Delhi air quality nears 'severe' zone as farm fires shoot up

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 central government's Air Quality Early Warning System for Delhi said a significant increase in the number of “fire points” was observed over Punjab (around 3,000), Haryana and Uttar Pradesh on Wednesday, which is likely to impact the air quality of Delhi-NCR and other parts of northwest India.

Delhi recorded an air quality index (AQI) of 397 at 11 am on Thursday. 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.

Sixteen monitoring stations, including at Shadipur (406), Patparganj (411), Jahangirpuri (429) and Vivek Vihar (432), recorded the 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 said the wind speed dipped on Wednesday which allowed accumulation of pollutants.

“Following slight relief, the air quality again entered the ‘very poor' category by the evening,” he said.

PM10 levels in Delhi-NCR stood at 420 microgram per cubic metre (mg/m3) at 10 am -- the highest this season so far, according to CPCB data.

PM10 levels below 100 mg/m3 are considered safe in India.

PM10 is particulate matter with a diameter of 10 micrometres and is inhalable into the lungs. These particles include dust, pollen and mould spores.

The levels of PM2.5 -- finer particles which can even enter the bloodstream -- were 227 mg/m3.

The PM2.5 levels up to 60 mg/m3 are considered safe.

NASA's satellite imagery showed a large, dense cluster of fires that covered most parts of Punjab and some regions of Haryana.

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 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 factors for the increase in PM2.5 levels.

According to 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 in this season so far.

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

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 Thursday -- 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, Delhi Environment Minister Gopal Rai said 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 considering the seriousness of the situation due to the Covid-19 pandemic.