Incidents of stubble burning in Haryana and Punjab have increased. (Representational)
Haze returned to Delhi and its suburbs on Tuesday due to raging farm fires in neighbouring states and a fall in temperature and the wind speed.
Weather experts said the national capital recorded a minimum of 11.7 degrees Celsius, the season's lowest so far, Tuesday morning. It is two notches below normal for this time of the year.
A dip in the wind speed and temperature makes the air cold and denser, leading to accumulation of pollutants, said Kuldeep Srivastava, head of the India Meteorological Department's regional weather forecasting center.
On Tuesday, Delhi's overall air quality index read 417 at 12.30 pm. It was 360 at 4 pm on Monday.
The levels of PM 2.5 -- tiny particulate matter less than 2.5 microns in diameter that can enter deep into the lungs and even the bloodstream -- shot up to 292 micrograms per cubic meter, around five times the safe limit of 0-60 micrograms per cubic meter.
The levels of PM10 increased to 436 micrograms per cubic meter, more than four times the safe limit of 100 micrograms per cubic meter.
Most of the 37 air quality monitoring stations across Delhi recorded air quality in the severe category.
Bawana was the most-polluted area in the city with an AQI of 445, followed by Anand Vihar (442), Wazirpur (442) and Dwarka Sector-8 (442).
Faridabad (404), Ghaziabad (445), Greater Noida (436), and Noida (436) also choked on extremely polluted air.
An AQI between 201 and 300 is considered ''poor'', 301-400 ''very poor'' and 401-500 ''severe''. An AQI above 500 falls in the ''severe plus'' category.
The experts said the spike in pollution levels can be attributed to a significant decline in the wind speed -- from 20 kilometers per hour over the last two days to 10 kilometers per hour on Monday and Tuesday.
Incidents of stubble burning in Haryana and Punjab have increased and northwesterly winds have been bringing more farm fire plume to the Delhi-NCR region, they said.
Stubble plume intrusion is expected to increase and a decrease in surface wind speed over the Delhi region is predicted for the next two days.
According to the government's air quality monitor, SAFAR, the share of stubble burning in Delhi's pollution is predicted to be 25 per cent on Tuesday, up from 18 per cent on Monday.
A pungent smog lingered over Delhi-NCR for around a week after Diwali as a result of emissions from firecrackers, stubble burning and unfavourable weather.
A Supreme Court-mandated panel declared a public health emergency as air pollution neared the emergency levels on November 1 and the administration ordered closure of schools till November 5.
Last week, the apex court had pulled up the Centre and state governments for their inability to curb stubble burning in Punjab and Haryana and bring air pollution in Delhi under control. It had asked the governments if they feel ashamed that people are no longer safe even in their houses.
The top court had also ordered that all farmers be given a Rs 100 per quintal incentive to prevent them from setting their fields on fire in preparation for the next crop, and provide them free machines to get rid of the agriculture residue.
The period between October 15 and November 15 is considered critical as maximum number of stubble burning incidents take place in this span in Punjab and adjoining states, which is one of the main reasons for alarming spike in pollution in Delhi-NCR.
Despite a ban on stubble burning in Punjab and Haryana, farmers continue to defy it amid lack of financial incentives.
State governments are providing 50 to 80 per cent subsidy to farmers and cooperative societies to buy modern farm equipment for in-situ management of paddy straw, but farmers say use of machines increases the input cost manifold.
According to an affidavit filed by the Centre in the Supreme Court, Haryana and Punjab distributed around 63,000 machines to farmers during 2018-19. In 2019-20, as many as 46,000 machines have been distributed.
The Delhi Environment minister had on Monday said, "There are around 27 lakh farmers in Punjab alone. If the same speed continues, it will take another 60 years for the machines to reach every farmer...It seems stubble burning will continue at the same pace next year too."