PM2.5 readings at many of Delhi’s air monitoring stations climbed towards 1,000 micrograms per cubic metre on Diwali night and then went blank, while the stations went on sending readings for gases, HT’s analysis of records kept by the Central Pollution Control Board (CPCB) shows.
The blanks came as readings were at their highest. The air quality index (AQI) is CPCB’s scale for turning pollution levels into a single score. HT estimated each blank stretch with a straight line from the station’s last reading before it to its first reading after. The scale tops out well below the levels recorded that night.
No PM2.5 reading from any Delhi station went past 1,000 this year. The highest were exactly 1,000.0, at Sirifort and ITO, both sent in the early morning after those stations had come back from their gaps. Last Diwali, 29 of the 39 stations recorded at least one hour above 1,000, the highest 2,224 at NSIT Dwarka, according to a copy of CPCB’s live feed kept by OpenAQ, an open-data platform. Counting only the hours recorded, HT’s estimate of Delhi’s air quality index for the 24 hours to 9am on November 9 is 437; with the blank hours estimated from the readings either side of them, it is 471. Each pollutant’s 24-hour average at a station is given a score, the station takes the score of its worst pollutant, and Delhi’s figure is the average of its stations. North Campus recorded 999 at 10pm and 824 at 4.30am, so its blank hours are put at 972 at 11pm, 918 at 1am and 851 at 3.30am. A straight line cannot rise above the readings on either side, so the estimate leaves out any readings past 1,000, of the kind 29 of these stations recorded last Diwali. If the air went past 1,000 µg/m³ in those hours, HT’s estimate would be higher than 471, though no higher than 500, where the scale stops. In the hour from 2am, 14 of the 39 stations were still sending PM2.5, and their median reading, the middle value once the readings are put in order, was 693 µg/m³. That puts the hour past the top of the scale, at 500, the AQI an hour at that level would score. The AQI is a 24-hour average, so this is not Delhi’s AQI for the day. Several of the stations that had gone blank were reading close to 1,000 just before their gaps, so the median of those still reporting probably understates the hour.
The bands run from “good” (0-50) through “satisfactory”, “moderate”, “poor” and “very poor” (301-400) to “severe” (401-500). CPCB’s “severe” band begins at 250 µg/m³ of PM2.5, which scores 401. Carrying on the rate at which scores rise through the “very poor” band, HT puts the top of the scale, 500, at 380 µg/m³.
