The national capital on Tuesday witnessed the cleanest air quality on the day after Deepavali in the past seven years, according to the Central Pollution Control Board (CPCB) data.
Though people across the city violated a ban and burst firecrackers on Deepavali night, leading to a main pollutant rising up to 10-17 times, the permissible limit in different parts of the city, the air quality improved with time due to favourable meteorological conditions, according to experts.
Better air quality this year is also because the air quality on the day before Deepavali (Sunday) this year was the cleanest recorded in Delhi a day before Deepavali in the last seven years.
Every year, Delhi experiences extreme air pollution during winter due to internal and external factors, including stubble burning in neighbouring States.
Rise and fall
As people burst firecrackers, the level of PM2.5 — a major pollutant — on Monday night and early hours of Tuesday rose, but it gradually fell as the day progressed, shows Delhi Pollution Control Board (DPCB) data.
PM2.5 (Particulate matter 2.5) are fine inhalable particles, which can get into lungs and bloodstream and lead to a range of respiratory and other diseases.
For instance, the PM2.5 level in Okhla at 12 a.m. on Tuesday was 1,042 ug/m3 — 17 times the permissible value of 60 ug/m3. It was 964 ug/m3 in Jahangirpuri at 11 p.m. on Monday and 616 ug/m3 at Anand Vihar at 12 a.m. on Tuesday. But they all fell as the day progressed.
Why better?
Anumita Roychowdhury, executive director, research and advocacy, CSE, said the air quality was better as Deepavali happened early this year, when conditions are comparatively warmer and windier, which aid dispersion of pollutants.
“Generally, as temperature drops, wind speed also drops. As wind speed drops, pollutants are not effectively dispersed and this leads to accumulation of pollutants,” she said.
As temperature drops, the mixing height also tends to reduce. Mixing depth is the height, measured from the surface of the earth, up to which pollutants can be dispersed in the atmosphere. During summers, the mixing height is generally higher.
“When wind speed falls and mixing height lowers at the same time, air gets trapped and this leads to a higher pollution. But this year as it was warmer, the wind speed was better and mixing height was also not so low. That is why though firecrackers were burst, there was dispersion of pollutants and it did not lead to a build up,” she said.
A DPCB analysis also stated that the better air quality was due to faster winds and higher temperature among other factors.
Stubble burning
Also, the effect of stubble burning in neighbouring States on the air quality of Delhi was “meagre” on Tuesday, according to the Central government-run monitoring agency SAFAR (System of Air Quality and Weather Forecasting and Research).
The expert said the spike in PM2.5 levels at night had a direct connection with firecrackers being burst on Monday.
About possible spikes in air pollution this winter, she said, “We could now see a spike in air pollution when stubble burning spikes and also in December-January when temperature drops drastically.”
The air quality is likely to get better on Wednesday and Thursday, as per SAFAR.
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