The air quality in Delhi-NCR recorded a marginal improvement but remained in the “very poor” category on Saturday as stubble burning incidents were “unusually high”.
Air quality is expected to get better in the coming days due to favourable wind speed, said forecasting agency SAFAR. The Capital recorded a 24-hour average Air Quality Index of 367 on Saturday, it was 374 on October 31.
Fire count in Punjab on Friday was 4,266, Haryana (155), Uttar Pradesh (51). The concentration of major air pollutants PM2.5 and PM10 was also high in cities neighbouring Delhi.
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’.
SAFAR said Delhi’s AQI did not improve much despite better ventilation, mainly because of highly favourable North-Northwesterly boundary level wind direction and perfect wind speed for the intrusion of pollutants from farm fires in Delhi.
Increased local surface wind speed with faster dispersion conditions are counteracting with increased fire-related emission and associated intrusion. But still, AQI is likely to improve for the next two days as dispersion will eventually play a lead role, it said.
According to the India Meteorological Department, the predominant wind direction was northwesterly and the maximum wind speed was 15 kmph. The minimum temperature was recorded at 13 degrees Celsius.
Calm winds and low temperatures trap pollutants close to the ground, while favorable wind speed helps in their dispersion.
The city’s ventilation index — a product of mixing depth and average wind speed — was around 9,000 meter square per second on Saturday — 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.
A ventilation index lower than 6,000 sqm/second, with the average wind speed less than 10 kmph, is unfavourable for dispersal of pollutants.
