CPCB Explains Blank Diwali AQI Readings In Delhi, Flags Outdated 2015 Validation System

Delhi’s blank Diwali pollution readings were linked to the current AQI system, which leaves out hourly particulate matter data above 999 µg/m³. The Central Pollution Control Board issued an 11-point clarification on its official X handle on Tuesday. The board stated that the current tool was developed in 2015, and the monitoring system was originally built to handle concentrations in the 0-1,000 µg/m³ range.

The clarification followed air pollution concerns and alleged data discrepancies reported after Diwali in Delhi. CPCB acknowledged that readings of particulate matter above 999 µg/m³ are left out of Air Quality Index calculations under the current method.

The board also stated it is reviewing the existing methodology, with internal deliberations and consultations underway on how to process data beyond the system’s designed measurement range.

How the system works

The limit is built into the system. When the National Air Quality Index was introduced in 2015, particulate matter analysers could measure pollution only up to 1,000 µg/m³, CPCB stated.

As a result, the system automatically marks hourly readings above that level as out of range and keeps them out of AQI calculations. “During extreme episodes, which are occasional, like Diwali firecracker bursting, this may appear as a data gap at the particular station concerned, and those hours do not contribute to the 24-hour average of relevant pollutant for concerned station,” CPCB stated.

That explanation matters because 45 of Delhi’s 59 monitoring stations had at least one pollutant with missing hourly readings. Experts also noted that several stations showed data gaps as PM2.5 concentrations neared 1,000 µg/m³.

The data still exists. “It is not that the data is missing. We have the data. However, that data is not used in the calculation,” a ministry source told ANI.

The source repeated the point in similar terms. “Those data missing bands that you see on CPCB, we have the complete data. However, that data is not used in the calculation,” the source added.

Outdated validation framework

According to ANI, citing sources, the Environment Ministry has acknowledged that the 2015 validation framework is outdated and needs revision. The ministry’s position is that monitoring instruments have been upgraded over the years, but the methodology used for data validation has not changed.

CPCB also stated that the NAQI works on a 0 to 500 scale, with the upper end marking the highest level of health concern.

The index is capped by design. “Beyond this level, the AQI remains in the 'Severe' category and extrapolation of the index would not alter the assigned health impact category,” CPCB stated.

That cap was part of the 2015 NAQI methodology, the board stated. CPCB added that the method was developed in line with prevailing international practices, including the US Environmental Protection Agency’s AQI scale.