This year, Delhi has seen the earliest spell of severe AQI streak

With average air quality index (AQI) of 421 for the 24 hours ending at 4 pm, November 6 became the fourth consecutive “severe” AQI day in Delhi. While Delhi has had longer stints of consecutive “severe” AQI days earlier, this year is the earliest (in the year) that air quality has been so bad for such a long spell.

The November 3-6 period spell of back-to-back “severe” AQI days is the fifth longest consecutive “severe” AQI spell in Delhi since January 2016, the earliest period for which HT analysed CPCB bulletins.

There have been 63 spells of back-to-back “severe” air quality days in Delhi since January 2016. Only four of them have been longer than the current spell of four days. These were the two seven-day spells ending November 11, 2016 and November 13, 2017; and the two six-day spells ending November 10, 2020 and December 26, 2021. The current four-day spell is tied with two spells of similar length in November 2019 and December 2018. Since the November 2019 spell started on November 12 and the current one on November 3, this is the earliest in the year that such a long spell has been observed. In fact, the current spell has come earlier than even the seven-day and six-day spells listed above. And, based on current forecasts, it may end up being longer than all of them.

To be sure, the longer spells in 2016 and 2017 are not strictly comparable. The number of air quality stations operating in “severe” days in November of those years averaged six and 15. Active stations doubled to the 30-40 range from 2018 onwards.

As far as overall number of “severe” AQI days is concerned, Delhi has recorded around 6-15 days of “severe” air in relatively good years (2017, 2020, and 2022) and 20-26 days of “severe” air in the relatively bad years. All such days have also almost entirely come in the relatively colder October- January period. 2023 has recorded seven days of “severe” air so far. However, given the fact that “Severe” AQI spell has come early this year, it remains to be seen whether 2023 will end up being one of the most polluted years for Delhi.