PM2.5 readings of 1K+ excluded from AQI: CPCB clarification after HT reports on ‘dark monitors’

HT earlier reported that more than two-thirds of Capital’s 59 air-quality monitoring stations went offline between 10pm and 6am on Diwali.

The Central Pollution Control Board (CPCB) on Tuesday said it has capped the upper limit of readings from all continuous ambient air quality monitoring stations (CAAQMS) at 1,000 micrograms per cubic metre (μg/m³), meaning readings beyond this threshold will not be used to calculate the National Air Quality Index (NAQI). The board said the index is intended to reflect “prolonged exposure” rather than “momentary readings”.

CPCB said its methodology remains under review in light of advances in monitoring technology and international practices. (Sakib Ali/Hindustan Times)

At the same time, the pollution watchdog said its methodology remains under review in light of advances in monitoring technology and international practices.

The clarification came a day after HT reported that more than two-thirds of the Capital’s 59 air-quality monitoring stations went offline between 10pm and 6am on Diwali, when exposure to pollution was at its highest. Data later released by the Delhi Pollution Control Committee (DPCC), and acknowledged by CPCB on Tuesday, showed readings as high as 3,744 μg/m³ at west Delhi’s Mundka at 2am on Monday, 3,023 μg/m³ at south Delhi’s JNU at 4am and 3,008 μg/m³ at NSUT West Campus at 5am. These readings were not used to calculate the AQI.

“The NAQI system has defined measurement ranges for all pollutants. The particulate matter analysers deployed at the time of introduction of the NAQI were calibrated for a measurement range of 0–1,000 μg/m³. Accordingly, consistent with this, the designed system of automated data validation treats hourly readings beyond the calibrated range as outside range, and such readings are not used in the computation,” the CPCB said in a statement released late on Tuesday.

It added that during occasional extreme episodes, such as Diwali firecracker bursting, this could result in a data gap for the station concerned, with those hours excluded from the calculation of the 24-hour average for the relevant pollutant.

“The NAQI recognises that prolonged exposure, rather than a momentary reading, is the more relevant indicator of ill effects on human health,” the board said, citing the National Ambient Air Quality Standards (NAAQS), 2009, which prescribe both short-term and long-term weighted averages. It described spikes during firecracker bursting as episodic and short-lived.

“Internal deliberations and consultations with respect to processing of data beyond system design range are underway,” the statement added.

Launched in 2015, NAQI generates an hourly AQI based on sub-indices for eight pollutants: PM2.5, PM10, carbon monoxide (CO), nitrogen dioxide (NO2), sulphur dioxide (SO2), ammonia (NH3), ozone and lead. The sub-index for each pollutant is calculated using its average concentration over the preceding 24 hours, except for CO and ozone, for which an eight-hour average is used. The highest sub-index among the pollutants determines the AQI for that hour.

CPCB said a sub-index could be generated with at least 16 hours of available data out of 24, except for CO and ozone. At least three pollutants must have sufficient data, including either PM2.5 or PM10.

The board also clarified that AQI has an upper limit of 500, beyond which readings remain in the “severe” category. “Beyond this level, the AQI remains in the ‘Severe’ category and extrapolation of the index would not alter the assigned health impact category,” it said.

Experts, however, said CPCB’s responsibility extended beyond publishing AQI figures. As the government’s technical arm, it must also collect and analyse data to assess whether pollution-control measures are working. “Withholding the exact numbers cripples the analysis needed to judge measures taken on the ground, such as restrictions on firecrackers or an outright ban.

This doesn’t handicap CPCB alone. It also hurts every other stakeholder who has a real interest in the data, including judiciary, researchers, data analysts, institutions and journalists,” said Sunil Dahiya, founder and lead analyst at Envirocatalysts. The actual exposure people experience, he said, should be available in the public domain.