Delhi AQI Data Under Scrutiny as Monitoring Stations Go Offline on Diwali Night

New Delhi: Delhi recorded its most polluted Diwali in five years but the extent of its severity was likely worse than official figures showed as several monitoring stations went offline on Sunday night during the peak period of firecracker use in the Capital, raising questions over the reliability of the AQI tracking exercise itself.

AQI (air quality index) readings put out by private agencies did, in fact, paint a much more unpleasant picture than what the official figures reflected, with most parts of Delhi-NCR reporting extremely hazardous air pollution levels, leading experts to cast doubts over government data.

Experts tracking the numbers in real time said key monitoring stations stopped reporting data during the night - when pollution levels shot through the roof as the Supreme Court-mandated time restrictions on the use of fireworks went up in smoke - while flagging significant discrepancies between the readings reported by government and private monitoring agencies.

Ruing the absence of credible official data during the worst period of pollution, Anumita Roychowdhury, executive director of the Centre for Science and Environment (CSE), said the monitoring network's failure to provide real-time readings was inexplicable despite substantial public investment in its establishment and maintenance.

"When substantial public funds have been invested in establishing Delhi-NCR's air quality monitoring network - the densest in the country - backed by active operation and maintenance contracts, a data eclipse precisely during peak Diwali pollution is inexplicable," she said.

She said real-time data should have driven targeted public health advisories and calibrated emergency responses.

"While the severity of exposure is evident on the ground, credible official data is needed for institutional accountability, public awareness and mitigation. Losing data does not reduce the public health emergency," Roychowdhury said.

Responding to a query about the issue at a press conference, Delhi cabinet minister Kapil Mishra asked reporters to share details of AQI monitoring stations reported to be non-functional, saying he would seek action on the matter.

From midnight to 2 am, more than 20 stations were not reporting data during peak smog hours, according to an analysis by the Centre for Research on Energy and Clean Air (CREA). Data from Delhi's 59 air quality monitoring stations showed that the number of stations with missing readings increased sharply as Diwali night progressed, peaking in the early hours of Monday.

While only one station had missing data at 7 pm on Sunday, the number rose to seven by 10 pm and 23 by 11 pm. By midnight, 29 stations were not reporting data, with the number rising further to 36 at 1 am on Monday. This meant that only 23 of the city's 59 monitoring stations were reporting readings at that time.

The gap remained high at 2 am, when 35 stations had missing data, before gradually declining. At 3 am, 30 stations were not reporting, while the number dropped to 20 by 4 am, 19 by 5 am and 16 by 6 am.

The situation gradually improved through the morning, with 13 stations having missing data at 7 am and seven at 8 am. By 9 am, only two stations were not reporting data, while one station continued to have missing readings until 11 am.

The average AQI in Delhi dropped to 344 by 4 pm on Monday, compared with 376 reported at the corresponding time on Diwali day. Notably, only 47 monitoring stations were recording data at 4 pm, of which 40 were in the very poor category and seven in the poor category. Bawana recorded the worst AQI at 396, followed by Burari at 393.

The Opposition hit out at the authorities, with senior Aam Aadmi Party (AAP) leader Saurabh Bharadwaj accusing the Delhi government of "data fraud" and alleging that it was manipulating air quality readings to understate pollution levels, putting people's lives at risk.

The concern came amid wide variations in air quality readings reported by different agencies on Monday, a day after Diwali.

While official data showed Delhi's AQI at 370 at 8 am on Monday, private monitoring platforms reported considerably different figures.

Swiss air quality monitoring platform IQAir recorded an AQI of 245 on the US scale in the 'very unhealthy' category at 8.30 am, with PM2.5 concentration at 170 micrograms per cubic metre (µg/m³), around 34 times the World Health Organisation's annual guideline value.

Meanwhile, AQI.in recorded an air quality reading of 637 in Delhi on Monday morning, classifying the air quality as "hazardous". The platform also reported that the city's AQI had peaked at 2,283 after Diwali.

The differences in readings brought the monitoring network, data availability and variations in measurement parameters and averaging methods used by government and private agencies into focus - not for the first time, though.

A similar controversy had erupted after Diwali last year, when the Aam Aadmi Party accused the BJP-led Delhi government of deliberately shutting down air quality monitoring stations to conceal rising pollution levels.

The AAP had alleged that dozens of monitoring stations operated by the Central Pollution Control Board (CPCB), Delhi Pollution Control Committee (DPCC), India Meteorological Department (IMD) and Indian Institute of Tropical Meteorology (IITM) had gone offline as pollution levels rose on Diwali night and resumed functioning the following morning. Experts tracking the data in real time corroborated the same.

Commenting on Sunday's data hole, Radhey Shyam Sharma, head of the Department of Environmental Studies at Delhi University, said transparency in air quality figures was essential, particularly during an emergency when people needed reliable information about their exposure to pollution.

"Transparency in data should be there. There is also the question of accountability. We are talking about an emergency situation, a severe emergency," he said.

Sharma said the missing readings could create confusion and panic, emphasising that the absence of data was not a solution to the Capital's annual air crisis.

Reliable information was important for policy formulation and assessing the severity of the situation, he added.

He also pointed to the differences in the way government and private agencies report air quality, including variations in averaging methods.

"Private agencies do not necessarily average the readings in the same manner. The official data should clearly specify the location and time of the readings. The exposure levels need to be clear so that there is a better picture of the situation," he said, adding that average AQI is bound to differ from peak pollution levels when exposure to the associated health risks is at its highest.

Sharma said the monitoring system and its management needed to be handled better instead of allowing data to go missing, adding that the focus should remain on people exposed to pollution and the accountability of those managing the network.

Going by official data, the national capital recorded its most polluted Diwali in five years, with an average AQI reading of 376. The morning after was only marginally better at 371, recorded at 10 am on Monday when a blanket of smog and fog blanketed the city.

The morning-after figure of 371 was also the worst for a post-Diwali day since 2021.

The absence of data followed a similar controversy earlier pertaining to the use of large water sprinklers near monitoring stations, purportedly to lower the pollution readings.

"They are constantly sprinkling water near the Jahangirpuri AQI station so that the average comes down. Similarly, the government switched off AQI monitoring stations on Diwali night and committed data fraud. How do you trust such a government?" Bharadwaj said.

Comparing AQI monitoring stations to blood sugar monitoring devices, he said, "If anyone has diabetes or a blood pressure problem, they check that every day because their medication depends on the levels.

"People check their diabetes levels regularly because what they eat and when they take medicines depends on it. If their machine is tampered with and shows normal levels, they will not take their medicines; they will eat sweets and die. This is what the government is doing right now."

He said the government had installed 20 new AQI monitoring stations in open and green areas with less traffic and lower population density to bring down the overall average and paper over the extent of the crisis.