As winter sets in, North India’s air is yet again severely contaminated by a combination of factors: crop stubble burning, garbage incineration, industrial pollutants, vehicular exhaust and dust from construction sites.

Over the past four years, air pollution levels in the region have started rising sharply towards the end of October through early November and got worse with the firecrackers burst at Diwali. This year, the day after the festival was celebrated on November 7, the air quality index in some parts of Delhi touched a high of 500 (the safe limit is below 100), as a thick smog descended on the city. This level, categorised by the Central Pollution Control Board as “severe”, poses the risk of respiratory illness.

The graphic below highlights the highest air quality index readings in Delhi for every day since February 2015. It depicts levels of PM 2.5, or hazardous particles in the air with a diameter of less than 2.5 micrometres. Even on better air-quality days, Delhi records readings higher than the level considered safe. Delhi and 13 other cities North India are among the 20 most polluted cities in the world ranked by the World Health Organisation.

Though the biggest spikes in air pollution occur around Diwali, North India’s air is polluted all year round. Most solutions are seasonal and are direct responses to public outrage.

Over the past few years, air pollution has forced schools in Delhi to shut down on especially bad days. In addition, old diesel-run vehicles have been banned from the roads, construction work halted and coal-fired plants closed. Last week, the Environment Pollution Control Authority suggested that private vehicles should be kept off the roads in order to control the pollution.

Many people blame farmers in Punjab and Haryana for the increase in air pollution. After the harvest, these farmers begin burning their crop stubble from late September to early November. This is the cheapest method for them to prepare their fields for the rabi or winter crop. Satellite images show that this stubble burning generates pollution across Punjab, which eventually permeates through the Indo-Gangetic plain.

Stubble burning on November 5 in Punjab's Bhatinda district.
Stubble burning on November 5 in Punjab's Bhatinda district.

In 2015, the National Green Tribunal directed Delhi and its neighbouring states to ban stubble burning. Earlier this year, the green tribunal ordered these states to formulate a comprehensive policy to stop farmers from clearing their fields this way. But Punjab Chief Minister Amarinder Singh has said he cannot stop stubble burning unless farmers are adequately compensated to use other, more expensive methods of stubble disposal.

There has been a huge spike in the burning of crop residue this season, according to satellite imagery. Over 10,000 cases have been recorded as of November 2. Though the focus is on Delhi, the smoke eventually reaches even Bihar and Jharkhand.

NASA EOSDIS Worldview.
NASA EOSDIS Worldview.

Winter and the wind

In his book the Great Smog of India, writer Siddharth Singh writes that the most fundamental parameter to impact air quality is the speed and direction of wind.

“Cities in northern India, unfortunately, have no such luck. While there are several contributory sources of air pollution in the region, pollution levels remain at critical levels to a considerable extent due to geographic and meteorological conditions. And that is Delhi’s – and northern India’s – geographical and meteorological misfortune.

In Delhi, the average wind speed in winter ranges between one and three metres per second, which is nearly one-third of the average speeds in the summer months and much lower than the average wind speed in Chennai.”

Singh also notes that Chennai, unlike Delhi, is situated near the sea and has a continuous stream of wind. “The lack of winds that can carry away pollutants is one of the most important factors impacting air quality,” he said.

The lack of wind coupled with weather conditions in winter and the geographical location of the national capital mean that pollutants remain trapped in the plains. Satellite imagery from November 5-November 8, 2017, shows how the topography of the region leads to pollution spreading through the plains.

NASA EOSDIS Worldview.
NASA EOSDIS Worldview.

As crop burning begins in September through to November, slow winds from the north west begin to slowly push the air towards the plains of Haryana, New Delhi and Uttar Pradesh.

NASA EOSDIS Worldview.
NASA EOSDIS Worldview.

With the Himalayas forming a huge barrier to the north, the smog follows a path through the plains hugging the bottom of the mountain range. The air begins to accumulate in the region and is unable to escape.

NASA EOSDIS Worldview.
NASA EOSDIS Worldview.

Because of the high mountain range and cooler winter air, the smog is unable to rise (hot air rises, cool air settles), keeping it trapped within the plains. This is known as temperature inversion. Since the winds are relatively slower during the winter, they slowly funnel eastward towards Bangladesh.

At the same time, North India and Delhi cannot blame the poor geography as the primary reason for the bad air quality. Man-made factors like power plants contribute too. According to an IIT Kanpur study, 13 power plants lie within a 300 km radius of Delhi, close enough to be a major contributor to the air pollution in the region.

Then there is Diwali. The festival always coincides with the onset of winter and the crop stubble burning season. Every year unfailingly, education campaigns, health warnings and even Supreme Court restrictions on the bursting of crackers literally go up in smoke on Diwali day.

It does not help that North India’s pollution problem has become worse with each passing year. The aerosol optical depth – the number of particles such as dust and smoke in the atmosphere that block sunlight – has increased over the last decade. NASA’s Earth Observations has been measuring this globally since 2000 and the spike in the optical depth in North India is nearly at 1. The average for most countries is 0.1 to 0.15.

The government has stepped in to erect smog towers and has instituted measures like sprinkling water in highly-polluted areas in the hope that the pollution will settle, but these are not long-term solutions.

Though early indicators suggest that the air quality is better this year for Delhi and North India than in previous years, a lot depends on which way the wind blows and whether the people who live in these areas are motivated enough to ensure that an already bad situation does not become worse.

Source: NASA Earth Observations
Source: NASA Earth Observations