New Delhi, Oct 29 (PTI) The national capital's air quality inched closer to the 'severe' category on Thursday, primarily due to calm winds and spike in farm fires.
The Ministry of Earth Sciences' monitoring agency, SAFAR, said the share of stubble burning in Delhi's PM2.5 pollution was 36 per cent on Thursday - the maximum so far this season.
Delhi recorded a 24-hour average air quality index (AQI) of 395. It was 297 on Wednesday, 312 on Tuesday, 353 on Monday, 349 on Sunday, 345 on Saturday and 366 on Friday.
Most of the monitoring stations, including at Shadipur (421), Patparganj (418), Bawana (453) and Mundka (427), 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 accumulation of pollutants.
'Following slight relief, the air quality again entered the very poor category by the evening,' he said.
The AQI entered the 'severe' category for a brief period in the afternoon before slipping back into the 'very poor' category.
PM10 levels in Delhi-NCR stood at 424 microgram per cubic meter (µg/m3) at 10 am - the highest this season so far, according to CPCB data.
PM10 levels below 100 µg/m3 are considered safe in India.
PM10 is particulate matter with a diameter of 10 micrometers and is inhalable into the lungs. These particles include dust, pollen and mold spores. The levels of PM2.5 – finer particles which can even enter the bloodstream – were 231 µg/m3. PM2.5 levels up to 60 µg/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 36 per cent on Thursday.
It was 18 per cent on Wednesday, 23 per cent on Tuesday, 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 were the major factors for the increase in PM2.5 levels.
'Extremely calm surface and boundary layer winds along with the low nighttime boundary layer height resulted in low ventilation… practically stagnant, no dispersion condition has resulted in the current situation,' it said.
An increase in surface wind speed and better ventilation conditions are likely to 'significantly' improve the situation by Saturday, SAFAR said.
According to the India Meteorological Department (IMD), the predominant wind direction was northerly and the maximum wind speed was 8 kilometers per hour. The minimum temperature was recorded at 12.5 degrees Celsius - the lowest in October in 26 years.
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 - was around 4,000 meter 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.
The Centre introduced a new law through an ordinance that put in place a powerful oversight body to curb air pollution.
Under the ordinance released by the Union ministry of law and justice on Thursday, the Environment Pollution (Prevention and Control) Authority (EPCA) has been dissolved and replaced by a commission comprising over 20 members.
The Commission for Air Quality Management in the National Capital Region and Adjoining Areas will have the power to lay down parameters of air quality, inspect premises violating the law, ordering closure of non-abiding industries/plants among others. PTI GVS KJ