As pollution soared on Diwali night, Delhi’s air monitors fail at key moments
As Delhi’s skyline lit up unabated on Diwali night, the levels of pollutants skyrocketed far beyond permissible limits across the city. But at the very moment when tracking dangerous pollution peaks was most critical, the majority of Delhi’s 39 continuous ambient air quality monitoring stations (CAAQMS) went offline for hours, creating significant data gaps.
An analysis of Central Pollution Control Board (CPCB) and Delhi Pollution Control Committee (DPCC) data by HT revealed that only nine stations – barely 23% of the city’s monitoring network – recorded continuous data.
The remaining stations experienced blackouts ranging from one to nine hours during the crucial 36-hour period between midnight on Monday and 11am Tuesday.
Dwarka Sector 8 recorded the poorest performance, with data available for only 27 of the 36 critical hours. Monitoring stations at Jawaharlal Nehru Stadium, Nehru Nagar, Patparganj, and RK Puram each lost approximately eight hours of readings.
Four other stations missed only one hour, but 10 crucial monitoring points went dark for six hours or more during the period when firecracker emissions typically peak and accumulate.
Data gaps and pollution spikes
Experts stressed that the missing data is not a new phenomenon. Every year, several stations stop reporting when pollution levels spike during Diwali, suggesting the system’s inability to handle pollution loads beyond a certain threshold.
“The observed pattern shows that most stations stopped providing data when they approached 1,000 µg/m³, while there were stations such as Anand Vihar, Nehru Nagar, Mundka, that recorded values above 1,000 µg/m³, indicating that the technology of monitoring isn’t an issue, at least in terms of being available. We have observed this over the past years too,” said Sunil Dahiya, founder and lead analyst of think tank EnviroCatalysts.
Dahiya said the problem cuts across agencies. He referred to previous years, which had similar data gaps observed not only for stations under the Delhi Pollution Control Committee (DPCC), but also those under the Indian Institute of Tropical Meteorology (IITM), the Central Pollution Control Board (CPCB) and the India Meteorological Department (IMD).
“This isn’t data being withheld. It is a technological limitation in the sense some stations are not able to record complete and accurate data beyond a certain limit,” Dahiya said, calling for stations to be suitably upgraded.
Where data was available, the picture was alarming. DPCC’s Mandir Marg station recorded PM2.5 concentrations of 1,066 µg/m³ at midnight before going offline from 1am to 4am. At Nehru Nagar – Delhi’s most polluted station on Diwali night – PM2.5 touched 1,763 µg/m³ at 10pm, with no data for the next hour and a four-hour blackout between 1am and 5am.
CPCB’s ITO station registered PM2.5 levels of 923 µg/m³ at 1am and went dark. It worked again at 7am when it logged 967 µg/m³. At Dwarka’s NSIT station, data vanished from midnight to 5am, while readings at IMD’s Ayanagar were unavailable for up to five hours.
An expert, well acquainted with the CAAQMS across NCR, said for hourly readings – stations spend close to 40 minutes to collect particulate matter, with the remaining time spent to calibrate and analyse it. “Stations are designed to record extremely high values – even up to 10,000µg/m³. With the rise in concentration, accuracy may reduce but it does not mean values are not released. It simply means the accuracy is reduced marginally – by 1% or 2%. In this case, it is plausible a certain threshold has been set beyond which values will not be captured,”
The cost of missing data
These gaps, experts warned, undermine public health response and scientific understanding of pollution’s real impact. “Such missing data hampers insights, makes it impossible to assess exposure accurately, and gives a false sense that air quality is slightly better than it actually is,” said Dahiya.
Activist Bhavreen Kandhari called it a “worrying breach of transparency.” “Reliable, continuous monitoring is the foundation of any clean-air strategy. Transparency in air quality data should be treated as a public health imperative, not a discretionary choice,” she said.
Anumita Roychowdhury, executive director for research and advocacy at the Centre for Science and Environment (CSE), said such lapses were particularly concerning during “extreme pollution episodes” like Diwali. “High pollution peaks are expected during such extreme pollution episodes. It is thus highly important to keep the data open and transparent even if extreme peaks are recorded, to understand the magnitude of the impacts and to inform policy and people for adequate and effective health safeguards,” she said.
When asked about the missing data, Delhi’s environment minister Manjinder Singh Sirsa dismissed the findings. “All data is available on CPCB and on DPCC websites. There is no missing data. You have to check. Those who are saying there is missing data, you know what their intentions are,” he said at a press conference on Tuesday.
Officials at CPCB and DPCC did not respond to HT’s queries, nor did the Commission for Air Quality Management (CAQM).
Scientists note that this pattern of blackout during peak pollution is not unique to this Diwali. Every year, as particulate matter levels surge, monitors in Delhi and NCR experience data losses. Similar outages were reported in 2022 and 2023, particularly during smog episodes following Diwali and in early winter.
Delhi’s 39 CAAQMS stations — run by multiple agencies — are designed to collect real-time data on particulate matter, gases such as NO2 and SO2, and meteorological factors like humidity and wind speed. This network forms the backbone of CPCB’s daily Air Quality Index (AQI) bulletins and helps determine emergency responses such as bans on construction or traffic restrictions under the Graded Response Action Plan (GRAP).
But without reliable data, scientists said, both short-term interventions and long-term planning become blindfolded. “Such missing data hampers data-driven insights, understanding the contribution of certain activities, their impact on health, as well as skews the daily pollution level recordings, giving a false sense of lower air pollution levels,” Dahiya said.
