Delhi's official air quality index (AQI) may have understated the severity of pollution during Diwali celebrations after monitoring stations excluded several hours of exceptionally high particulate concentrations from their calculations, according to an analysis published on Wednesday.
The findings have raised questions about the Central Pollution Control Board's (CPCB) methodology, particularly its treatment of pollution readings exceeding the calibrated measurement range of monitoring equipment.
An analysis by Hindustan Times, based on CPCB and Delhi Pollution Control Committee (DPCC) data, found that including the excluded hours would have increased Delhi's AQI for the 24-hour period ending at 9 am on November 9 from the reported 370 to 393.
Although both readings fall within the "very poor" category, the difference was more pronounced at individual monitoring stations, several of which would have been classified as experiencing "severe" pollution.
𝗪𝗵𝘆 𝘄𝗲𝗿𝗲 𝗲𝘅𝘁𝗿𝗲𝗺𝗲 𝗿𝗲𝗮𝗱𝗶𝗻𝗴𝘀 𝗲𝘅𝗰𝗹𝘂𝗱𝗲𝗱?
In a statement issued on November 10, CPCB explained that its particulate matter analysers were calibrated to measure concentrations up to 1,000 micrograms per cubic metre.
Readings exceeding this limit were considered outside the instruments' calibrated range and excluded from the 24-hour AQI calculation.
The pollution watchdog also maintained that India's AQI scale was designed with a maximum value of 500 and that differences between its readings and those published by other monitoring organisations reflected variations in methodology rather than deliberate under-reporting.
However, the analysis questioned the justification for excluding extreme readings when the AQI calculation already places an upper limit on their contribution.
Under the methodology examined, hourly PM2.5 concentrations of approximately 420 micrograms per cubic metre or above receive the maximum AQI score of 500.
Consequently, concentrations of 450 and 3,744 micrograms per cubic metre would contribute the same maximum score, despite representing substantially different pollution levels.
The analysis argued that including readings exceeding 1,000 micrograms per cubic metre would therefore not distort the index beyond its established ceiling.
𝗛𝗼𝘄 𝗱𝗶𝗱 𝗶𝘁 𝗮𝗳𝗳𝗲𝗰𝘁 𝗗𝗲𝗹𝗵𝗶'𝘀 𝗔𝗤𝗜?
During Diwali night, monitoring stations recorded exceptionally high particulate concentrations, with a DPCC monitor at Mundka registering PM2.5 levels of 3,744 micrograms per cubic metre at 2 am.
Between 9 pm on November 8 and 7 am on November 9, CPCB's system assigned the maximum hourly AQI score to 167 readings below 1,000 micrograms per cubic metre but recorded no AQI score for another 226 hourly observations.
Subsequent DPCC data provided concentration readings for 112 of those excluded hours, all exceeding 1,000 micrograms per cubic metre.
The analysis found that excluding these hours reduced the calculated pollution levels at several monitoring locations.
At Nehru Nagar, for instance, the official AQI declined from 387 at 10 pm to 344 at 6 am, even though particulate concentrations during eight excluded hours ranged between 1,658 and 2,808 micrograms per cubic metre.
Including those hours at the maximum AQI score would have produced a reading of approximately 396 instead of 344.
Across Delhi, the recalculation increased the 9 am AQI from 370 to 393. The official 4 pm bulletin reading of 344 would similarly have risen to 372.
𝗪𝗵𝘆 𝗱𝗶𝗱 𝗺𝗼𝗻𝗶𝘁𝗼𝗿𝗶𝗻𝗴 𝘀𝘁𝗮𝘁𝗶𝗼𝗻𝘀 𝗱𝗶𝘀𝗮𝗽𝗽𝗲𝗮𝗿?
A separate concern involved monitoring stations being excluded entirely from Delhi's overall AQI calculation.
Under the applicable methodology, a station must have at least 16 hours of valid observations within a 24-hour period to generate an AQI.
When extreme pollution readings were excluded, some stations fell below this threshold and consequently stopped contributing to the city's average.
By 9 am on November 9, 10 of Delhi's 59 monitoring stations had been excluded, including those at Nehru Nagar, RK Puram, Dwarka Sector 8 and NSUT Jaffarpur.
By 4 pm, the number of excluded stations had increased to 12.
The analysis estimated that nine of the 10 stations excluded by 9 am would have recorded severe pollution had the missing hours been included.
It also found that the number of stations classified under the severe category at 9 am would have increased from 14 to 27.
𝗪𝗵𝗮𝘁 𝗱𝗼𝗲𝘀 𝘁𝗵𝗶𝘀 𝗺𝗲𝗮𝗻 𝗳𝗼𝗿 𝗽𝗼𝗹𝗹𝘂𝘁𝗶𝗼𝗻 𝗿𝗲𝗽𝗼𝗿𝘁𝗶𝗻𝗴?
The findings suggest that the exclusion of extreme readings can make official AQI figures less representative of pollution conditions during particularly hazardous episodes.
CPCB has argued that the index is intended to represent prolonged exposure rather than brief pollution spikes. However, the analysis noted that the 24-hour averaging process already accounts for the duration of exposure.
The study also found that the official post-Diwali AQI declined by 32 points, from 376 on Diwali to 344 the following day. Including the excluded hours would have reduced that decline to approximately four points.
Importantly, the analysis did not establish any deliberate manipulation of pollution data. CPCB has maintained that its automated methodology has been in operation since the AQI system was introduced.
The pollution watchdog has also acknowledged that the treatment of readings exceeding the system's measurement range is under review, with internal consultations underway.
The findings nevertheless highlight the importance of ensuring that extreme pollution episodes are accurately represented in public air quality reporting, particularly when such measurements influence health advisories and pollution-control responses.