On Diwali night, Delhi's PM2.5 readings climbed towards 1,000, then went blank

HT's analysis of CPCB's records shows fine-dust readings at many stations stopped just short of 1,000 µg/m³ while their gas monitors kept reporting. 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.A child bursts firecrackers while wearing a face mask during the Diwali festival celebrations, near Akshardham Temple in New Delhi, late Sunday. (PTI)PM2.5 is fine dust, particles 2.5 micrometres across or smaller, and micrograms per cubic metre (µg/m³) measure how much of it is in the air. HT compared 39 Delhi stations that also reported last Diwali. For the 1am hour on November 9, 13 of them sent a PM2.5 reading and 38 sent a reading for nitrogen dioxide, a gas.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.ALSO READ | Delhi records 3rd worst Diwali pollution since '21, data gaps cloud full pictureThe pattern fits a limit at about 1,000 µg/m³ on the dust readings, which could lie in the monitors or in CPCB's data system. Stations that lose power or connection stop sending all their readings; these kept sending gas readings through the night. Why the limit exists, and whether it is new, are for CPCB to answer. HT has sent queries and the story will be updated with any response.The blanks came as readings were at their highest. 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.The readings rose steeply before they stopped. At North Campus, a station run by the Indian Institute of Tropical Meteorology (IITM), PM2.5 climbed from 326 µg/m³ at 8pm to 999 at 10pm, and the next readings were blank. At CRRI Mathura Road, also an IITM station, it went from 394 to 981 in the two hours to 10.30pm. ITO, a CPCB station, last sent 994 at 10.45pm.Across all 59 Delhi stations, 43 had a gap in PM2.5 between 10pm and 6am. At 14 of those 43, the last reading before the gap was 900 or more, within 100 of the 1,000 mark. Stations run by all three operators went blank: CPCB, IITM and the Delhi Pollution Control Committee (DPCC), the city's own pollution regulator.ALSO READ | SC turns focus to vehicular pollution, seeks action plan from Delhi, 3 statesThrough the night, at least 37 of the 39 stations kept sending readings for nitrogen dioxide, carbon monoxide and ozone. PM10, a wider measure of dust that counts all particles up to 10 micrometres, including PM2.5, gives a higher reading than PM2.5 in any given air, so a limit at 1,000 would cut it off first. PM10 readings did drop out sooner and further: for the 1am and 2am hours, only 10 of the 39 stations sent one.Readings darkened towards 1,000, then went blank. Each row is one of 39 Delhi stations; each cell is 15 minutes of PM2.5 from 8pm on November 8 to 7am on November 9. Hatched cells have no reading. DPCC stations report hourly, so their cells come in blocks of four. The darkest cells are readings of exactly 1,000; none went past it. Source: CPCB's Central Control Room records as read by HT, 9.32am, November 9. (Provisional)From 10pm to 6am, the 39 stations should have sent 351 hourly PM2.5 readings, one from each station for each of the nine hours. HT counts each of these as a station-hour. Of the 351, 148, or 42%, were blank. On ordinary nights in the days before Diwali, these stations left 3.9% blank. Last Diwali, at least 31 of the 39 sent PM2.5 in every hour after 10pm; this year, the count fell to 13.Last Diwali, 29 of these stations went past 1,000. This year, none did. Each of the 39 stations' highest hour of PM2.5 on Diwali night, 2025 against 2026. Sources: OpenAQ's copy of CPCB's live feed (2025); CPCB's Central Control Room records as read by HT (2026) (Provisional)What the blanks do to the AQIThe air quality index (AQI) is CPCB's scale for turning pollution levels into a single score. 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. The bands run from "good" (0-50) through "satisfactory", "moderate", "poor" and "very poor" (301-400) to "severe" (401-500).ALSO READ | AQI of 387 in India would be nearly 1,500 in US and 10 in UK. Why the range range is so wideHT applied CPCB's method to the 39 stations for the 24 hours to 9am on November 9. Counting only the readings recorded, HT's estimate of Delhi's AQI is 437. With the blank hours estimated, it is 471. CPCB's official AQI at 9am, for all Delhi stations, was 370; on the 39 stations alone, it was 381.The blank hours lower HT's estimate in two ways. They drop out of each station's 24-hour average, which is then built from the hours that were recorded, most of them outside the night's worst stretch. And seven stations, including Nehru Nagar and Pusa (IITM), which sent no PM2.5 at all from 10pm to 6am, fall short of the 16 hours of readings a station needs to count, so they leave the city's average altogether.HT estimated each blank stretch with a straight line from the station's last reading before it to its first reading after. 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.HT's estimate is above 400 even counting only the hours recorded. Whether CPCB's figure of 370 was computed without the blank hours is among the questions HT has put to it.The blank night hours take 34 points off an estimate of Delhi's AQI. HT's estimates for the 24 hours to 9am on November 9 on 39 stations, as recorded and with blank hours estimated, against CPCB's official figure. HT's estimates, not CPCB figures. Source: CPCB's Central Control Room records as read by HT. (Provisional)The scale tops out well below the levels recorded that night. 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. 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.Deep DiveBinayak Dasgupta is a senior editor at Hindustan Times, New Delhi, where he has worked since 2013. He heads the paper's Page 1 team and is part of the editorial leadership overseeing daily news operations. He has previously led the national news desk. His work sits at the intersection of specialist depth and emerging public consequence — stories where complexity and nuance are in themselves the journalistic challenge. He shapes Hindustan Times's coverage and editorial positions on technology and artificial intelligence, information security, environment, health and science — steering these subjects from the commissioning of reporting to editing and the framing of the paper’s institutional view. His work spans analysis, commentary and reportage and in the past has led the paper's coverage of Covid-19 pandemic science. His interviews and hosted sessions include Nobel laureates, technology leaders, economists and climate justice scholars. In recent months, Binayak has overseen the coverage of the Air India Flight 171 crash and its investigation, India's energy exposure during the West Asia conflict, the trade dispute with the US, Delhi's air quality emergency and the evolving conversations around AI, including the IndiaAI Summit.Read More