Post-stubble burning, why Delhi sees a second spike in pollution in December
It is the second spell that is more worrying in the pollution fight as it occurs due to high background emissions in the region
New Delhi: Delhi’s winter smog has become commonplace in November, when a sharp spike in the number of paddy fields are burnt in Punjab and Haryana to remove stubble, and the festival of Diwali, which adds emissions from firecrackers to the already polluted air.
Over the past five years, Delhi has recorded its longest and most severe smog episodes in November, none more prominent than the spell between November 2 and November 11, 2016, which had nine "severe" air days, including one which saw the air quality index (AQI) touch a reading of 497 — the highest ever for the Capital. However, pollution data gathered from 2001 till 2016 by the Indian Institute of Technology (IIT), Delhi, as part of a study carried out in 2019, shows the Capital tends to see another episodic smog event in late December and early January, pushing Delhi’s PM 2.5 concentration around 350 micrograms per cubic metre — nearly six times the national standard.
Unlike the first smog episode, which IIT Delhi pegged to be most prominent between October 29 and November 4, the second smog episode — said to be most prominent between December 31 and January 6, is all down to local sources of pollution, aided by unfavourable meteorological conditions which include low temperature and dense fog.
Professor Sagnik Dey, associate professor at the Centre for Atmospheric Sciences at IIT-Delhi and one of the authors of the study, says both smog events remain consistent over the 15-year period analysed, with Delhi’s PM 2.5 levels touching nearly 500 micrograms per cubic metre during the first episodic smog event from October 29 and November 4.
However, he says it is the second spell that is more worrying in the pollution fight as it occurs due to high background emissions in the region.
“The first spell is attributed to stubble burning and generally coincides with Diwali. We start to see a drop in temperature during this period and emissions from stubble burning and firecrackers create a thick haze which only disperses if wind speeds pick up. By the second smog event towards the end of December, stubble burning is no longer a factor, nor are firecracker emissions,” explains Dey, stating only local emissions are in play in the last two weeks of December.
“We observe a spike which is not as sharp as the first smog build-up but PM 2.5 levels of around 350 micrograms are still recorded during this second spell, which is damaging even for healthy people. Over time, local emissions can get trapped due to a combination of low temperature and possibly fog,” says Dey.
As per the Graded Response Action Plan (Grap) — an emergency level plan used to counter a spike in pollution in Delhi NCR — measures under its most stringent — "severe-plus" or "emergency" category can be imposed if PM 2.5 levels remain over 300 micrograms per cubic metre for over 48 hours.
While low temperatures slow down the air and make it heavier, they also bring down mixing height — an invisible layer of the atmosphere within which dispersion takes place. The lower the mixing height, the lesser the room for pollutants to disperse and move freely.
This year, Delhi has already seen this episodic smog build-up occur nearly a week earlier. The Capital recorded six consecutive "severe" air days between December 21 and 26, with the AQI peaking at 459 on December 26, before rains came to Delhi’s rescue. This was December’s worst spell of air quality, with previously, four consecutive "severe" air days recorded in December 2018. Light showers on Sunday night however meant Delhi’s air quality was back in the ‘poor’ category, with a reading of 283 (poor) recorded on Monday as per the Central Pollution Control Board’s index.
Gufran Beig, Founder project director says dense fog also plays a key role in the second spell seen in late December, with an increase in moisture trapping pollutants closer to the surface.
“The more the moisture, the greater the chance pollutants will not disperse but stay floating in the air. If the moisture content is high and cloudy conditions prevail over the day, Delhi tends to see a haze-like situation, which only traps more pollution and leads to a smog episode,” said Beig, stating in such a scenario, local emissions find no outlet to escape.
Delhi is not the only contributor to this second spell, with pollution from NCR floating towards Delhi just as much. As per a 2018 source apportionment study carried out by The Energy and Resources Institute (TERI) found Delhi only contributes to 36% of its pollution levels in winter, with the remaining 64% coming from NCR and beyond.
In this, 34% of the pollution load comes from NCR. Similarly, Delhi was found to be contributing to 40% of Noida’s total winter pollution load.
“The issue largely comes down to an air-shed, which includes NCR and beyond. To tackle pollution, we need to control sources of pollution not just in Delhi but all of NCR. This spike in pollution seen in late December and early January only occurs because the background emissions in NCR are very high and even a slight drop in temperature and wind speed leads to smog build-up,” says Anumita Roychowdhury, executive director, research and advocacy at the Centre for Science and Environment (CSE).