Diwali pollution data 'gap': 30 of 39 AQI monitors went offline for hours when Delhi needed them most

As Diwali lights illuminated Delhi’s skyline on Monday night, pollution levels surged across the city, reaching dangerous peaks. Yet, at the very moment when monitoring this spike was most critical, most of Delhi’s continuous ambient air quality monitoring stations (CAAQMS) went offline for hours, leaving major gaps in data.

According to a report by Hindustan Times, only nine out of 39 stations, just 23% of the city’s network, provided continuous readings during the crucial 36-hour window from Monday midnight to Tuesday 11 am.

The remaining stations experienced blackouts ranging from one to nine hours. Dwarka Sector 8 recorded data for only 27 of the 36 hours.

Stations at Jawaharlal Nehru Stadium, Nehru Nagar, Patparganj, and RK Puram each lost about eight hours of readings. Four other stations missed just an hour, but 10 key monitoring points went dark for six hours or more, the period when firecracker emissions typically peak.

Why data gaps occur during high pollution

According to several experts, this problem is not new. Every year, several monitoring stations stop reporting during Diwali as pollution spikes, indicating that the system struggles to handle extremely high pollution levels.

“The 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,” HT quoted Sunil Dahiya, founder and lead analyst of think tank EnviroCatalysts, as saying.

He noted that the problem affects stations run by different agencies, including DPCC, IITM, CPCB, and IMD. “It’s not data being withheld; it’s a technological limitation. Some stations can’t record complete readings beyond a certain threshold,” he said.

How monitoring stations work

A CAAQMS station typically spends about 40 minutes collecting particulate matter, using the remaining time for calibration and analysis.

“Stations can record values up to 10,000 µg/m³. Accuracy may slightly drop, by 1-2%, when concentrations are extremely high, but data should still be released. It appears a threshold is set beyond which readings are not captured,” an expert familiar with NCR stations was quoted by HT as saying.

Impact on public health and policy

Missing data has real consequences. Scientists warn it undermines public health responses and skews understanding of air pollution’s effects. “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.

Delhi’s 39 CAAQMS stations track particulate matter, gases like NO2 and SO2, and weather conditions. This information feeds into the Air Quality Index (AQI) and informs emergency measures, such as traffic restrictions or construction bans, under the Graded Response Action Plan (GRAP).

According to the scientists, both short-term interventions and long-term planning are compromised without reliable data. “We cannot make data-driven decisions, assess which activities contribute most to pollution, or design effective policies if readings are missing during peak pollution,” Dahiya added.

When questioned, Delhi’s Environment Minister Manjinder Singh Sirsa dismissed concerns. “All data is available on CPCB and DPCC websites. There is no missing data. Those who claim it is missing have their own intentions,” he said.

Meanwhile, scientists note that monitoring gaps during Diwali and early winter are a recurring problem, reported in 2022 and 2023 as well.