Delhi-NCR’s ‘severe’ air quality likely to improve by Monday but may still remain in ‘poor’ category, says IMD

The situation in the National Capital remained grim as Delhi’s air quality turned “severe” on Saturday. Neighbouring cities of Ghaziabad, Greater Noida and Noida also entered the “severe” zone, with the air quality index (AQI) recorded at 446, 401 and 406 respectively.

With slow wind speed allowing the accumulation of “locally generated” pollutants, Delhi’s AQI was 401 at 10 am on Saturday, according to India Meteorological Department (IMD) officials.

The 24-hour average AQI was 382 on Friday, 341 on Thursday, 373 on Wednesday, 367 on Tuesday, 318 on Monday and 268 on Sunday. 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’.

Wind speed remains slow

“The wind speed remained slow allowing the accumulation of pollutants generated locally. Easterly winds carrying moisture also led to the formation of secondary particulate matter. All these factors together pushed the air quality in the ‘severe’ zone,” VK Soni, head of the IMD’s environment research centre said.

Soni, however, said the air quality is likely to improve to the ‘poor’ category by Monday as the wind speed is expected to pick up. According to the IMD, the maximum wind speed is likely to be 8 kmph on Saturday, 10 kmph on Sunday and 15 kmph on Monday.

Minimum temperature at 11° Celsius

The minimum temperature in Delhi was 11° Celsius on Saturday, and the maximum is expected to settle around 28° Celsius, the weather department said. The minimum temperature, however, is likely to dip again with the wind direction changing to northwesterly by Monday, according to IMD officials. Westerly and northwesterly winds blow from snow-laden western Himalayas towards the plains.

Shallow to moderate fog is also predicted over the next two days, the IMD said. The minimum temperature this season has remained 2-3 notches below normal in the absence of a cloud cover on most days, according to IMD officials.

Dispersal of pollutants

Calm winds and low temperatures trap pollutants close to the ground, while favourable wind speed helps in their dispersion.

The central government’s Air Quality Early Warning System for Delhi said the city’s ventilation index — a product of mixing depth and average wind speed — is likely to be 2,500 m2/s on Friday. Mixing depth is the vertical height at which pollutants are suspended in the air. It reduces on cold days with calm wind speed.

A ventilation index lower than 6,000 sqm/second, with the average wind speed less than 10 kmph, is unfavourable for the dispersal of pollutants.

(With inputs from PTI)

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