Hazy picture: Delhi’s sharp AQI spike puts data under spotlight

spike over the past 24 hours, and previous underreporting. After all, Wednesday (October 29), looked visibly like a 350-plus day, as longtime sufferers in the Capital can vouch.

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AQI readings on Oct 28, 29 and 30 point to this There has been a visibly dense haze in Delhi for the past three-four days. Anecdotally speaking, the official AQI numbers for October 28 and 29 the 24-hour averages at 4pm were 294 and 279 seemed to underestimate the ambient pollution. On October 30, the haze was largely similar to the previous two days, but the AQI reading increased to 373 in the Central Pollution Control Board bulletin, a jump of almost hundred points. What really led to this?

Not a calculation error

HT independently calculated the 24-hour average of AQI from sub-indices of individual pollutants at each station to check if there was a calculation error that had kept the AQI low in the past two days. No such error was found. The 24-hour average AQI at 4 PM on October 28 and 29 as calculated by HT was 294.9 and 278.8. Both numbers are within a point of the bulletin readings. At noon on October 30, HT’s calculation showed an AQI of 375.7, also similar to the then reading on the Sameer App, and close to the 373 reading in the bulletin published including the next four hours.