New stations on periphery may be clouding AQI data in Delhi, HT analysis shows

Delhi’s official air quality index (AQI) hit 382 on Friday, the city’s worst since January 19, and may still be understating pollution levels after 13 new stations were added to the monitoring network in October, an HT analysis of Central Pollution Control Board (CPCB) data has found. Without the new stations, Friday’s 4pm bulletin would have read 393.

Most of the new monitors, set up by the Delhi Pollution Control Committee (DPCC), the city government’s pollution regulator, are on the city’s periphery, but older stations just as far out, in areas just as built-up, show much higher readings. HT’s analysis found the levels of PM2.5 — the biggest contributor to pollution during the winter smog spell --- at the new stations are about 35% lower than those at their neighbouring stations.

Their effect on the AQI readings showed on Friday amid Dhanteras, which is typically accompanied by heavy traffic on the city’s roads. CPCB’s official figure is a plain average of every station’s AQI, each a rolling 24-hour average (the figure at 4pm becomes the day’s bulletin). From 7am to noon it stayed between 389 and 391, in the “very poor” band.

Without the 13 new stations, which averaged about 350 at 10am, it would have been 401 to 403, in the “severe” band, the worst on the scale.