The data gaps reported at several air quality monitoring stations in Delhi during the post-Diwali pollution spike have brought renewed scrutiny to India's decade-old air pollution monitoring methodology, with Environment Ministry sources telling news agency ANI that the data-validation framework needs to be revised despite upgrades to monitoring instruments.
The particulate matter analysers deployed when the National Air Quality Index (NAQI) was introduced in 2015 were calibrated to measure concentrations between 0 and 1,000 micrograms per cubic metre (µg/m³), according to the Central Pollution Control Board (CPCB).
In an 11-point statement issued on November 10, the board said its automated data-validation system classifies hourly readings beyond the calibrated range as out-of-range and excludes them from AQI calculations.
The board said that during extreme pollution episodes, such as Diwali firecracker bursting, this could appear as a data gap at an individual monitoring station because the affected hourly readings are not included in the 24-hour average for the pollutant concerned.
However, Environment Ministry sources told ANI that while monitoring instruments have been upgraded over the years, the methodology governing data validation has remained unchanged.
"However, over the years, the instruments have been upgraded, but the methodology has remained the same," a source told ANI.
The issue came into focus amid reports that 45 of Delhi's 59 monitoring stations recorded missing hourly readings for at least one pollutant, with experts flagging data gaps as PM2.5 concentrations approached 1,000 µg/m³.
Explaining the apparent data gaps, a Ministry source told ANI that the readings were available but were not being used in the AQI calculation.
"It is not that the data is missing. We have the data. However, that data is not used in the calculation," the source said.
"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.
On the need to revise the methodology, another source told ANI that internal discussions were underway, with two or three scientists already examining the issue. However, no external expert group had officially been constituted to review the framework.
"What I know is that internally, two-three scientists are already working on this," the source said.
"Officially, as of now, no external expert group, like the one formed in 2015, has been constituted," the source added, arguing that an expert group should be set up 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.
The source also questioned the continued use of the old methodology despite investments in monitoring infrastructure.
"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 said.
CPCB, in its November 10 statement, said it was keeping the methodology under review in light of advances in monitoring technology and international practices. It confirmed that internal deliberations and consultations were underway on processing readings beyond the system's designed measurement range.
The board said the country's network of Continuous Ambient Air Quality Monitoring Stations (CAAQMS) had expanded from 30 in 2014-15 to 634, while Delhi's network had grown from 10 to 59 stations over the same period.
CPCB defended the use of a 24-hour running average under the NAQI, saying prolonged exposure to pollutants, rather than momentary spikes, was more relevant to assessing potential health impacts. The index uses sub-indices for eight pollutants — PM2.5, PM10, carbon monoxide, nitrogen dioxide, sulphur dioxide, ammonia, ozone and lead — and takes the highest sub-index as the AQI for a given hour.
The calculation uses the preceding 24 hours of data, with at least 16 hours of available readings required to generate a sub-index, except for carbon monoxide and ozone.
The board also defended the AQI ceiling of 500, saying it represents the highest level of health concern and that readings beyond it would remain in the 'Severe' category. It said extending the index would not alter the assigned health impact category.
On differences between government and private air quality platforms, CPCB attributed variations to differences in measurement techniques, averaging periods and methodologies. It said some platforms use instantaneous hourly readings and low-cost sensors, which may have calibration and data-quality limitations.
CPCB said the NAQI is computed through an automated system without human intervention, while individual pollutant concentrations recorded at monitoring stations are also made available in the public domain.
(With inputs from ANI)