The national capital's overall air quality inched closer to the 'severe' category on Thursday morning as pollution levels rose sharply after a marginal reduction, primarily due to calm winds and spike in farm fires.
The city recorded an air quality index (AQI) of 392 at 10 am. The 24-hour average AQI was 297 on Wednesday, 312 on Tuesday, 353 on Monday, 349 on Sunday, 345 on Saturday and 366 on Friday.
Fourteen monitoring stations, including at Shadipur (406), Patparganj (411), Jahangirpuri (429) and Vivek Vihar (432), recorded air quality in the 'severe' category.
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'.
A senior scientist at the India Meteorological Department (IMD) said the wind speed dipped on Wednesday which allowed the accumulation of pollutants.
“Following slight relief, the air quality again entered the very poor category by the evening,” he said.
According to the Ministry of Earth Sciences' air quality monitor, SAFAR, the share of stubble burning in Delhi's PM2.5 concentration was 18 per cent on Wednesday.
It was 23 per cent on Tuesday, the maximum this season so far, 16 per cent on Monday, 19 per cent on Sunday and 9 per cent on Saturday.
SAFAR said the accumulation of locally generated pollutants and increased external intrusion due to north-north westerly boundary level winds from regions where stubble is burnt will be major reasons for the increase in PM2.5 levels.
According to the IMD, the predominant wind direction was northerly and the maximum wind speed was 8 kilometres per hour.
The minimum temperature was recorded at 12.5 degrees Celsius – the lowest this season so far.
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 around 4,000 metre square per second on Wednesday, unfavourable 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.
On Wednesday, Environment Minister Gopal Rai said that only 'green' firecrackers can be manufactured, sold and used in the national capital.
He also said the Delhi government will launch an anti-firecracker campaign from November 3 and requested people not to burn crackers at all considering the seriousness of the situation due to the COVID-19 pandemic.
