Delhi’s Sept AQI consistently over 100, first since 2016

Experts said conditions shifted after early September showers gave way to drier northwesterly winds. Rain intensity fell sharply and the monsoon officially withdrew on September 24 -- its earliest retreat since 2002.

Since September 12, Delhi has seen its longest run of September days where the air quality index (AQI) has stayed above 100 in nearly a decade -- a streak unmatched since 2016, when the city logged 29 such days.

Delhi recorded just five consecutive days above 100 in 2024, 10 each in 2023 and 2022, two in 2021, and 14 in 2020. (Hindustan Times)

Data from the Central Pollution Control Board (CPCB) shows the contrast clearly. Delhi recorded just five consecutive days above 100 in 2024, 10 each in 2023 and 2022, two in 2021, and 14 in 2020.

For Delhi, where winter pollution often pushes AQI levels beyond 400, an index above 100 may not appear alarming. Yet for September, usually cleaner due to monsoon rains, it signals an unusually polluted month.

Experts said conditions shifted after early September showers gave way to drier northwesterly winds. Rain intensity fell sharply and the monsoon officially withdrew on September 24 -- its earliest retreat since 2002.

“Even before the monsoon officially withdrew, we had a dry period. This meant consistent rain was missing - to wash away pollutants. Post withdrawal, we also see clear skies and increased heat, which also accelerates ozone formation,” said Dipankar Saha, former head of CPCB’s air laboratory.

The city’s highest AQI this month was 169 on September 27, while the lowest was 52. The monthly average stood at 104.7 — marginally better than 2023 (107.8) and 2024 (105.4), but worse than 2022 (104.4). CPCB classifies 51-100 as “satisfactory”, 101-200 as “moderate”, 201-300 as “poor”, 301-400 as “very poor”, and over 400 as “severe”. Forecasts suggest “moderate” air quality will continue into early October.

September was also hotter than usual. The mean maximum temperature was 34.9°C, above both last year (33.8°C) and the long-period average of 34.1°C, though cooler than 2023, when it reached 35.6°C. On September 28, the mercury hit 38.1°C, the hottest September day in two years, followed by 37.5°C on Monday. IMD has forecast a marginal dip of 1–2°C and chances of scattered drizzle on Tuesday.