Delhi's air-quality readings went blank on Diwali night

TL;DR in 30 sec

• Of the PM2.5 readings that 39 Delhi stations should have sent between 10pm on Diwali and 6am, 42% were blank, against 3.9% on ordinary nights, according to a Hindustan Times analysis of CPCB records.
• By 1 am on Monday, 36 of Delhi’s 59 monitoring stations were not reporting, according to an analysis by CREA cited by OrissaPOST.
• Counting only recorded hours, HT estimated Delhi’s 24-hour AQI to 9am on 9 November at 437; CPCB’s official figure at 9am was 370.
• AAP leader Saurabh Bharadwaj alleged “data fraud”, while Delhi minister Kapil Mishra asked reporters to share details of non-functional stations and said he would seek action, OrissaPOST reported.

What happened

Delhi’s air got worse through Diwali night, and many of the city’s monitors stopped saying by how much.

At many stations, PM2.5 readings climbed towards 1,000 micrograms per cubic metre and then went blank, Hindustan Times reported from its analysis of Central Pollution Control Board (CPCB) records. The same stations kept sending readings for gases such as nitrogen dioxide through the night, HT said.

HT compared 39 stations that also reported last Diwali. From 10pm to 6am they should have sent 351 hourly PM2.5 readings; 148 of them, or 42%, were blank, HT found. On ordinary nights before Diwali, the same stations left 3.9% blank.

A separate count by the Centre for Research on Energy and Clean Air (CREA), cited by OrissaPOST, looked at all 59 Delhi stations. One station had missing data at 7 pm on Sunday, seven by 10 pm and 29 by midnight. At 1 am on Monday, 36 were not reporting.

Outlook reported that stations at RK Puram, Nehru Nagar, Mundka, Sonia Vihar and Dwarka stopped showing readings after 9 pm. RK Puram had gone from 504 micrograms per cubic metre at around 8 pm to 924 by 9 pm before its data stopped, Outlook said. The JNU station kept reporting, and its reading rose from 161 at 8 pm to 786 by 11 pm.

Why it matters for India 🇮🇳

The air quality index (AQI) is CPCB’s scale for turning pollution levels into a single score, running from “good” (0-50) to “severe” (401-500), HT explained.

Reliable figures matter most in an emergency, when people need to know their exposure, Radhey Shyam Sharma, head of environmental studies at Delhi University, told OrissaPOST. Stations without recent readings are left out of Delhi’s hourly AQI, according to the city’s official air-quality portal as cited by Outlook. So when the worst-hit stations go quiet, the city’s figure can look better than the air.

HT’s numbers show the size of that gap. Counting only the hours recorded, HT estimated Delhi’s AQI for the 24 hours to 9am on 9 November at 437; with the blank hours estimated, at 471. CPCB’s official AQI at 9am was 370, HT reported.

“Losing data does not reduce the public health emergency,” Anumita Roychowdhury, executive director of the Centre for Science and Environment (CSE), said, according to OrissaPOST.

Background

HT said the pattern fits a limit at about 1,000 micrograms per cubic metre on the dust readings, which could lie in the monitors or in CPCB’s data system. No PM2.5 reading from a Delhi station went past 1,000 this year, while last Diwali 29 of the 39 stations recorded at least one hour above it, HT reported, citing a copy of CPCB’s feed kept by OpenAQ. HT said it has sent questions to CPCB.

Stations run by all three operators went blank, according to HT: CPCB, the Indian Institute of Tropical Meteorology (IITM) and the Delhi Pollution Control Committee (DPCC).

Missing readings can come from problems with equipment, power supply, calibration or data transmission, Outlook noted, adding that no station-specific explanation was available. Gaps are not new: The Indian Express reported on 7 November that 11 stations had missing readings on 6 November, according to Outlook. A similar row followed Diwali last year, OrissaPOST reported.

Official data put Delhi’s average AQI on Diwali at 376, OrissaPOST reported, calling it the most polluted Diwali in five years. HT put the city’s AQI at 366, in the “very poor” category, at 6:15 am on Monday.

Different perspectives

The Aam Aadmi Party’s Saurabh Bharadwaj alleged that the government “committed data fraud” by switching off stations on Diwali night, OrissaPOST reported. He also alleged that water was being sprinkled around the Jahangirpuri station and that 20 new stations had been placed in greener, less crowded areas to pull the average down, HT reported. These are allegations; the reports reviewed do not show evidence of deliberate tampering.

On new stations, an HT analysis found that 13 newly installed stations on the city’s periphery may be affecting the overall reading: Delhi’s AQI of 382 on 6 November would have been 393 without them.

For the Delhi government, cabinet minister Kapil Mishra asked reporters at a press conference to share details of stations reported to be non-functional and said he would seek action, OrissaPOST reported. No explanation from CPCB was found in the reports reviewed.

Experts quoted by OrissaPOST also pointed to a second source of confusion: private platforms. IQAir and AQI.in reported readings very different from official figures, and Sharma said private agencies do not necessarily average readings the same way.

Open questions 🤔

• Why did so many Delhi stations stop sending PM2.5 readings on Diwali night while still sending gas readings?
• Is there a limit of about 1,000 micrograms per cubic metre on PM2.5 readings, in the monitors or in CPCB’s data system?
• Was CPCB’s official AQI of 370 for the morning after Diwali calculated without the blank hours?

What to watch next

Hindustan Times said it has sent its questions to CPCB and will update its report with any response. Delhi minister Kapil Mishra said he would seek action once details of the non-functional stations are shared, OrissaPOST reported.