Even as farm fires continued to rage in neighbouring States, the national Capital witnessed a dip in pollution levels on Monday with high wind speed aiding dispersion of pollutants.
The city recorded a 24-hour average air quality index (AQI) of 293, which falls in the “poor” category.
It was 364 on Sunday, with stubble burning contributing 40 per cent to Delhi’s pollution and negating the effect of better ventilation.
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 Air Quality Early Warning System for Delhi said a “significantly” large number of fires were observed over Punjab, Haryana, Uttar Pradesh and Madhya Pradesh on Sunday which is likely to impact the air quality in Delhi-NCR and northwest India.
Higher wind speed and better ventilation index will be favourable for dispersion of pollutants on Tuesday, it said.
According to the Ministry of Earth Sciences’ air quality monitor, SAFAR, the share of stubble burning in Delhi’s pollution was 16 per cent on Monday. It had soared to 40 per cent on Sunday, the maximum so far this season.
Stubble burning accounted for 32 per cent of Delhi’s PM2.5 pollution 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.
According to Punjab Pollution Control Board, the State has recorded 33,165 “fire events” this season so far, while Haryana has reported 6,034 farm fires during the period.
NASA’s satellite imagery also showed a large, dense cluster of fire dots covering Punjab and parts of Haryana and Uttar Pradesh. According to the India Meteorological Department, the predominant wind direction was westerly-northwesterly and the maximum wind speed was 18 kilometers per hour on Monday. The city recorded a minimum temperature of 10.8 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 dispersion.
According to the Central Government’s Air Quality Early Warning System for Delhi, the city’s ventilation index - a product of mixing depth and average wind speed - was around 12,000 meter square per second on Monday - 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.
