Data Gaps During Post-Diwali Smog Spike Put India’s Air Quality Methodology Under Scrutiny

New Delhi: The Central Pollution Control Board (CPCB) said on Tuesday that readings above a monitor’s calibrated range are left out of Air Quality Index calculations, explaining why many Delhi stations showed missing data during the post-Diwali pollution spike.

Reports said 45 of Delhi’s 59 monitoring stations recorded missing hourly readings for at least one pollutant, as PM2.5 concentrations approached 1,000 micrograms per cubic metre.

In an 11-point statement on November 10, the CPCB said the particulate analysers deployed when the National Air Quality Index was introduced in 2015 measure concentrations between 0 and 1,000 micrograms per cubic metre. Its automated validation system marks hourly readings beyond that range as out of range and excludes them from the 24-hour average, which can appear as a data gap at a station during extreme episodes such as firecracker bursting.

Environment Ministry sources told ANI that the readings are available but are not used in the calculation. They said that although instruments have been upgraded, the data-validation methodology has not changed. One source said two or three scientists were examining the issue internally and that no external expert group had been constituted, unlike in 2015.

The CPCB said it is keeping the methodology under review and holding internal consultations on processing readings beyond the instruments’ range. It said the number of continuous monitoring stations has grown from 30 in 2014-15 to 634 nationwide, and from 10 to 59 in Delhi. It defended the 24-hour running average, arguing prolonged exposure matters more for health than brief spikes, and the 500 ceiling, saying readings above it stay in the severe category. It attributed differences with private platforms to variations in measurement techniques, averaging periods and the use of low-cost sensors, not under-reporting.