CPCB explains why Delhi's air readings went blank on Diwali night

TL;DR in 30 sec

The Central Pollution Control Board (CPCB) said its dust monitors were calibrated for 0 to 1,000 micrograms per cubic metre, and hourly readings above that are left out of the AQI, The Hindu reported.
CPCB said this is why stations showed data gaps on Diwali night, and that those hours do not count towards the 24-hour average, according to The Hindu and The News Mill.
All 125 station-hours missing from CPCB’s records read above 1,000 in delayed data released by the Delhi Pollution Control Committee (DPCC), WION reported.
CPCB said internal discussions on handling readings beyond the system’s range are under way, The Hindu reported.

A tall measuring tube on an air monitor fills with grey haze that rises past a red cap line and spills out at the top, above a city skyline at night. — Illustration

What happened

The Central Pollution Control Board has explained why many Delhi stations stopped sending fine-dust readings on Diwali night: the readings went off its scale.

In a statement on Tuesday, CPCB said particulate matter (PM) analysers in use when the National Air Quality Index (NAQI) was introduced in 2015 were calibrated for a range of 0 to 1,000 micrograms per cubic metre, The Hindu reported. Its automated data checks mark any hourly reading above that range as “outside range”, and those readings are not used in the AQI, the statement said.

“During extreme episodes, which are occasional, like Diwali firecracker bursting, this may appear as a data gap at the particular station concerned,” the statement said, according to The Hindu. Those hours do not count towards that station’s 24-hour average, CPCB added.

The board posted the 11-point statement on X, The News Mill reported, with inputs from ANI. CPCB said internal deliberations and consultations on processing data “beyond system design range” were under way, according to The Hindu.

Why it matters for India 🇮🇳

Experts cited by The Hindu said the gaps had made the official AQI artificially lower, so the air looked better than it was.

And the readings left out were very high. In delayed hourly data released by the DPCC on Monday evening, all 125 station-hours missing from CPCB’s records were above 1,000 micrograms per cubic metre, WION reported. Mundka recorded 3,744 micrograms of PM2.5, and 11 DPCC stations went above 2,000, according to WION.

The limit also shapes the published index. The NAQI runs from 0 to 500, and readings beyond 500 stay in the “Severe” category, CPCB said, according to The News Mill. CPCB said the cap was a design feature of its 2015 method, which followed international practice, including that of the US Environmental Protection Agency.

Background

CPCB’s 2015 method calculates the AQI only when data for at least three pollutants are available, and one of them must be PM2.5 or PM10, The Hindu reported.

Each hour, the highest of the sub-indices for eight pollutants becomes the AQI, according to The News Mill. A sub-index needs at least 16 hours of data in the past 24, the same report said.

The DPCC runs 46 of Delhi’s 59 monitoring stations, and CPCB uses their data to calculate the AQI, The Hindu reported. The DPCC itself does not calculate the AQI, the same report said.

An official told The Hindu that the method was never updated even though the instruments improved. “But now there are discussions of raising this cap so that peak values above 1,000 µg/m³ could be measured,” the official said.

Environment Ministry sources quoted by The News Mill said the data still exists. “It is not that the data is missing,” one source said. Another said two or three scientists were working on the issue internally, but no external expert group had been formed, according to the same report.

Delhi’s air improved to “poor” on Tuesday, with a 24-hour average AQI of 299, The Hindu reported.

Different perspectives

CPCB said the gaps between its figures and those of private air-quality platforms come from different measuring methods, “and not to under-reporting”, The News Mill reported. It said the AQI is computed by a fully automated system without human intervention, according to the same report.

The Aam Aadmi Party had alleged “AQI data fraud” by the ruling BJP government, WION reported. WION also argued that a single cut-off at 1,000 does not explain every gap: RK Puram’s last reading before it went blank was 924, it noted.

Open questions 🤔

Will CPCB raise the 1,000 microgram limit, and from when?
Will CPCB publish a recalculated AQI for Diwali night that includes the readings above 1,000?
Why did some stations stop reporting before their readings crossed 1,000?

What to watch next

CPCB said internal deliberations and consultations on processing data beyond the system’s range are under way, The Hindu reported. None of the reports reviewed gave a date for a decision.