Delhi, considered among the most polluted cities in the world, sees two spikes in pollution during winter months, a new study has shown. In both spikes, lasting a week each, pollution levels shoot up at least five times above the permissible limits, forcing authorities to implement emergency measures.

Researchers from Indian Institute of Technology (IIT)-Delhi, the University of California, University of Illinois, and other research organisations found that while the first peak hits the National Capital Region (NCR) sometime around late October and early November, the second one, comparatively milder, occurs around December-end and early January.

The researchers analysed 16 years of satellite data (from 2001 to 2016) to obtain weekly concentration of PM2.5 levels in Delhi-NCR between October and May of the following year.

The research was published in Atmospheric Environment, a peer-reviewed scientific journal, in February 2019.

PM 2.5 — particulate matter that are 2.5 microns or less in width — are pollutants that can penetrate deep inside the lungs.

“The study indicates two peak pollution episodes in Delhi-NCR. The first peak occurs in the week of October 29-November 4. The second peak is slightly lower than the first peak and hits between December 30 and January 5,” said Sagnik Dey, associate professor at the Centre for Atmospheric Studies in IIT-Delhi.

Pollution levels starts peaking from October 15- 21 and peak between October 29 and November 4. This peak continues till around November 11 after which pollution levels start declining. The concentration of PM 2.5 again starts building up from around December 17-23 and hits a peak around December 30-January 5.

In 2018, Delhi witnessed one peak on November 9, a day after Diwali, when pollution hit the emergency level. Another peak was witnessed between December 23 and 25. Pollution levels hit the emergency levels at least two times after that, largely because of unfavourable wind conditions. Delhi encountered its worst fog in 17 years in the last week of October and the first week of November in 2016.

During the first episode, stubble burning plays a major role with northwesterly winds bringing in toxic fumes from the stubble burning regions of Punjab and Haryana (where farmers burn stubble from the previous crop to get ready for the new season). The second episode is triggered by multiple factors, including local emissions, unfavourable meteorological conditions, and pollutants coming in from outside Delhi.

“Satellite data shows that pollution from stubble burning shot up by at least 9% between 2009 and 2016. The Punjab government implemented the Sub-soil Water Preservation Act which reduced the time period between wheat and paddy crops [meaning farmers have less time to clear the previous crop]. This could have led to a rise in stubble burning,” said Dey.

The research also shows that places located downwind of Delhi become more polluted because the wind transports the pollutants locally emitted by vehicles and industries within Delhi. This, researchers term as ‘Megacity Outflow’.

An earlier study done by The Energy Resources Institute (TERI) and Automotive Research Association of India said that while Noida receives 40% of its pollution from Delhi, Gurugram and Faridabad receive around 15% and 17% of their pollution from Delhi.

While east and north-east Delhi remain more polluted during the October-November episode, the pollution rises across the national capital, except South Delhi, during the December- January episode.

“Pollution levels in Delhi are governed primarily by two factors — meteorology and ground-level activities. If we can manage the ground level activities, pollution can be brought down. If we can plan our actions keeping in mind these two periods, we will able to further bring down pollution levels,” said D Saha, former head of the Central Pollution Control Board’s air quality laboratory.

First Published: Mar 02, 2019 08:42 IST