Layers of smog and the acrid sting of pollution cloaked the Capital on Diwali eve, prompting the Commission for Air Quality Management (CAQM) to invoke stage 2 measures under the Graded Response Action Plan (Grap), as residents braced for yet another festive day marred by toxic air before the celebrations kicked off.

The 24-hour average Air Quality Index dipped to 296 (poor) at 4pm, from Saturday’s 268 at the same time, according to data from the Central Pollution Control Board. (Arvind Yadav/HT PHOTO)The 24-hour average Air Quality Index dipped to 296 (poor) at 4pm, from Saturday’s 268 at the same time, according to data from the Central Pollution Control Board. (Arvind Yadav/HT PHOTO)

The 24-hour average Air Quality Index dipped to 296 (poor) at 4pm, from Saturday’s 268 at the same time, according to data from the Central Pollution Control Board. However, the number at 4pm progressively began to surge upwards after dusk, breaching the “very poor” threshold to touch 300 by 6pm. It settled at 310at 11pm. Weather experts warned that the situation is unlikely to abate till the end of the week. This is the first time since February 2 that Delhi’s AQI turned “very poor”. It was 326 on the day.

The Air Quality Early Warning System (AQEWS) for Delhi has predicted the number will touch the dirtier side of the “very poor” zone on Diwali. With an increase in firecracker emissions on the day of, AQI levels will be pushed into the “severe” zone on Tuesday, it said. Experts said the impact of the additional emissions is set to hang in the air this week as surface winds will stay calm till Friday. Seasonal transport winds, predominantly northwesterlies, also aid in bringing stubble fire smoke from neighbouring Punjab and Haryana.

Twelve preventive measures are undertaken to mitigate air pollution under Grap stage 2: Among them, stricter enforcement on restrictions related to diesel generator sets; restricting interstate buses (other than EVs/CNG/BS-VI diesel) from entering Delhi and enhancement of parking fees to discourage use of private transport are key.

Also Read | External factors behind Capital’s ‘very poor’ AQI: DSS report

The harmful conditions are usually down to a toxic cocktail of stagnant surface winds, low temperatures, emissions from tailpipes and widespread bursting of firecrackers and the uptick in stubble fires in the farmlands of Punjab. However, data from the Centre’s Decision Support System (DSS) showed that Sunday’s deterioration was largely due to cross-boundary pollution from NCR towns factoring in 67.89% of Delhi’s total PM2.5 load. The rest, 32.11%, was due to local sources.

“Delhi’s air quality is very likely to be in the ‘very poor’ category on Monday – on Diwali day. The air quality is very likely to be in the ‘severe’ category from Tuesday till Wednesday – in case of enhanced emissions from firecrackers,” said the AQEWS in its bulletin on Sunday. CPCB classifies air quality on a scale where 0-50 is “good”, 51-100 “satisfactory”, 101-200 “moderate”, 201-300 “poor”, 301-400 “very poor”, and 401-500 “severe”.

Also Read | Four UP cities record worse AQI than Delhi ahead of Diwali

According to Sunday’s data, the highest pollution load outside Delhi was attributed to the “others” category or unaccounted sources at 26.7%. This also put the contribution of stubble fires to 2-3%. Without adequate wind movement, pollutants from outside are trapped in Delhi’s air and the cumulative effect is magnified by local emissions, experts have pointed out.

To be sure, Diwali this year has arrived earlier than usual and farm fire numbers only pick up towards the end of October, peaking around mid-November.

Punjab on Sunday clocked 67 stubble fires — the highest single-day count so far this season for the state. There were 33 recorded fires on Saturday and 20 on Friday. Haryana logged seven farm fires on Sunday — the joint-highest single day count so far this season. The total farm fires this season so far stood at 308, according to data from the Indian Agricultural Research Institute (IARI), while Haryana has logged 38 fires so far.

Widespread bursting of firecrackers, green and conventional, was seen in Delhi in the run-up to Diwali. Even though the Supreme Court guidelines only permitted the use of green firecrackers within particular windows on Diwali eve and Diwali day, multiple spot checks by HT showed that these rules were visibly flouted across the Capita.

Mukesh Khare, an air pollution expert from IIT Delhi, said an improvement this week is unlikely. “If current weather conditions persist — calm winds and reduced sunlight — the AQI will only worsen further through an increase in PM2.5 concentration in the ambient air. The primary source will be firecrackers. Though the authorities have permitted only green firecrackers, questions remain on effective implementation,” Khare said.

Yearly CPCB data has shown that PM 2.5 levels can spike between 20-30 times the permissible hourly limits across the city on Diwali day. When conditions are unfavourable, these emissions can take multiple days to disperse. Delhi’s AQI has worsened the day after Diwali in nine of the last 10 years, barring 2022, when strong winds helped AQI improve the day after.

Last year, Delhi’s AQI was 328 (very poor) on Diwali (October 31), but spiked to 339 (very poor) the day after. In 2023, it went from 218 (poor) on Diwali on November 12, to 358 (very poor) the day after. The last time Delhi’s AQI was in “severe” the day-after Diwali was in 2021, when it shot up from 382 (very poor) on Diwali on November 4 to 462 (severe) the day after.

Mahesh Palawat, vice president at Skymet meteorology, said despite stubble fires not playing a key role so far, local emissions have accumulated due to wind stagnation. “This is not allowing pollutants to disperse and we are seeing a slight haze during the day too,” he said, adding that no significant relief is in sight for the coming days.

Of the city’s 38 ambient air quality stations for which data was available on Sunday, one — Anand Vihar — was in the “severe” zone with a reading of 430 at 4pm, while 23 other stations were in the “very poor” zone. These included Wazirpur (375), Vivek Vihar (355), Dilshad Garden (352), Jahangirpuri (341), Ashok Vihar (330), RK Puram (329) and Dwarka Sector 8 (323), among others.

Anumita Roychowdhury, executive director, research and advocacy at the Centre for Science and Environment (CSE) said even though Diwali is early this year, cold and calm conditions have already begun to set in. “We have already noticed a 1.6 time increase in the daily average of PM2.5 levels since October 11 with AQI worsening to ‘poor’ category. This is when the contribution of farm fires to Delhi’s air quality is less than 1-2%,” she said.

Delhi has had some form of a ban since 2017, when the Supreme Court first asked the government to halt sales to study the impact on air quality. In 2018 and 2019, limited use of green firecrackers was allowed, but police and local authorities struggled to distinguish between them and conventional ones. In the years that followed, even with a complete ban in place, the Capital saw widespread violations, with pollution levels spiking sharply the day after Diwali.