The air quality in Delhi again deteriorated on Thursday, reaching the brink of the “very poor” category as a reduction in wind speed and a change in wind direction brought in pollutants from stubble fires in Haryana and Punjab that caused a spike in pollution levels throughout the country’s northwest.

The condition is expected to further deteriorate over Friday and Saturday as conditions will remain unfavourable for the dispersion of pollutants, government agencies have warned.

In Delhi, the overall air quality index (AQI) on Thursday was 296, considered in the “poor” zone, as per the Central Pollution Control Board (CPCB) reading. An AQI of 201 to 300 is considered “poor” while an AQI of 301 to 400 is considered “very poor”, and is associated with respiratory illness to people on prolonged exposure.

Several parts of northwest India — particularly areas in the National Capital Region such as Faridabad, Ghaziabad, Greater Noida, Panipat — recorded “very poor” air on Thursday.

Wind direction was variable with easterly in the morning, and north-westerly in the afternoon. North-westerly winds tend to blow fumes from stubble fires from northern India towards the Capital. Stubble fires touched the highest of the season on Thursday as 1,428 fires were observed in Haryana and Punjab.

“Winds were calm in the afternoon and blowing at a speed of only 5 kmph in the day… The air quality is expected to deteriorate further on October 23-24 on account of reduced wind speed. We are expecting wind direction to change tomorrow to north-easterly again but it will not have any significant impact on air quality because winds will remain calm. Conditions are not favourable for dispersion (of pollutants),” said Vijay Soni, scientist, India Meteorological Department (IMD).

Senior Delhi government officials blamed crop stubble burning in neighbouring Haryana and Punjab as the major reason behind the plunging air quality in the Capital.

“We cannot control stubble burning in the neighbouring states, because of which air pollution is increasing in Delhi. We are repeatedly requesting the Central government and state governments to cooperate and to stop the stubble fires,” said Delhi environment minister Gopal Rai.

While the share of stubble burning to overall pollution in Delhi reduced to 9% on Thursday from 15% the day before, calm winds lowered the ventilation coefficient (thus not allowing dispersion of pollutants in the atmosphere), according to the forecast of the System of Air Quality and Weather Forecasting and Research (SAFAR), the Union ministry’s air quality forecasting wing.

The ventilation index was 5,000 sq metres per second on Thursday — a ventilation index below 6,000 sq m/s is considered not conducive for dispersal of pollutants in the air, according to CPCB. Ventilation index is a function of the mixing height and the wind speed and defines the ability of the atmosphere to disperse contaminants.

“Calm wind conditions mean the wind speed was zero and hence there was no dispersion. This is a usual phenomenon for the months of October and November, which is when air quality deteriorates. Air quality is likely to touch the ‘very poor’ zone and remain in the same category over the next two days. Some improvement is likely around October 26 when wind speed may pick up,” said Soni.

On Thursday, the minimum temperature settled at 17.2 degrees Celsius, a notch below normal. The maximum was 33.4 degrees Celsius, two degrees above normal.

A feeble western disturbance (WD) is also likely to impact northern India around October 25. “There can be some rain in the hills in Jammu & Kashmir and Himachal Pradesh but it is a weak system,” said K Sathi Devi, head, national weather forecasting centre adding that “intense weather and heavy rains are expected in northeast India and West Bengal this week.”

The rain may marginally bring down the minimum temperature across northern India over the next week, officials added.

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