CPCB Reviews Outdated AQI Methodology Amid Rising Pollution Concerns

The CPCB admits that air quality readings above 999 µg/m³ are excluded from AQI calculations due to an outdated 2015 methodology. Experts urge a revision as monitoring instruments have improved, but the data validation framework remains stagnant.

Methodology Limitations and Data Gaps

The Central Pollution Control Board (CPCB) on Tuesday acknowledged that particulate matter readings exceeding 999 micrograms per cubic metre (µg/m³) are excluded from the Air Quality Index (AQI) calculation under the existing methodology, as the monitoring system was originally designed to measure concentrations within a range of 0–1,000 µg/m³.

The pollution watchdog also indicated that it is reviewing the existing methodology, with internal deliberations and consultations underway on processing data beyond the system's designed measurement range. Sources in the Environment Ministry, meanwhile, acknowledged that the 2015 data-validation framework is outdated and needs to be revised, pointing out that although monitoring instruments have been upgraded over the years, the methodology governing data validation has remained unchanged.

Sources said the particulate matter analysers deployed when the National Air Quality Index (NAQI) was introduced in 2015 were calibrated for a measurement range of 0–1,000 µg/m³, and the methodology was developed accordingly. "However, over the years, the instruments have been upgraded, but the methodology has remained the same," a source said.

The CPCB Statement

In a detailed 11-point statement issued on X, CPCB said: "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."

"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," it added. This comes amid reports that 45 of Delhi's 59 monitoring stations recorded missing hourly readings for at least one pollutant, with experts pointing out that several monitoring stations showed data gaps as PM2.5 concentrations approached 1,000 µg/m³.

Explaining why such readings appear as missing data, a source in the ministry said: "It is not that the data is missing. We have the data. However, that data is not used in the calculation." "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.

Need for Methodology Revision

On the need to revise the methodology, another source said internal discussions were already underway, although no external expert committee had officially been constituted so far. "What I know is that internally, two-three scientists are already working on this."

"Officially, as of now, no external expert group, like the one formed in 2015, has been constituted," the source said, adding that an expert group should be formed to examine the issue. "It is the right time, rather high time. It is a late thing. This should have been revised two-three years ago. If they had revised it and incorporated everything properly, these issues would not have arisen," the source said.

"They are increasing the number of instruments, replacing the instruments, but the actual methodology is still the same old outdated methodology. The instruments have changed, but the methodology has not changed," the source added.

AQI Scale Clarification

CPCB, in its statement, also clarified that the NAQI operates on a scale of 0 to 500, with the upper limit representing the highest level of health concern. "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.

The board said the capping at 500 was a design feature of the 2015 NAQI methodology, developed in line with prevailing international practices, including the US Environmental Protection Agency's AQI scale. On differences between government and private air quality platforms, CPCB said variations could arise from differences in measurement techniques, averaging periods and methodologies.

"The difference in AQI values is thus attributable to differences in measurement of pollutant concentration and in methodology, and not to under-reporting," it said. Under the NAQI, the AQI is generated every hour using sub-indices for eight pollutants -- PM2.5, PM10, carbon monoxide (CO), nitrogen dioxide (NO2), sulphur dioxide (SO2), ammonia (NH3), ozone and lead.

The highest sub-index among the pollutants is taken as the AQI for that hour. The calculation is based on a running average of the preceding 24 hours, with at least 16 hours of available data required for generating a sub-index, except in the case of CO and ozone. CPCB said the NAQI is computed and disseminated through a fully automated system without human intervention, while individual pollutant concentrations recorded at Continuous Ambient Air Quality Monitoring Stations (CAAQMS) are also made available in the public domain. (ANI)

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