Delhi's air quality deteriorates to 'poor' zone

New Delhi: Delhi's air quality slipped back into the "poor" category on Saturday, a day after it had improved significantly due to favourable wind speed.

The city's air quality index (AQI) was 209 at 9 am. The 24-hour average AQI was 137 on Friday, 302 on Thursday and 413 on Wednesday. An AQI between zero and 50 is considered "good", 51 and 100 "satisfactory", 101 and 200 "moderate", 201 and 300 "poor", 301 and 400 "very poor", and 401 and 500 "severe".

Stronger winds and a drastic reduction in the contribution of stubble burning were two major reasons for cleaner skies over Delhi on Friday. The winds slowed down at night allowing the pollutants to accumulate. The maximum wind speed was 18 kmph on Friday. It is likely to be 15 kmph on Saturday, according to the India Meteorological Department (IMD).

The share of stubble burning in Delhi's pollution has also reduced significantly as the harvesting season has ended. The contribution of stubble burning in neighbouring states to Delhi's PM2.5 levels was just two per cent on Friday and one per cent on Thursday, according to the Ministry of Earth Sciences' air quality monitor, SAFAR.

The weather department said the maximum temperature was likely to settle around 26 degrees Celsius on Saturday. Calm winds and low temperatures trap pollutants close to the ground, while favourable wind speed helps in their dispersion.