The national capital's air quality slipped back into the “very poor” category again on Tuesday after recording a marginal improvement.

However, the share of stubble burning in Delhi's pollution dropped to 10 per cent during the day due to a change in the wind direction, a central government agency said.

Officials at the India Meteorological department said the air quality had improved on Monday with high wind speed aiding dispersion of pollutants. However, stagnant night-time conditions led to the accumulation of pollutants.

The city recorded an air quality index (AQI) of 332 at 10 am. It improved to 302 by 4pm as wind speed picked up.

The 24-hour average AQI was 293 on Monday which falls in the "poor" category. It was 364 on Sunday, with stubble burning contributing to 40 per cent of Delhi's pollution.

An AQI between 0 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''.

The Ministry of Earth Sciences' air quality monitor, SAFAR, said the share of stubble burning in Delhi's PM 2.5 pollution has "decreased significantly" due to a change in the wind direction and is estimated at 10 per cent for Tuesday.

It said 3,068 farm fires were spotted over Punjab, Haryana, Uttar Pradesh and Uttarakhand on Monday.

The boundary layer wind direction became southwesterly on Tuesday morning after a long spell which is unfavourable for the transport of pollutants from farm fires in Punjab and Haryana, according to the agency.

SAFAR said it was a typical example of high fire count and its low impact on Delhi's air quality due to unfavourable transport-level winds, "demonstrating how meteorology can play a decisive role".

Stubble burning accounted for 16 per cent of Delhi's pollution on Monday and 40 per cent on Sunday, the maximum so far this season. It was 32 per cent on Saturday, 19 per cent on Friday and 36 per cent on Thursday.

Last year, the farm fire contribution to Delhi's pollution had peaked to 44 per cent on November 1, according to SAFAR data.

SAFAR has predicted a marginal deterioration in the air quality on Wednesday and Thursday.

According to the India Meteorological Department, the maximum wind speed was 12 kilometers per hour on Tuesday. The city recorded a minimum temperature of 10 degrees Celsius, the lowest in the season so far.

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

According to the Air Quality Early Warning System for Delhi, the city’s ventilation index – a product of mixing depth and average wind speed – was expected to be around 8,000 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.

In a bid to control pollution, the Delhi government has also said that only “green firecrackers” can be manufactured, sold and used in the national capital in accordance with a 2018 Supreme Court order.

‘Green crackers’ are not as polluting as the conventional types of firecrackers and they contain at least 30 per cent less particulate matter such as sulphur dioxide and nitrogen oxide.

Fireworks can take place between 8 pm and 10 pm only on festivals like Diwali and Gurpurab etc. On Christmas eve and New Year eve, it would be from 11:55 pm till 12:30 am only, according to the Delhi Pollution Control Committee.

The National Green Tribunal has also issued notice to the Centre, Central Pollution Control Board, and the governments of Delhi, Haryana, Uttar Pradesh and Rajasthan, asking if crackers could be banned between November 7 and November 30 in the interest of public health and environment.?