Experts Explain Why Delhi's Air Quality Monitors Went Blank On Diwali Night

Delhi's air grew more polluted on Diwali night. Then, in several neighbourhoods, the readings meant to show how bad it had become, went blank.

At RK Puram, PM2.5 concentration rose to 924 micrograms per cubic metre before the station stopped displaying readings after 9 pm. Similar gaps were reported at Nehru Nagar, Mundka, Sonia Vihar and Dwarka.

Were the machines unable to measure such high pollution, or was the data failing to reach the public? NDTV spoke to three experts to understand what may have gone wrong.

What Happened On Diwali Night?

An analysis by the Centre for Research on Energy and Clean Air (CREA) found that the number of stations not reporting readings rose from seven at 10 pm on November 8 to 23 at 11 pm, 29 at midnight and 36 at 1 am on November 9.

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Across 59 stations, CREA counted 227 missing hourly observations between midnight and 5 am.

CREA analyst Manoj Kumar said Diwali's average PM2.5 concentration was 257 micrograms per cubic metre, making it the fourth most polluted Diwali since 2016.

However, Kumar cautioned that the timing of the missing data alone did not establish the cause.

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How Do Air Quality Monitoring Stations Work?

A monitoring station has different instruments, called analysers, that measure PM2.5, PM10 and gases such as nitrogen dioxide and carbon monoxide.

PM2.5 refers to fine particles that can enter deep into the lungs. Its concentration is measured in micrograms per cubic metre.

Readings are stored by a data logger and transmitted to servers and public dashboards. So a missing PM2.5 figure does not mean the entire station stopped working.

An analysis of 39 stations found that at 1 am, only 13 reported PM2.5, while 38 continued reporting nitrogen dioxide. At least some stations were still sending data.

Can Pollution Become Too High For A Monitor?

Anil Gupta, a member of the Delhi Pollution Control Committee (DPCC) and the Central Pollution Control Board (CPCB), attributed the gaps to unusually high PM2.5 readings.

"When there is a sudden spike in PM2.5 and it crosses 999, the monitoring station stops giving a reading," Gupta told NDTV.

"There is no deliberate move. There is no malfunction. It is a technical glitch," Gupta added.

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CPCB's 2019 technical specifications prescribe a measurement range of up to 1,000 micrograms per cubic metre for certain particulate matter analysers. Such instruments can reach their measurement ceiling during severe pollution spikes.

But this does not prove what caused the gaps at individual stations.

Former CPCB Additional Director Dr Dipankar Saha had a different take.

"In my opinion, a glitch has nothing to do with high or low values," he said.

Saha explained that technical problems could occur at either the analyser or display end. He also called for shortcomings in the monitoring system to be investigated regularly to maintain public confidence.

Could The Missing Data Affect Delhi's AQI?

Potentially. At Siri Fort, Kumar said, PM2.5 reached 1,000 micrograms per cubic metre, the maximum displayed on CPCB's public portal.

"There is a possibility that the official AQI does not fully capture the severity of the pollution episode," he said.

But PM2.5 concentration and AQI are different.

PM2.5 concentration measures fine particles in the air. India's Air Quality Index (AQI) converts pollutant levels into a score capped at 500.

AQI is calculated using readings averaged over specified periods, not simply the worst reading in an hour. Gupta said Delhi's overall AQI could still be calculated from stations that continued reporting.

However, if some of the highest pollution readings were missing, the available data might not show the full severity of the episode.

Can The Missing Readings Be Recovered?

Asked whether the figures were recorded internally, Gupta said, "There is no internal mechanism."

However, CPCB's published technical specifications provide for station-level data storage, as well as diagnostic records and error logs.

That does not necessarily mean valid measurements were recorded after an instrument exceeded its range. But examining those records could help establish whether the problem occurred during measurement, transmission or display.

The pollution did not disappear when the readings did. Until authorities explain the gaps and establish what happened to the missing data, Delhi cannot know whether its worst hours of pollution were fully captured.