The Central Pollution Control Board (CPCB) on Tuesday, November 10, defended its National Air Quality Index (NAQI) reporting after questions were raised over data gaps at Delhi monitoring stations on Diwali night and over why official readings differed sharply from those shown by private platforms. In a clarification on post-Diwali AQI reporting, the Board said the variations were attributable to differences in measurement and methodology, and not to under-reporting, news agency ANI reported.

Source: ANI via Daily Prabhat

The CPCB said the upper limit of 500 on the index was a design feature of the NAQI, developed in 2015 through the Indian Institute of Technology Kanpur under the guidance of an expert group of medical professionals, air-quality experts, academic institutions, NGOs and State Pollution Control Boards, and that 500 represented the highest level of health concern. The index, it said, was designed to reflect sustained pollution levels over a period of time rather than momentary spikes, such as those caused by firecracker bursting during Diwali.

Why the stations went blank

On the reported data gaps, the Board said the particulate-matter analysers deployed when the NAQI was introduced had a calibrated measurement range of 0 to 1,000 micrograms per cubic metre. Its automated data-validation system therefore treated hourly readings beyond that range as outside range, and such readings were not included in AQI calculations. During extreme pollution episodes, the CPCB said, this could produce data gaps at the affected stations, with those hours not contributing to the pollutant's 24-hour average.

The clarification followed a Hindustan Times analysis, reported by Business Today on Tuesday, which found that 125 hourly PM2.5 readings from Delhi's monitoring stations were missing from the CPCB's records for Diwali night, even though every one of those readings was above 1,000 micrograms per cubic metre on the Delhi Pollution Control Committee's own website. The highest figure in the DPCC data was 3,744 micrograms per cubic metre at Mundka at 2 am on November 9, and eleven DPCC stations recorded levels above 2,000 at some point that night. In the CPCB record, by contrast, the highest reading carried for the night was exactly 1,000, and at 1 am, 36 of Delhi's 59 stations had no PM2.5 reading at all.

Source: Business Today / Hindustan Times analysis via The Popular Story

Using the CPCB's own method with the missing readings put back, the 24-hour AQI to 9 am on November 9 works out to about 384, against the 370 the CPCB published — both in the 'very poor' band, because the index tops out at 500 and readings far above it barely move the final number. The Hindu BusinessLine, citing an analysis by the Centre for Research on Energy and Clean Air (CREA), had reported on November 9 that monitoring gaps across at least 44 of the city's 59 official stations may have obscured the full extent of the pollution spike, with PM2.5 concentrations topping 900 micrograms per cubic metre at several locations.

Source: The Hindu BusinessLine

Allegations, and the Board's answer

The missing readings had drawn sharp political allegations. Aam Aadmi Party leader Saurabh Bharadwaj alleged that the Delhi government had manipulated pollution figures — an allegation the CPCB's clarification implicitly rejected by attributing the gaps to calibration limits and methodology. The Board stated plainly: 'The difference in AQI values is thus attributable to differences in measurement of pollutant concentration and in methodology, and not to under-reporting.'

The clarification did not announce any change to the analysers' calibrated range, to the automated validation rule, or to the 500 cap on the index. What happens next will be watched closely: whether the Board revisits a measurement ceiling set in 2015, in a city whose Diwali-night pollution now routinely bursts through it.