NEW DELHI: After remaining in the 'severe' zone for six days on the trot, Delhi's air quality improved slightly on Wednesday as a change in the wind direction reduced the contribution of stubble burning to the pollution, though it was still in the 'very poo' category.
The city recorded an air quality index (AQI) of 344. The 24-hour average AQI was 476 on Tuesday.
An AQI between 201 and 300 is considered 'poor', 301-400 'very poor' and 401-500 'severe', while the AQI above 500 falls in the severe plus category.
The national capital had witnessed six consecutive 'severe' air quality days till Tuesday. It had recorded seven such days in November last year.
The neighbouring cities of Faridabad (327), Ghaziabad (360), Noida (309), Greater Noida (340), and Gurgaon (288), which fall in the National Capital Region (NCR), also recorded their AQI in poor and "very poor" categories on Wednesday.
The Central Pollution Control Board (CPCB) ordered the closure of hot mix plants and stone crushers in Delhi-NCR till November 17 in view of a likely increase in pollution levels during the coming days, when a number of festivals will be celebrated.
It also asked the governments of Punjab and Haryana to take immediate stringent actions to curb stubble burning and authorities in Delhi-NCR to strictly check biomass burning.
The levels of PM2.5 'which is about three percent the diameter of a human hair and can lead to premature deaths from heart and lung diseases' was 177 microgram per cubic meter (µg/m3) at 5 pm, below the emergency threshold of 300 µg/m3.
The safe limit is 60 µg/m3. On Tuesday, PM2.5 levels had soared to 528 µg/m3 in the afternoon. PM10 level stood at 343 µg/m3 at 5 pm.
It had peaked at 685 µg/m3 on Tuesday, according to CPCB data. PM10 levels below 100 µg/m3 are considered safe in India and 500 µg/m3 is the emergency threshold.
According to the Graded Response Action Plan (GRAP), the air quality is considered in the 'severe plus' or 'emergency' category if PM2.5 and PM10 levels persist above 300 µg/m3 and 500 µg/m3 for more than 48 hours.
GRAP recommends measures such as a ban on construction activities, entry of trucks and car rationing scheme in such a scenario.
The Commission for Air Quality Management in the National Capital Region and Adjoining Areas on Tuesday tasked the CPCB with operationalising and monitoring GRAP measures till a mechanism is set up by the newly-constituted panel.
The Ministry of Earth Sciences' air quality monitor, SAFAR, said the change in transport level wind direction has led to a significant decrease in stubble burning-related intrusion in spite of high fire counts.
The farm fire count in Punjab, Haryana, Uttar Pradesh, Uttarakhand and neighbouring areas was 2,422 on Tuesday and the share of stubble burning in Delhi's PM2.5 accumulation is almost negligible -- 3 percent on Wednesday due to unfavorable transport-level winds, it said.
"Deterioration (in air quality) is expected on Friday towards the higher end of the 'very poor' category," it said.
An official of the India Meteorological Department (IMD) said the predominant wind direction is east-northeasterly, which is not favorable for the transport of pollutants from farm fires in Punjab and Haryana.
The city recorded a minimum temperature of 11.4 degrees Celsius on Wednesday morning.
Calm winds and low temperatures trap pollutants close to the ground, while favorable wind speed helps in their dispersion.
The central government's Air Quality Early Warning System for Delhi said, "The change in wind direction and wind speed has positively impacted air quality. It is likely to remain in the upper end of the 'very poor' category on Thursday and deteriorate marginally on Friday."
Delhi's ventilation index -- a product of mixing depth and average wind speed -- is likely to be around 12,500 m2/s on Wednesday, favorable 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.
The ventilation index lower than 6000 m2/s with an average wind speed less than 10 kmph is unfavorable for dispersion of pollutants.