A cyclist rides through the heavy haze at Rajpath, New Delhi | PTI/Ravi Choudhary
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The Himalayas surround the Indo-Gangetic plains to form a land-locked Valley that leaves no space for Delhi’s emissions to escape.

New Delhi: What’s the first place that comes to mind when you hear the word ‘smog’? Chances are it’s New Delhi. The national capital is also the poster-city of urban air pollution in India, along with neighbouring cities like Kanpur, Faridabad, Varanasi and Gaya. Every year, the onset of winter is automatically associated with poor air, respiratory and visibility problems in these places.

But why does air quality drop during winter? And why is the northern part of the country more affected than the others? Here are the answers to all your questions about the link between winter and air pollution in India.

Why does air quality drop in the winter months?

Polluted air is a fact of life in Indian cities throughout the year. For example, even during the warmer months of March, April and May this year, Delhi did not enjoy a single day of ‘good’ quality air.

However, the pollution, especially over northern India, gets magnified during the post-monsoon months due to a combination of atmospheric and human factors. These include winter inversion, and the ‘valley’ effect (explained below), apart from the usual culprits like industrial and vehicular emissions. Seasonal factors like dust storms, crop fires, burning of solid fuels for heating, and firecracker-related pollution during Diwali, also aggravate the winter pollution crisis.

But before we get into that…is winter pollution unique to India?

No; heightened pollution in winter is a global phenomenon that first made headlines during the Great London Smog of 1952. The Smog, which killed around 12000 Londoners, came as a shock to Britain and the world. Since it was primarily caused by excessive coal burning after a particularly frosty spell, its aftermath forced Britain to enforce stricter air quality norms.

Closer home, China has been battling winter pollution for decades, particularly in its northern cities. However, in recent years, it has enforced emission cuts for industries and households during the winter, and experts say these steps are beginning to show results. Like India, one of China’s biggest challenges is controlling levels of PM2.5 (fine particulate matter with diameter up to 2.5 microns), which in large quantities can be a killer. China and India together contribute to more than half of the 41 lakh worldwide deaths attributable to PM2.5 in 2016.

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So is PM2.5 the worst winter pollutant? Where does it come from?

While the toxic soup over north India is composed of several types of pollutants such as sulphur dioxide (SO2), nitrogen dioxide (NO2), PM2.5, PM10 and Ozone, the most harmful ones are PM2.5 and NO2, says Vivek Chattopadhyay, programme manager, Centre for Science and Environment. The health impact of these pollutants can range from asthma to cancer, strokes and Alzheimer’s disease.

As far as sources are concerned, NO2 is mainly emitted by vehicles and power plants, while common sources of PM2.5 include industries, vehicles, power plants, crop and garbage burning, and diesel generators. These pollutants are often unable to disperse due to a phenomenon called winter inversion and remain closer to the ground, choking our cities.

Hold on. What is winter inversion?

In summer, air in the planetary boundary layer (the lowest part of the atmosphere) is warmer and lighter, and rises upwards more easily. This carries pollutants away from the ground and mixes them with cleaner air in the upper layers of the atmosphere in a process called ‘vertical mixing’.

During winters the planetary boundary layer is thinner as the cooler air near the earth’s surface is dense. The cooler air is trapped under the warm air above that forms a kind of atmospheric ‘lid’. This phenomenon is called winter inversion. Since the vertical mixing of air happens only within this layer, the pollutants released lack enough space to disperse in the atmosphere.

During summers, pollution levels decrease as the warmer air rises up freely, making the boundary layer thicker, and providing enough space for pollutants to disperse. The same thing happens during winter afternoons, when increased heat brings down pollution slightly.

The effects of inversion are stronger at night, which is why air quality levels drop overnight. This is also why experts ask people to refrain from early morning walks, as they could be exposed to much higher pollution levels at that time.

But if winter inversion happens everywhere, why do places like Delhi and Uttar Pradesh have the worst air in the country?

In cities closer to the coast, like Mumbai, the sea breeze and moisture help disperse pollution. However, the Indo-Gangetic plain, which includes Punjab, Delhi, UP, Bihar and West Bengal, is like a valley surrounded by the Himalayas and other mountain ranges. Polluted air settles in this land-locked valley and is unable to escape due to low wind speeds.

In major cities of this region, such as Delhi and Kanpur, high industrial and vehicular emissions coupled with biomass burning in surrounding areas cause more pollution that gets trapped due to this valley effect and inversion.

Summing it up, Dr. Vinoj V., Assistant Professor, School of Earth, Ocean and Climate Sciences, IIT Bhubaneshwar says, “Local pollution, topography and meteorology combine to worsen the pollution situation in Delhi and other northern cities.”

Is winter pollution getting better or worse?

Increased emissions and industrial activity are driving up pollution in most countries. And in the Indo-Gangetic Plains, the hub of winter air pollution, things seem to be getting worse.

“On average, 1-2 days of poor air quality are being added every year over the Indo-Gangetic Plains. This means we are losing an equal number of good/clean/satisfactory air days. This is a serious issue that needs to be addressed,” says Dr. Vinoj.

He also stresses the need to address the particulate matter problem. “Biomass burning that starts post-monsoon has been increasing at an alarming rate of almost 25% each year. Also, we have observed that the winter air is gradually getting more stagnant over the Indo-Gangetic plain in recent years, slowing the dispersal of pollutants. All these factors have made the northern cities ideal for high pollution loading,” Dr. Vinoj adds.

Great. So what could be the solution?

Last year, Delhi launched a ‘Graded Response Action Plan’ under directions from the Supreme Court. Implementation of the GRAP and releasing a list of approved fuels for vehicle and industrial use, are steps in the right direction, says CSE’s Chattopadhyay.

However he stresses that more needs to be done, “City and regional-level pollution control plans need to be drawn up, and there needs to be better coordination between states,” he says, adding, “While administrations like Delhi are preparing to deal with pollution, other states need to get more serious and not treat high levels of pollution as a normal part of winter.”

This article was originally published on Weather.com.

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