among adults, leading to premature death,” says Sagnik Dey from the Indian Institute of Technology (IIT) Delhi, who studies the interaction of aerosols with climate and their effects on human health. Dey was part of the Global Burden of Disease Study 2019, which found that aerosol pollution was responsible for almost a million deaths in 2019, with a 115% increase in the death rate as compared to three decades ago.

Not all cities are the same

“Most initiatives look at cities as a separate entity when it comes to measuring and taking steps to reduce air pollution. But local weather and the associated regional circulation have a very important role to play in its day-to-day variability and spatial distribution,” says Vinoj V., a climate scientist from IIT Bhubaneswar. To investigate this, he, and his student Soumya Satyakanta Sethi, carried out the study comparing patterns of aerosol pollution domes across 141 Indian cities. “When we started, we wanted to explore the relationship between aerosol pollution and climate,” explains Vinoj.

As a proxy for aerosol concentration, the researchers used satellite observations of aerosol optical depth, which quantifies how much light is scattered or absorbed by aerosols present in a column of the atmosphere. For each city, they considered a 50 km domain around its centre and averaged aerosol optical depth values across 18 years (2003–2020) for each kilometre-wide pixel within this area. To test for the urban aerosol island effect, the team calculated the difference in aerosol optical depth values between urban and surrounding non-urban areas. They also looked at how this intensity varies locally as one moves away from the city centre towards the periphery.

As expected, the study found signatures of urban aerosol pollution islands. What was surprising was that this was not found in all cities — these islands were apparent in southern and eastern India, where cities seem to be enclosed in a dome of higher aerosol pollution as compared to their surroundings. However, cities in north and north-western India emerged as urban aerosol clean islands. Despite having higher overall pollution than the cities in southern and eastern India, the cities in northern and north-western India had relatively cleaner air within them as compared to their rural neighbourhoods. “We were surprised as we expected each of these cities to be a pollution island. The results were counter-intuitive,” says Vinoj.

Digging deeper into seasonal patterns of aerosol pollution, they found that the number of urban aerosol clean islands increased dramatically during the dry pre-monsoon months. On the other hand, urban aerosol pollution islands were most prominent in low-dust conditions after the monsoon. Overall, there was a significant correlation between the urban aerosol island intensity and dust levels, indicating that processes larger than the scale of a city might be driving these patterns.

“We found a correlation between patterns of wind, dust levels, and the distribution of urban aerosol clean islands,” says Vinoj. In northern India, cities may physically block dust blowing from drier regions and emissions from surrounding satellite towns, leading to a buildup of aerosol pollution just outside cities. “We hypothesize that urbanization and the associated built-up structures reduce wind speed over the city, creating a convergence barrier on the upwind side of the city, which accumulates and/or redirects the transport of pollutants. The wind bifurcates around the city, carrying the pollutants with it,” he explains. Local gradients in aerosol pollution in and around cities in the Indo-Gangetic plain provide further evidence for this.

As wind patterns and dust levels change seasonally, these air pollution gradients vary in strength over time. In areas with urban pollution domes, cities may similarly create barriers that trap pollutants from escaping outside, leading to a sharp rise in pollution levels within the city. “Many studies in the past have reported this stagnation within cities,” says Vinoj.

Adopting a regional view of air pollution

Similar results have been reported by a global study, which found higher aerosol pollution in the surroundings of Indian and Chinese cities using more coarse-scale data. Another study across eight South Asian cities found distinct geographic and seasonal signatures of aerosol pollution. It reported a higher aerosol load over most cities and a seasonally varying local gradient, attributing it to transport of city-based emissions and the possible role of local or transported dust.

Recent research using ground-based measurements and satellite observations has even discovered wind-driven transport of aerosols from northern India all the way to the south. “In winter, winds over India often carry polluted air from the north to the southeastern coast, including Chennai. These air masses bring aerosols that settle 1–3 km above ground, warming the atmosphere and trapping pollution closer to the surface,” says Chandan Sarangi, an earth system scientist at IIT Madras, who was involved in the study.

“The signal is small but distinct,” says Vinoj, about the 2% (clean island) and 3% (pollution island) difference in aerosol pollution between cities and their surroundings. “We need to look at clean air programs not at the scale of a city but at a regional scale,” he emphasises. “This even has implications for planning clean and climate-resilient cities in the future,” he adds.

The National Clean Air Programme is a five-year plan initiated by the Ministry of Environment, Forest and Climate Change in 2019 to monitor air pollution and improve air quality in Indian cities. It is currently implemented across 130 cities with the aim of reducing PM₁₀ (particulate matter with a diameter of 10 micrometres or smaller) levels by 40% by 2026. As of last year, 55 of these cities showed a 20% or more reduction in PM₁₀ levels as compared to 2017–2018, although only 18 cities met the National Ambient Air Quality Standards (NAAQS) for PM₁₀ concentration.

“The entire Indo-Gangetic plain region, cutting across the urban–rural boundary, has air pollution levels almost double that of the NAAQS. Unless regional state-level planning is implemented with accountable measures and clear time-bound targets, progress will remain slow,” says Dey. “Air pollution policies need to be implemented at the scale of regional airsheds, transitioning from a city-centric focus.”

“Our monitoring networks mostly observe changes at 2 m in the atmosphere and air quality discussions are restricted to surface level changes. While emissions do occur at the surface, transported particles can be present at higher altitudes,” says Sarangi, “When haze layers transported over long range are present at high altitudes above the pollution domes of megacities, they make the atmosphere warmer relative to days without these layers,” he adds. His group’s work shows that the episodes of long-range transport of aerosols, which change atmospheric dynamics and have a cascading effect on particulate pollution, are becoming prolonged over the last decade.

The interaction of climate and cities means that the observed patterns of aerosol pollution may shift under a changing climate. Previous work from Vinoj’s research group shows evidence for reduced dust emission from arid regions of north-western India and its effect of weakening monsoonal circulation. “A changing climate is going to alter meteorology, which is not spatially uniform and depends on many local and regional factors,” says Dey.

“These complex pathways need to be understood and considered by future policies. Strong policy and technological interventions, transition to clean energy, and switching completely to clean cooking will further both the climate and air pollution agenda,” he adds. “We need stronger collaborations between urban planners, air quality experts, and climate scientists to shape the cities of tomorrow,” adds Vinoj.

(Published under Creative Commons from Mongabay India)