Delhi recorded a second consecutive “severe” air day on Tuesday due to a combination of calm winds, low temperatures, and fog, prompting experts to call for a long-term solution and cutting down pollution at the source instead of resorting to emergency level plans.
The Air Quality Index (AQI) deteriorated to 450 at 9am on Tuesday before improving slightly to 427 (severe) by 11am. On Monday, AQI sharply deteriorated to 434 from Sunday’s 371 (very poor). It was the poorest air quality since November 4 (447 in the severe category).
The air was relatively clean on January 1 with an AQI of 259 (poor). The air quality has gradually deteriorated since, touching the “severe” category for the first time this year on Monday.
The Commission for Air Quality Management (CAQM) in NCR was expected to hold a review meeting in view of the deteriorating air quality. On Monday, the Commission enforced a ban on BS-III petrol and BS-IV diesel vehicles as part of measures under the Graded Response Action Plan (Grap) to check the pollution.
Centre for Science and Environment executive director (research and advocacy) Anumita Roychowdhury underlined spikes in pollution will continue annually based on the meteorological conditions until there is a significant reduction in its sources. “We have seen high AQI each time winds become calm or the temperature dips, as local sources and even external sources are fairly high and can accumulate in the region. This will invariably lead to severe air…”
Roychowdhury underlined Grap is an emergency level plan and not a long-term solution, which will be in cutting down pollution at the source. “The move to ban coal and other dirty fuels across NCR from January 1 is a step in the right direction to tackle one such source,” she said.
Experts said vehicles remain the most prominent source of local pollution. Data from the Union earth sciences ministry’s Decision Support System (DSS) showed stubble burning is no longer a source of pollution as it was on November 3. Pollution from outside Delhi was contributing to Delhi’s PM 2.5 more than local sources.
Sources outside Delhi were estimated to have contributed 70% to the pollution in Delhi on Tuesday. The transport sector contributed around 40% of the total pollution load in Delhi and industries around 33.33%.
The two sources were projected to contribute between 75 and 80% of the pollution in Delhi for the next three days. Open waste burning was only contributing around 3-5% of the total pollution.
Northwesterly winds were bringing pollutants from other cities. Pollutants from Jhajjar were estimated to have contributed around 18% to Delhi’s PM 2.5 concentration on Tuesday, followed by Rohtak (5.3%) and Sonipat (3.6%), DSS data showed.
Experts said sources of pollution have largely remained similar, barring stubble burning, which has reduced even as open burning and the contribution of pollution from industries has risen compared to November.
Open burning is also highest during this time of the year when temperatures are low, the official added.
The meteorological conditions were most unfavourable on Monday, which led to the accumulation of pollutants closer to the surface. There was barely any sunlight on Monday due to fog and low clouds. This kept the mixing height, an invisible layer in the atmosphere within which pollutants get trapped, to less than 100 metres.
The greater the mixing height, the more room there is for pollutants to disperse or escape. During sunlight, parts of the earth heat up unevenly, causing winds, which further help disperse pollutants. But on Monday, there were calm winds in Delhi.
Recent studies have shown Delhi seems to record two spells of pollution each winter. The first spell is led by stubble burning and the second is largely due to open burning, low temperatures, and fog.
A 2019 Indian Institute of Technology (IIT) Delhi study based on an analysis of 15-year annual data found two prominent spikes in air pollution each year. The first spike was noticed from October 29-November 4 and the second was lower than the first from December 31-January 6.
During the first spell, the peak PM 2.5 concentration breached 500 micrograms per cubic metre. In the second spell, the peak concentration of PM 2.5 was around 350 micrograms per cubic metre.
Sagnik Dey, the lead researcher of the IIT Delhi study, said the data assessed was for a substantial period of time, indicating the pollution spike largely came around the same period each year. “The first spell is more to do with Diwali and stubble burning. We found that while meteorological conditions are less adverse, these emissions play a key role in driving up pollution levels suddenly.” Dey said in the second spell, it is all down to local emissions and low temperatures.