E-Paper
What worked and what went against Delhi
New Delhi: Even as air quality in the national capital improved to ‘very poor’ on Saturday from the ‘emergency’ zone a day before, a number of factors helped mitigate
New Delhi: Even as air quality in the national capital improved to ‘very poor’ on Saturday from the ‘emergency’ zone a day before, a number of factors helped mitigate pollutants while some others continue to play foul. The city’s 24-hour average air quality index (AQI) on Saturday was 399, just two notches short of the ‘severe’ zone.
As per the bulletin released by the System of Air Quality and Weather Forecasting Research (Safar), a unit under the union ministry of earth sciences, there was a significant improvement in air quality on Saturday, even though it remained close to severe levels. Traces of rain and surface winds speeds picking up helped the air quality. However, smoke from stubble burning in Haryana and Punjab and westerly winds continue to add pollutants to Delhi air.
Factors that helped
Surface wind speed
Good average surface wind speed is one of the most crucial factors for dispersal of pollutants trapped in the air. On Saturday afternoon, the average wind speed was around 15kmph, which helped in flushing out pollutants and helped in lifting air quality from the ‘severe plus’ zone.
Expert Speak
“In meteorological terms, surface winds mean the winds between the ground and 10 metres up in the atmosphere, which directly affects breathing. An average wind speed of 10kmph is required for a consistent duration to disperse all the pollutants accumulated in the air,” said Kuldeep Srivastava, head, regional weather forecasting centre (RWFC), IMD.
Rainfall
While no rainfall was recorded at the Safdarjung station, which represents Delhi’s weather, traces of rain at the Lodhi Road and Ridge weather monitoring stations contributed marginally to the improvement of air quality in the city.
Expert Speak
According to a senior scientist at Safar, rainfall washes away the tiny pollutants hanging in the air. “These small particles, which are not gaseous, are the pollution that we breathe. Rain helps clean the air and results in pollutants settling down. If rain is accompanied with strong winds, it has a significant impact on improving air quality,” the scientist said.
Factors that played spoilsport
Stubble burning counts in nearby states
On Saturday, even though the share of crop residue burning contributing to overall air quality of Delhi was 17% — much lower than 46% on Friday, which was the season’s highest, it still added to the overall pollution load in the city. Upper-level winds from the northwestern region are carrying the smoke to Delhi, creating a situation unsuitable to the dispersal of pollutants.
Expert Speak
“Smoke from farm fires travelling to Delhi accumulates here and the air becomes still and heavy as the weather turns colder. This is in addition to the already polluted air from local emissions,” said D Saha, former head, Central Pollution Control Board’s air laboratory.
Low mixing layer height
The mixing layer height is an important meteorological factor affecting air quality. As per the Safar bulletin, it was low and hence unfavourable for dispersion of pollutants.
Expert Speak
“During this time of the season when air is colder, the mixing height reduces to less than one kilometre. When the boundary layer is shallow, pollutants are stagnated and come closer to the surface and hence could be felt while breathing. It is like a roof, which on reducing, increasing the concentration of pollutants. It rises with a rise in temperature and good sunlight allowing dispersion,” said Sagnik Dey, associate professor, IIT-Delhi’s Centre for Atmospheric Sciences.
