Delhi's air quality slips back into 'poor' category
Delhi's air quality was recorded in the poor category on Thursday morning and is likely to deteriorate further amid a dip in the minimum temperature.
The minimum temperature in the national capital dropped to 9.4 degrees Celsius on Thursday morning, the lowest this season so far, due to cold winds blowing from snow-capped western Himalayas.
A change in the wind direction has also increased the share of stubble burning in the city's pollution slightly.
Delhi's air quality index was recorded at 272 at 9 am. The 24-hour average AQI was 211 on Wednesday and 171 on Tuesday.
An AQI between zero 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".
V K Soni, the head of the IMD's environment research centre, said the northwesterly winds increased the stubble contribution slightly on Wednesday.
According to the Ministry of Earth Sciences' air quality monitor, SAFAR, stubble burning accounted for 8 percent of Delhi's PM2.5 pollution on Wednesday. It was 3 percent on Tuesday.
Around 800 fire points were observed in Punjab, Haryana and western Pakistan on Wednesday. However, their impact on Delhi-NCR's air quality will not be significant, Soni said.
The central government's Air Quality Early Warning System for Delhi also said Delhi-NCR's air quality is likely to remain in the moderate and the lower end of the poor category on Thursday and Friday.
However, SAFAR said it may deteriorate to the lower end of the very poor category on Friday and Saturday, as the conductive meteorological environment created after rainfall is slowly retreating .
The minimum temperature settled at 9.4 degrees Celsius on Thursday the lowest this season so far -- and the maximum wind speed was predicted to be 10 kmph, according to IMD.
Calm winds and low temperatures trap pollutants close to the ground, while favourable wind speed helps in their dispersion.
Delhi's ventilation index -- a product of mixing depth and average wind speed was around 5,000 m2/s on Wednesday and is likely to be 7,500 m2/s on Thursday.
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 IMD had earlier said the minimum temperature in Delhi will drop to 9 degrees Celsius by Saturday, as cold winds have started blowing from hilly regions, which have witnessed a fresh bout of snowfall.
The minimum temperature this month, barring on November 16, has remained 2-3 degree Celsius below normal in the absence of a cloud cover, according to IMD officials.
The minimum temperature on Thursday was three notches below normal in the nattional capital. The city recorded a low of 10.6 degrees Celsius on Wednesday.
The average minimum temperature from November 22 to November 26 is 11.3 degrees Celsius, according to the India Meteorological Department.
The IMD had earlier predicted a drop in the minimum temperature in Delhi as cold winds have started blowing from hilly regions, which have witnessed a fresh bout of snowfall.
Clouds trap some of the outgoing infrared radiation and radiate it back downward, warming the ground.