New Delhi: Delhi, Gwalior, Lahore, and Dhaka share a common problem — toxic air pollution. Vehicle emissions, stubble burning, construction debris and industry emissions are all culpable, but another factor — geography — is at play.
All these cities that consistently top the list of worst-polluted fall within the Indo Gangetic Plain — a low-lying, bowl-like trap in the Indian subcontinent, that amplifies pollution levels in winter.
During peak winter, the region faces a dangerous cocktail of meteorological factors, which prevent particulate matter from dispersing, and a seasonal increase in emissions, with the particulate nature of pollutants like soot exacerbating the problem. The region is also susceptible to a unique phenomenon spurred by the behemoth Himalayas, keeping all emissions at the level initially emitted.
Environmental scientists across the border have also highlighted this topographical disadvantage, often calling it the “valley effect”.
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As winds blow from the South and the West in the winter, greenhouse gases get trapped at the foothills of the Himalayas. “Then, cool winds start blowing southward from the Himalayas, and this creates a layer of cold air, two to three km above the land surface, called an inversion layer,” explained Chandan Sarangi, an IIT Madras earth systems scientist, specialising in meteorology and air quality.
“The land surface tends to be cooler than the immediate air above because of reduced absorption of solar radiation. Therefore, the warm layer of air, with pollutants, remains trapped between cold layers of air,” Sarangi added.
‘Bowl’ of Indo-Gangetic Plain
The nature of particulate emissions such as soot, carbon, and smoke particles — dark in colour— is such that they absorb more heat. As they are emitted within the warm layer close to surface, they absorb heat, increasing the temperature difference between the warm and cool layers of air, thus increasing the stability and thus longevity of the winter stagnation of polluted, warm air.
The particulate emissions can even induce the phenomenon of an inversion layer by the absorption of heat, making the air below warmer than the air above, explained Sarangi.
The year-long particulate emissions from vehicles and industries mix with dust from the Thar desert, loose soil from the Himalayas, road dust, and emissions from stubble and biofuel burning in the winter. Since the particles cannot disperse as they do for the rest of the year, they stay trapped within the air column closest to the ground.
The situation gets worse because the Indo-Gangetic Plain (IGP) is a low-lying “bowl”, effectively sucking in pollutants from across the land in winter.
In the winter, Westerlies are the prevailing wind, blowing from West to East. These winds, coming through Pakistan, are constrained from reaching the South by The Vindhyas, which direct them instead towards the Bay of Bengal, explained Sarangi.
Along this path lies the Indo-Gangetic Plain (IGP), which sits at a lower elevation and where smoke particles flow into as the wind brings them in. The IGP stretches in a hook shape, carved out by the mountains, starting in Pakistan and curving while passing through India to Bangladesh, Bhutan, and Nepal. Its lowest points lie in northern and northeastern India and Bangladesh, with winds originating from Pakistan blowing in pollutants.
In the bowl-shaped IGP, Sarangi explained, the cold air above effectively keeps the warm air below trapped with particles, creating a saucer-like thick haze of smoke and smog nearly one km high. A lack of rainfall and precipitation prevents the clearing of particulates.
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Lahore’s ‘locational disadvantage’
The phenomenon persists across India’s neighbouring countries. Given Lahore’s high pollution levels, scientists in Pakistan have also been studying the phenomenon.
“Lahore is situated in the Indo-Gangetic Plain in a way that makes it prone to temperature inversion and pollutants’ trapping. Air quality issues are also transactional between South Asian countries,” said Sarah Asif, an environmental engineer based in Lahore.
As the Indian cities in the IGP, Lahore faces a “locational disadvantage” that exacerbates pollution from rapid urbanisation and increasing vehicular and industrial emissions. “Crop burning is the largest contributing factor, with vehicular emissions being the second largest pollution source,” Asif added.
In a January 2024 research study, scientists from Pakistan explained that the highly irrigated region of the Indo-Gangetic Plain (IGP) provides abundant cloud condensation nuclei — tiny particles in the atmosphere that attract water molecules essential for clouds to form — which also provide enough moisture to accumulate pollutants. The loose alluvial sediments in this region and others from anthropogenic activities tend to latch onto these moisture droplets, especially around the post-monsoon and winter periods, increasing the pollution load.
“Since the past few years, the recurring air pollution episodes have devastated the lives of citizens across South Asia and, due to frequent occurrences, have also been labelled ‘the fifth season’ in the region,” the study read. Scientists said the “fifth season” — observed in October and November across the IGP — is marked by low visibility conditions, driven by increased concentrations of atmospheric pollutants.
Pakistan faces comparatively fewer notorious smog days due to the westerly winds, which blow from the direction of Pakistan to Bangladesh. While these winds clear some particulates, increasing emissions worsen the winter stagnation.
How to check winter ‘stagnation’
How the subcontinent could manage the pollution levels due to ‘winter stagnation’ of polluted air is a matter of great concern to scientists.
Experts said the pollution management plans in India will have to go beyond just Delhi-NCR. Governments need to expand the network of air quality monitoring in the IGP, ensuring there is holistic data, based on which, it could tailor larger policy decisions.
“North India has a locational disadvantage, but it should not stop the government from developing long-term pollution management plans. Our approach now is very responsive. We need action throughout the year to control our emissions,” said Dipankar Saha, former chief of the Central Pollution Control Board (CPCB) air lab.
A recent study from August this year found that weather models based on nearly all scenarios of emissions predicted the worsening of winter stagnation over the next few years.
Many studies and analyses of case studies have shown the need for local and central governmental action to address the sources of pollutants that build up in winter. These include emissions from industries, dust from unpaved roads and construction material, vehicular emissions including deadlier diesel emissions, emissions from burning of coal and biofuel such as cow dung, especially in the winter, seasonal dust from the Thar Desert and the Himalayas, seasonal emissions from crop stubble burning across swathes of agricultural areas, and to a small but significant extent, a sharp spike in particulate emissions during seasonal Diwali celebrations.
Particulate emissions, especially PM 2.5 contribute significantly to adverse health effects in the region, home to nearly 40 percent of India’s population. Atmospheric pollution, studies have shown, shortened the lives of residents in the area, including Delhi, by an average of 6.3 years. However, challenges to controlling pollution sources in an all-rounded manner include rapid urbanisation, socio-economic factors, and lack of adequate enforcement of regulations.
The immediate and urgent need is to reduce emissions year-long across the board across various sources, said Sarangi.
“Meteorology, especially regional meteorology, plays a crucial role, and presently, studies indicate that it is moving towards more stagnation,” he said. “But the reduction of emissions is what is crucial. More research might be required into the policy of where and how to reduce emissions. But without reducing emissions, there is no other solution.”
(Edited by Madhurita Goswami)
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