Delhi woke up to Diwali on Sunday amid mounting concerns over a further deterioration in air quality, after the national capital recorded its most polluted pre-Diwali day in a decade. A combination of rising farm fires in neighbouring Punjab and Haryana, falling wind speeds and the first round of permitted firecracker bursting pushed pollution levels sharply higher on Saturday, even before the main celebrations began.
The 4pm Air Quality Index (AQI) stood at 369 on Saturday, placing Delhi in the 'very poor' category on the Central Pollution Control Board's (CPCB) scale of 0 to 500. This was the highest AQI recorded on the day before Diwali since 2016, when it touched 404. The index has steadily increased from 261 on November 1.
The deterioration was evident in the concentration of fine particulate matter, or PM2.5, which can penetrate deep into the lungs. Between 10am and 11am on Saturday, the average PM2.5 concentration across Delhi's 59 monitoring stations reached 295 micrograms per cubic metre. As many as 36 stations recorded levels above 250, the threshold at which the CPCB classifies PM2.5 pollution as severe.
Although Saturday afternoon had the week's best ventilation conditions, with the atmospheric mixing layer returning to its usual depth after remaining at roughly half that level for four days, pollution continued to worsen. Every hourly reading between 11am and 5pm was higher than the corresponding figure on Friday.
The situation deteriorated further after the permitted evening firecracker window opened at 8pm. An analysis of CPCB's 15-minute readings showed that PM2.5 levels across 55 reporting stations increased from 177 micrograms per cubic metre at 7.45pm to 208 at 8pm, before climbing at every subsequent 15-minute interval to reach 289 by 10pm.
By comparison, PM2.5 concentrations during the same period on Friday had risen only from 202 to 229.
Residential areas record sharp pollution spikes
Several residential neighbourhoods experienced particularly severe increases. In Punjabi Bagh, PM2.5 concentrations nearly tripled within an hour, rising from 208 micrograms per cubic metre at 7pm to 593 at 8pm. RK Puram recorded 621 at 9pm, more than twice its reading at the same time on Friday.
By 9pm, nine monitoring stations had registered PM2.5 levels above 400, compared with none on Friday. From 10pm onwards, even the median monitoring station recorded concentrations exceeding 250. Mundka reported an alarming reading of 718.
The Saturday evening increase occurred during the hours in which the Supreme Court has permitted the use of firecrackers. The court's order, made public on November 2, allows certified green crackers to be burst for one hour in the morning and two hours in the evening on both Saturday and Sunday.
The bench observed that a complete prohibition had neither significantly improved air quality nor prevented the illegal sale of conventional firecrackers. It also underlined that the right to religious practice was subject to the fundamental right to health.
Delhi Police have deployed more than 250 special enforcement teams, with at least one assigned to every police station, to ensure compliance with the restrictions. However, officers have acknowledged that enforcing the rules across the entire city would be difficult.
One police officer told Hindustan Times that it was nearly impossible to establish whether a firecracker being burst by an individual was green or conventional. The officer also described monitoring the entire city during the designated time slots as something that was not humanly possible.
There have already been violations. Conventional firecrackers were found available on request in markets including Chandni Chowk and Sadar Bazaar. Boxes without the QR codes mandated by the Supreme Court were also found being sold. The Supreme Court itself took note of a report highlighting these violations on November 3.
Weather conditions could make Diwali night worse
The prospect of further deterioration is particularly concerning because the main firecracker activity is expected on Sunday night, when weather conditions are forecast to remain unfavourable.
The Indian Institute of Tropical Meteorology (IITM) has projected Delhi's AQI to climb throughout Sunday night, potentially reaching 406 by 4am on Monday. This would push the capital into the 'severe' category, the worst classification on the air quality scale.
The forecast comes against an already elevated pollution baseline. Last year, Delhi's AQI stood at 296 on the eve of Diwali and rose to 351 the following day, as the post-Diwali bulletin incorporated pollution from the festival night.
The overnight pollution spike last year was particularly pronounced. At 2am, the average PM2.5 concentration across the 27 monitoring stations reporting through the night had reached 1,137 micrograms per cubic metre, compared with 535 at 10pm, the end of the permitted firecracker window. India's prescribed limit for a 24-hour average PM2.5 concentration is 60 micrograms per cubic metre.
This year's combination of weaker winds and a higher starting pollution level raises the possibility of an even worse outcome.
Sunil Dahiya, founder and lead analyst at the think-tank Envirocatalysts, warned that the next 48 hours could be particularly difficult for the capital. He said weak winds and existing forecasts pointed towards 'severe' air quality on Diwali and the day after.
Dahiya noted that Delhi's air quality had worsened on the day after Diwali compared with the preceding day every year since 2015. He said reducing firecracker use, limiting the number of trucks entering the city and preventing waste burning over the next two days were among the measures most likely to help.
Farm fires in Punjab and Haryana reach seasonal high
The worsening air quality has coincided with a sharp increase in agricultural fires across Punjab and Haryana, adding another source of pollution to the capital's already hazardous air.
Data from INSAT-3DS, an Indian Space Research Organisation (Isro) satellite that scans the region every 30 minutes, showed 415 fire pixels across the two states on Saturday. This was 5.5 times the satellite's count on November 1.
Of the 652 fire detections recorded by the satellite, 647 occurred between 4pm and 6pm. This was after NASA's satellite had completed its early-afternoon observations. NASA's widely cited fire count had also increased, reaching 1.7 times its November 1 level.
The two satellites use different methods to identify fires, meaning their absolute detection figures cannot be directly compared. However, the increase in their respective counts indicates that farm fires are intensifying.
Weather forecasts offer little immediate relief. Saturday's run of the Global Forecast System (GFS), the global weather model operated by the US National Weather Service, projected light winds of around 3 kmph through the lowest layer of the atmosphere on Sunday afternoon. It also indicated no rainfall through November 14.
With limited wind movement, smoke from agricultural fires is likely to accumulate beneath a layer of warmer air that forms over Delhi during calm nights. This atmospheric inversion traps pollutants close to the ground until sunlight in the morning breaks it up.
On Saturday morning, this warmer layer was located 578 metres above the ground, with temperatures 5.2 degrees Celsius higher than those at the surface.
Wind direction offers limited relief
One potentially favourable change in the forecast concerns the direction of winds carrying smoke from the farm fires.
Throughout the week, north-westerly winds at altitudes of one to three kilometres had transported smoke from Punjab and Haryana towards Delhi. However, Saturday's GFS forecast suggested that these winds would shift to a south-south-easterly direction on Sunday afternoon, potentially reducing the movement of agricultural smoke towards the capital.
Nevertheless, the change is unlikely to clear the pollution already trapped close to the ground.
Historical data also points to the risks associated with the current weather conditions. During Diwali in 2020, the last year when winds were similarly weak, Delhi's AQI stood at 414 on the festival day and worsened to 435 the following day.
In the 11 years since 2015, the capital's AQI has crossed 400 on the day after Diwali on four occasions.
The timing of this year's celebrations adds to the concern. Diwali falls within November 6 to 9, historically the most polluted stretch of Delhi's annual air quality calendar. Since 2015, the average AQI during these four days has been around 376, higher than the average recorded on any other dates of the year.
With farm fires continuing, firecracker activity expected to intensify and weak winds limiting the dispersal of pollutants, Delhi faces the prospect of entering the severe air quality category as the festival celebrations reach their peak.