Calm winds and low temperatures trap pollutants close to the ground, while favourable wind speed help in their dispersion.
The city recorded an AQI of 335 at 10 am. The 24-hour average AQI was 353 on Monday. It was 349 on Sunday, 345 on Saturday and 366 on Friday.
An AQI between 0and 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’.
According to the Ministry of Earth Sciences’ air quality monitor, SAFAR, the wind direction and speed were favourable for the transport of pollutants from farm fires in Punjab, Haryana and neighbouring regions.
It, however, said an improvement in local wind speed will counter the effect.
The share of stubble burning in Delhi’s PM2.5 concentration was 16 percent on Monday. It was 19 percent on Sunday and 9 percent on Saturday.
NASA’s satellite imagery also showed a very dense cluster of fires in Punjab, Haryana and nearby regions.
According to the Punjab Pollution Control Board, the state has recorded 14,461 incidents of stubble burning between September 21 and October 25 as compared to 9,796 during the corresponding period last year.
Haryana has recorded around 4,284 incidents of stubble burning this season so far.
The central government’s Air Quality Early Warning System for Delhi said Delhi’s air quality was likely to remain ‘very poor’ till October 31.
The air quality warning system said the ventilation index – a product of mixing depth and average wind speed – was expected to be around 11,500 metre square per second on Tuesday – favourable for dispersion of pollutants.
Mixing depth is the vertical height in 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 dispersal of pollutants.
Severe air pollution in Delhi is a year-round problem, which can be attributed to unfavourable meteorological conditions, farm fires in neighbouring regions and local sources of pollution.
The Centre had on Monday told the Supreme Court that it will bring a legislation to curb air pollution in the Delhi-NCR.
According to an analysis by the Council on Energy, Environment and Water, a Delhi-based think tank, transportation contributes the most — 18 to 39 per cent — to Delhi’s air pollution.
Road dust is the second largest source of air pollution in the city (18 to 38 per cent), followed by industries (2 to 29 per cent), thermal power plants (3 to 11 per cent) and construction (8 per cent).